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2/4 The Frobenius Structure Conjecture for Log Calabi-Yau Varieties

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2/4 The Frobenius Structure Conjecture for Log Calabi-Yau Varieties
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Skeletal curves: a key notion in the theory
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2
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We show that the naive counts of rational curves in an affine log Calabi-Yau variety U, containing an open algebraic torus, determine in a surprisingly simple way, a family of log Calabi-Yau varieties, as the spectrum of a commutative associative algebra equipped with a multilinear form. This is directly inspired by a very similar conjecture of Gross-Hacking-Keel in mirror symmetry, known as the Frobenius structure conjecture. Although the statement involves only elementary algebraic geometry, our proof employs Berkovich non-archimedean analytic methods. We construct the structure constants of the algebra via counting non-archimedean analytic disks in the analytification of U. We establish various properties of the counting, notably deformation invariance, symmetry, gluing formula and convexity. In the special case when U is a Fock-Goncharov skew-symmetric X-cluster variety, we prove that our algebra generalizes, and in particular gives a direct geometric construction of, the mirror algebra of Gross-Hacking-Keel-Kontsevich. The comparison is proved via a canonical scattering diagram defined by counting infinitesimal non-archimedean analytic cylinders, without using the Kontsevich-Soibelman algorithm. Several combinatorial conjectures of GHKK follow readily from the geometric description. This is joint work with S. Keel; the reference is arXiv:1908.09861. If time permits, I will mention another application of our theory to the study of the moduli space of polarized Calabi-Yau pairs, in a work in progress with P. Hacking and S. Keel. Here is a plan for each session of the mini-course: 1) Motivation and ideas from mirror symmetry, main results. 2) Skeletal curves: a key notion in the theory. 3) Naive counts, tail conditions and deformation invariance. 4) Scattering diagram, comparison with Gross-Hacking-Keel-Kontsevich, applications to cluster algebras, applications to moduli spaces of Calabi-Yau pairs.
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Transcript: English(auto-generated)
Thank you very much for coming to the second lecture of this mini-course. Here is the plan for today. First, I'll explain the theory of Tamkin's metrization
and the Kondasevich-Soberman essential skeleton. Second, I'll introduce skeletal curves, which is a key notion in the theory. Third, I'll explain where do skeletal curves come from in practice, natural sources of skeletal curves.
And fourth, I will introduce naive counts of skeletal curves. And finally, I will give a proof of the symmetry theorem by skeletal curves
as an application of the theory of skeletal curves. We will see other applications of skeletal curves in the next lectures. Okay, so let's start with the first part, Tamkin's metrization
and the Kondasevich-Soberman essential skeleton. The idea is the following. Belkovich's non-archimedian analytic spaces have very complicated underlying topological spaces.
For example, the analytic P1 is an infinite tree containing infinitely many vertices and infinitely many branches. And the Belkovich analytic elliptic curve is infinitely many trees attached to a circle.
It is impossible to visualize Belkovich's analytic spaces in higher dimensions, but they contain very nice piecewise linear subsets called skeletons.
In general, skeletons are not unique. They depend on the choice of formal models. But if we are given a volume form omega on the analytic space, then we can define a unique skeleton as k of omega associated to the volume form omega.
Thus, for Calabi-Yau variety, where we have a unique volume form up to scaling, we have a canonical skeleton called the essential skeleton.
For example, the circle inside this elliptic curve is the essential skeleton of the elliptic curve. Here is the history of essential skeleton.
In 2000, Kondasevich and Soberman constructed an essential skeleton inside non-archimedian analytic Calabi-Yau space X over C double parenthesis T of maximal degeneration. Here, C double parenthesis T denotes the field of formal Laurent series.
Their method is the following. First, they define a weight function psi on divisorial points, x div inside x using semi-stable models of x.
And then, they define the essential skeleton S k of x inside x to be the closure of the minimum locus of psi.
After that, in year 2012, Mustata Niiges extended the weight function psi to the whole analytic space. So it is no longer necessary to take closure for defining the essential skeleton.
Then, in 2017 and 2018, Brown-Mason and Murray-Mason-Stevenson, they extended the weight function and the essential skeleton to pairs.
And in 2014, not really in chronological order, Michael Temkin made a vast generalization. He bypasses completely the use of semi-stable models.
In this way, he is able to extend the theory of weight function and essential skeleton to any non-archimedian base field, not necessarily of characteristic zero or discrete valuation.
And moreover, his theory works in the relative situation for any analytic space x over another analytic space S. His method is the following.
First, he provides the sheaf of K-ler differentials on x, omega x with the maximum semi-norm called K-ler semi-norm, which is the maximum semi-norm making the differential d
from the sheaf of functions on x to the sheaf of differentials on x, a non-expansive map. He calls this maximum semi-norm K-ler semi-norm. Then, this gives rise to a semi-norm on the canonical bundle Kx
by taking top exterior power. Of the sheaf of K-ler differentials. Now, if we have a volume from omega, and if we apply this semi-norm to omega, we obtain a real-valued function.
And Temkin proved that this real-valued function is equal up to a constant to the Condesevici-Soberman-Mustat-Anikas weight function in the situations where the weight functions are well-defined.
And the essential skeleton in Temkin's language is just the maximum locus of this K-ler semi-norm of this volume from omega.
This is roughly his method. And since we will need Temkin's formulation to establish some properties of essential skeletons for some of our proofs, here, let me give more details of Temkin's construction.
So first, let us define some semi-norm at the level of rings. Definition, given a semi-norm the ring B
and a homomorphism of rings, phi from A to B, we equip omega B over A, the module of relative K-ler differentials with the K-ler semi-norm by the following formula.
For any element x in this module of relative K-ler differentials, we define its K-ler semi-norm to be follows. First, we write x as sum of Ci dBi,
where Ci and Bi lie in B. Ci and Bi are elements of B. And we take the maximum over i of the norm of Ci times the norm of Bi.
And then we take inf of this maximum over all possible ways of writing x as sum of Ci dBi. So this gives the definition of K-ler semi-norm at the level of rings. And Temkin proves that gives a canonical characterization
of this K-ler semi-norm defined by the explicit formula. He proved that this K-ler semi-norm is the maximum semi-norm that makes the differential d from B
to the module of relative K-ler differentials a non-expansive a-homomorphism. Now let's consider the global geometric situation.
Given f-amorphism of K-analytic spaces, where K is any non-archemedian base field, we apply this above definition at the level of rings, we obtain immediately a pre-shift of K-ler semi-norms
on arvinoid domains. Then via shiftification, we obtain so-called K-ler semi-norm on this shift of relative K-ler differentials.
And similarly, we have a canonical characterization as in the lemma above. Temkin shows that this norm defined from shiftification is simply the maximum semi-norm on this shift of relative K-ler differentials,
making the map d from the shift of functions to the shift of differentials a non-expansive map. And now if we take top exterior power
and arbitrary tensor product, we obtain the K-ler semi-norm on Plurie volume forms. There is a small technical point is that in fact we have to consider
so-called geometric K-ler semi-norm after passing to algebraic closure in order to get better properties. Now here is a theorem of Temkin.
For any Plurie volume form omega, we can take its K-ler semi-norm and we obtain a real valued function on x. Temkin's theorem says that this K-ler semi-norm of omega
is an upper semi-continuous function. The theorem says that this real valued function, semi-norm of omega, is an upper semi-continuous function.
Now we make the following definition. We define the skeleton of x associated to any Plurie volume form omega to be simply the maximum locus of the K-ler semi-norm of omega. It's possibly empty if the maximum doesn't exist.
And we denote this skeleton associated to omega by S-K of omega, considered as a subset of x.
So this definition of skeleton depends on the choice of some Plurie volume form. And now let's introduce the definition of essential skeleton, which is just a union of all such skeletons
over all possible volume forms. Here it is. Definition essential skeleton. Let K be a non-archemedian field of characteristic zero and let x be any smooth K variety.
We define the essential skeleton of x denoted as S-K of x to be simply the union of skeletons associated to all omega
where over all log-Plurie volume form omega. And by definition, a log-Plurie volume form is just a section of this line bundle
which is some arbitrary tensor product of the logarithmic canonical bundle. And here we take any SNC compactification x in y and so y is any SNC compactification of x
and d is the complement of x. And one can show that this space of sections is independent of the SNC compactification we choose. So we can just choose any SNC compactification
consider all Plurie volume forms as sections of any tensor powers of the logarithmic canonical bundle. Take the associated skeleton and then take union. This is by definition the essential skeleton of x.
Since we have taken union over all volume forms it's just canonically associated to x. Let's introduce a notation for later use. When a compactification x in y is fixed,
it's usually quite natural to consider the closure of the essential skeleton of x inside the identification of y. And we denote this closure by Sk bar x.
Sometimes we call it the closed essential skeleton. So that makes sense if we have a compactification fixed. Let's give some examples of essential skeletons.
First example, we take x to be the algebraic torus. In this case, the essential skeleton of x is homomorphic to Rn
and it lives in the identification of the algebraic torus. One can show that the essential skeleton of x is in fact a birational invariant. With respect to volume forms, of course.
So if u is a logarithm variety containing a Zariski open torus tm, m being the co-character lattice as in the previous talk, then the essential skeleton of u is just equal to the essential skeleton of the torus
and it's homomorphic to mR, the lattice m tensored with R. So it's just Rn.
So for our localabial, the essential skeleton is very simple, just Euclidean space. Second example, we take x to be p1 minus some closed points.
In this case, the essential skeleton of x is equal to the convex hull of these points. So recall that the analytic p1 is an infinite tree with infinitely many vertices and infinitely many branches.
And we take out some closed points from this tree. The closed points, they are points on the boundary of this disk. Then the claim is that the essential skeleton of the punctured p1 is equal to the convex hull of these points.
So here we take out four points, four closed points, and then the essential skeleton is the convex hull of these four points, which is this red subtree inside this infinite tree.
Example three, we take x to be an elliptic curve with bad reduction, whose analytification is infinitely many trees attached to a circle.
In this case, the essential skeleton is just the circle inside and it's inside this analytic space. Then we have a two-dimensional analog of this example three,
where we take x to be a K3 surface with maximal degeneration. And in this case, the essential skeleton is homomorphic to S2, two-dimensional sphere inside the analytification of x.
The final example we want to give is the following. We take x to be m0n, the moduli space of p1 with n marked points.
Then we show that the essential skeleton of x is homomorphic to trop0n, the moduli space of rational tropical curves with n legs. So, we show this by considering the classical D'Elima for compactification,
m0n bar of x consisting of stable n-pointed rational curves. And then we show that it gives rise to a minimal compactification
and we further deduce that the essential skeleton is just the usual skeleton associated to the compactification. And that skeleton was previously studied in the work of
Abramovich, Caporoso, and Payne. So that's, for the moment, that's what I want to explain for the theory of Temkin's metrization and essential skeleton.
Now let's turn to the next section. We will introduce the notion of skeletal curves, which is a key notion in the theory. The idea is the following.
Let's consider an analytic curve C in a log-kalabi-Yau variety U-analytic. We have our log-kalabi-Yau variety U-analytic, and the identification of our log-kalabi-Yau variety,
and inside we have this blue essential skeleton, piecewise linear subset embedded in this analytic space, this blue essential skeleton. And we consider some analytic, this red analytic curve C inside our kalabi-Yau.
If the dimension of U is greater or equal to two, then by dimensional reason, the curve C never meets the essential skeleton, because the points in the essential skeleton are valuations on the generic point of the variety U.
And the points in the curve C is a one-dimensional subspace. The points of the curve C, they are at most of dimension one, while the points in this essential skeleton is of top dimension.
So this curve C has no chance to meet this essential skeleton,
just because of dimension reason. But we can let the curve C touch the essential skeleton SKU, essential skeleton of U, if we allow the curve C to be defined over a big non-archemedian field extension,
k in k, k prime of k. And here is the surprise. And soon as some k point of the curve C touches the essential skeleton of U, then the whole skeleton of the curve C must lie in the essential skeleton of U.
So we observed that in general, by dimensional reason, there's no chance for a curve C to touch this green essential skeleton. But if we allow the curve C to be defined over a big enough non-archemedian field extension,
then as soon as some k point of C touches this essential skeleton SKU, then the whole skeleton of C will lie in the essential skeleton of U.
Now let us give the precise statement. We fix some log-Calabi-Yau variety U, the orange U over k, some volume form, omega on U,
U in Y, here Y, some SNC compactification, and let D be the divisors at infinity. We denote by D essential inside D the union of essential divisors.
By essential divisor, we mean divisors where the volume form omega has a pole. So here in the picture, these dark blue curves denote essential divisors, while this light blue,
light blue curve is a non-essential divisor. And now we will consider some curve C in Y, this red curve that touches some points of the boundary divisor.
So as we said, we must, if we want the curve to touch the skeleton, we must pass to a big enough field, base field extension. So let k and k prime be a non-archemedian field extension,
and we choose C, a rational nodal curve over k. We consider F a k prime analytic map from the base change,
from the base change of C to the base change of Y, such that the pre-image of F, the pre-image by F of the divisor D is equal to the pre-image by F of the essential part.
In other words, the curve C meets only essential divisors at infinity. And furthermore, we ask that the pre-image of the essential divisors is some linear combination of k points p i in C,
such a curve which mainly lies in the interior U. And when it hits the boundary divisor, it hits only the essential part at some k rational points with some multiplicities.
So F is a k prime analytic map between the base changes, and we consider the composition of F with this natural projection map
given from the base change. So we have made a base change, and we have the natural projection of base change, and we consider the composition which we denote by f y.
Now the claim is that if f y of x lies in the essential skeleton of U for some k point x, then f y of the essential skeleton of the base change of the punctured curve
C naught is just C minus the marked points. So then f y of the skeleton of the punctured curve will lie totally in the essential skeleton of U.
In other words, the whole skeleton of the curve lies in this essential skeleton of U. And recall from the example that we mentioned above,
the essential skeleton of such a punctured curve is just equal to the convex hull of all the marked points in the analytic space. So this is a precise statement, and we call such F skeletal curves.
Here is an example of a skeletal curve. We take U to be the algebraic torus, and we have seen from the examples above that the essential skeleton of the algebraic torus
is just Rn, this blue plane, essential skeleton homomorphic to Rn. And we take our curve C to be just P1. So it's an infinite tree, and we choose four marked points in P1.
One, two, three, four, four marked points in P1. Then the essential skeleton of the punctured curve C naught, C minus the four marked points, is just the convex hull of these four marked points, which is this red sub-tree inside this infinite.
A tree. And now we consider a map from this P1 to the algebraic torus. So as we said, in general, this map, the image of this P1,
has no chance to meet this blue essential skeleton, just because of dimensional reason. But if we pass to a big enough base field extension, then it might happen.
And the theorem says that if some k-point of the curve C hits the blue essential skeleton, then the whole skeleton, this red sub-tree, the whole skeleton of the curve will lie in the essential skeleton of U.
The major advantage of skeletal curves is that they have canonical tropicalization. Since the map Fy maps the skeleton of the curve into the essential skeleton of U,
we can just restrict this map Fy to the essential skeleton of the curve, and we get some tropical object from some finite tree, some tree, which we denote by gamma, to this polyhedral object.
And this restriction is independent of any choice of retraction map from the identification of U to the essential skeleton of U.
So in general, for general curve, this image of the skeleton of the curve does not lie in the essential skeleton of U. Therefore, to get anything tropical,
we must further compose with a retraction from the identification of U to the essential skeleton of U. But this retraction is not canonical. For example, different minimal compactification U in Y
gives different retraction maps. So then for general curve, different retraction maps gives different tropicalizations. But for skeletal curves, the compactification does not matter. We always have a canonical
tropicalization. And we call this restriction the spine associated to the skeletal curve. So in the example above, the associated spine is simply the map from this red sub-tree
to the blue plane, this red curve. And this is canonical, independent of any choice of retraction. Now let me explain the idea
of the proof of the skeletal curve theory. Let's first recall the statement. We have some non-archemedian field extension, k prime of k, and a rational curve, nodal rational curve
c over k. And we consider a k prime analytic map of the base change of c to the base change of y, such that the curve meets only essential boundary divisors
at some k points. And we consider the composition of f with the projection map
of from the base change. The claim is that if f y of x, if f y sends some k point of the curve to the essential skeleton of U, then f y sends the skeleton of
the base change of the punctured curve, which is just c minus all the marked points, to the essential skeleton of U. In other words, the whole skeleton of the curve lies in the essential skeleton of U. Here is the idea of the proof. So for the proof,
we put the map f above into a family, and we consider the skeleton of the family and also the skeleton of the base. We want to relate various skeletons together.
In order to put the map into a family, very naturally we consider a home scheme consisting of all maps from the curve to y-analytic, and we consider the subspace of
all maps f from c to y-analytic of the same curve class and the same intersection pattern with d as the given one. We have the following diagram. So h is some space of maps,
over h we have the universal curve, which is just a product. Since it's just a space of maps,
the domain curve doesn't change, so it's just a product c times h. We have two projections, p c to c, p h to h. Then we have the universal map from the universal curve to y,
which we denote by e, and we consider also the map phi from the universal curve to c times y, whose first factor is projection to c and the second factor is given by the universal map.
By the deformation theory of curves, we can show that the map phi is a tau over some dense Zariski open subset of the target. It's generically tau. Furthermore, using
deformation theory of curves by computing the tangent spaces of h, we show that the volume from omega on u in y, the volume from omega on u gives rise to a volume from omega h
on h. So it induces a natural volume from omega h. Then we do an explicit computation. One can see that the pullback of omega, omega is here, the pullback of omega
by e and the pullback of omega h, omega h is on h, by the projection map ph, they agree on ph horizontal tangent spaces of the universal curve. So they may not completely
agree but they agree on horizontal tangent spaces. This implies that for any one form alpha on the punctured curve c, if we pull back alpha by the projection map phi
and we wedge the pullback of omega by e, this is equal to the pullback of alpha by p c and wedge the pullback of omega h by ph.
It's just because they agree on the horizontal tangent space, the two forms, so if we wedge anything vertical, we get equality. We denote this by equality star. And the second,
for any k rational point, k point x in c, which is not the marked point p i,
we consider the evaluation map at x ev x, which is a map from this space of maps h to u, just evaluating at x to u-analytify. Then since such x gives a horizontal section,
gives a horizontal section of this projection ph, we see, since these two forms agree on the horizontal tangent spaces, and
if we pull back using the horizontal section, we see immediately that the pullback of omega by ev x is equal to, is just equal to the volume from omega h.
And that implies that the pre-image of the essential skeleton of u by ev x is equal to the, is just equal to the essential skeleton, to the skeleton of h associated to the volume from omega h.
This is because, again, by the deformation theory of curves, one can see that the evaluation map ev x is generically et al, and that implies that pullback of skeleton is equal to skeleton of pullback. So here, pre-image of skeleton
by the et al-ness of ev x, pre-image of skeleton is just a skeleton. Skeleton of pullback. So this omega h is pullback. And we denote this equality by double star.
Now let's pick one fiber of our family. So we choose any point f in h. h is the space of maps, we choose any point f. We denote by c f the fiber of the universal curve at f. So recall
that the universal curve is just a product, so the fiber at f is just some base change of c. And f h is the space of maps, f is a point in the space of maps,
so f gives a map from the fiber c f to Y analytic, which is just the restriction of the universal map e from the universal curve to Y analytic.
There should be no c here. And it's natural to denote this map f, because it's really given by f. Now assume that f x lies in the essential skeleton of u
for some k rational point x. Since f x is just evaluation of f at x,
so this equality double star implies that f lies in the skeleton associated to the volume from omega h. Because we assume f x lies in the skeleton of u, and f x is just evaluation of
x at f. So by this equality, we know that evaluation e v x of f lies here means that e v x of f lies in skeleton of u means that f lies in the pre-image of the skeleton of u
by e v x, which means that f lies in the skeleton associated to omega h. So we get a very nice characterization of f now just from our hypothesis.
And recall our goal is to show that f of the skeleton of the punctured fiber c naught f lies in the essential skeleton of u.
So in order to show that, let's compute this pre-image by phi of the product of the skeleton of c naught times this the skeleton of u. We want to show this,
we compute this product, we will use the map phi. By definition, the essential skeleton of c naught is just a union of skeletons associated to all possible log-volume forms on c naught.
Here, taking union over log-volume forms or log-plurie forms, they are the same. So first equality is by definition of essential skeleton. Next, using Tamkin's theory of metrization, one can show that skeleton of product is equal
to product of skeleton. So here we have product of skeleton and it's equal to skeleton of product with respect to the wedge of the volume forms.
And next, recall that phi is generically et al by deformation theory. This implies that pullback of skeleton is equal to skeleton of pullback. So here we have pre-image of skeleton
by some et al map and this is equal to skeleton of the pullback of the form by this map. Now recall, by definition, phi has two factors. First factor is the projection to c,
second factor is the universal map e. So by definition of phi, this is just equal to the
PC wedge pullback of omega by e. And now we apply our explicit computation, this equality of
forms on horizontal vector spaces. We apply our explicit computation, we deduce that this is equal to skeleton of this wedge product. So we replace this wedge, this pullback of omega by e by this pullback of omega h by ph. And then to summarize, this by definition again is just
the essential skeleton of c naught of the punctured curve times the essential skeleton of omega h. And we observe that by Tamkin's metrization theory, a point z lies in the
essential in the skeleton of a product x times y if and only if z projects to a point y in the skeleton of y, of big y, and z lies in the skeleton of the fiber
x, y. So a point lies in skeletal product if and only if it projects to skeleton of the base and moreover it lies in skeleton of the fiber. Therefore, since f lies in the skeleton
associated to the form omega h, so we think h as the base here, therefore for any x in the skeleton of the fiber, the skeleton of the fiber, the punctured curve at f,
this computation, the equality above between this one and this one shows that x just lives in the pre-image of the product of skeleton. Because by what we just said,
f already lives, so we look at this line, f already lives in the skeleton of the base. Now if we choose any point in the skeleton of the fiber,
then this point actually lies in the skeleton of this total space. And that is just equal to the pre-image of this product of skeleton. So this shows that x lies in the pre-image
by phi of this product of skeleton. And we deduce that, just to record the definition of phi, we deduce that f of the skeleton of the punctured curve at f lies in the essential skeleton of u. In other words, the skeleton of the curve
maps to the essential skeleton of u. So proof complete. Remark, by adding extra k points to our curve C as marked points, the above argument has a
stronger and perhaps more surprising result. We can show that the convex hull of all
k rational points inside the fiber C-F maps to the closed skeleton, S-K-U, the closed essential skeleton, which is just the closure of the essential skeleton
in this fixed compactification, y-analytic. So not only the skeleton of the curve maps to the skeleton of the target localabial, but the convex hull of all k points will lie there.
So that is all I want to say for the proof of the skeletal curve theorem. And if you did not follow every line of the proof, no worries.
And now we will move to the next topic. So the question is, the skeletal curves seem so nice. They have canonical tropicalization and we will be using them for many purposes.
So the natural question is, where do they come from in practice? In the next section, we will talk about natural sources of skeletal curves. Let's first make five minutes break before moving on to the next section.
So the skeletal curves, they seem so nice, but where do they come from in practice? And that's what I will explain in the next part of this lecture. So let's explain where do skeletal curves come from.
Recall from the Frobenius structure conjecture that we are interested in counting rational curves in Y with prescribed intersections with the boundary D. So we have Y, some SNC compactification of our localabial U, and we are interested in counting
this kind of red curves whose intersection numbers with the boundary divisors are fixed. Or if we can also phrase it in terms of the interior, in other words, we are interested
in punctured rational curves in U with prescribed asymptotics at the punctures. Anyway, U is what we ultimately care about. So let's fix some notations for convenience.
We have a tuple, bold P, consisting of Pj, where Pj are integer points in the skeleton. So in my last lecture, I gave an explicit formula for this SKUZ, which is just
zero disjoint union with positive integer multiples of essential divisorial valuation. And now, in this lecture, I explained the theory of essential skeleton,
and they are just integer points inside the essential skeleton. So we fix this tuple in order to prescribe intersections of our red curve with the boundary D. Some Pj can be zero, and we call such j internal marked points.
For example, we can have an internal marked point, P4, internal because they map to the interior.
And for non-zero Pj, we call such j boundary because these marked points are supposed to go to the boundary, and we write Pj as, in this explicit form, multiples of some divisorial valuation Mj times Nu j,
and the divisorial valuation is just some divisor at infinity. So we can always assume that Nu j is given by some component of our boundary D after making some blow up.
Now let's consider the moduli space, the moduli stack, Mu both P beta, consisting of n-pointed rational stable maps from some nodal rational curve C with marked
points Pj to Y of class beta, such that each boundary marked point Pj meets the interior of the divisor Dj with tangency order Mj and no other intersections with D.
So exactly the sort of moduli stack we consider in the Frobenius structure conjecture. And if we pick an internal marked point Pi, then we can evaluate at this internal marked point,
and we obtain something in U. And we can also take the domain and take the stabilization of the domain, we obtain a point in the Deling bound for the stack of
n-pointed stable rational, stable n-pointed rational curves. So recall that the domain of a stable map may not be stable. Thus, we need to take a further stabilization in order to get
a stable curve. And we put them together, we have the natural map of phi i. It's very analogous to the map phi we considered in the proof of the skeletal curve theorem. And now we have the theorem, source of skeletal curves, which says that
phi i over the skeleton inside the target has finite fibers, and moreover the fibers, they consist of skeletal curves, which just means that the pre-image of phi i by phi i of this
skeleton inside the product consists of skeletal curves. So that's the way we produce skeletal curves in practice. And just a small
point, here we consider closure of the skeleton, so it's a bit stronger than we just consider skeleton. And that is important in the theory because we also want to consider
degenerate domains. And in the proof of associativity, for example, and also just in the classical theory of Gromov-Witten invariance,
it's useful sometimes to degenerate stable maps and to break them apart. So that's why we also consider the closure of skeleton, which will contain these degenerated curves.
So that's the way we produce skeletal curves in practice. And the proof is the following. For finiteness, we again use the deformation theory. We can show that for any fixed modulus of domain, the fiber of phi i at mu is finite
eta over some Zariski-dense open subset of the log-Kalabi-Yau. And
skeletoness follows from the skeletal curve theorem. So here finiteness allows us to count curves naively without using virtual fundamental classes. And let me explain now
how do we count them naively using this finiteness result. So let me explain now naive counts of skeletal curves. The above theorem, source of skeletal curves,
suggests a simple definition of naive counts associated to spines in the essential skeleton of U, which we explain now. And the study of properties of such counts is the main technical foundation of our theory. So recall we have our natural map phi i going from the modularized
space of stable maps to the modularized space of stable curves by taking domain modulus and
to our log-Kalabi-Yau by taking evaluation of some internal marked point. Now any stable map inside the pre-image by phi i of the skeleton in the target
is skeletal by the above theorem. So we have a canonically defined spine, which is just we take restriction of f to the skeleton of our curve and this maps to the
skeleton of U by the skeletal curve theorem. So here we take the closure of the skeleton, it doesn't change much, just more convenient to work with because otherwise it's just infinite curves like in Rn. If we take closure, it's just more convenient for notation,
we can say where infinite point goes. So that's a very minor point. And conversely, given any abstract spine h from some graph, some tree to the skeleton of U
and some curve class beta, we want to count all skeletal curves of class beta giving rise to
spine h. So this is our goal now. We want to define the count n h beta, which is supposed to be the number of skeletal curves with spine h and the curve class beta.
So first question, what is an abstract spine in the essential skeleton of U? First, observe that the essential skeleton of U has an intrinsic conical piecewise integral
linear structure. The idea is the following. So if we take any SNC compactification of U, we obtain a simplicial cone complex structure on the essential skeleton. And now,
such structures given by two different SNC compactifications, they are just related by some piecewise integral linear map. Therefore,
this essential skeleton has some intrinsic piecewise integral linear structure. And thus, it makes sense to define a spine in the essential skeleton to be a piecewise
integral affine map h from some nodometric tree to the essential skeleton. Here is a picture. Now we consider such nodometric tree gamma. This is our essential skeleton and we consider
a spine inside. And we denote by Vj the set of one-valent vertices of gamma. Let us first consider the case of extended spine. In other words, let's assume that all
the Vj's are infinite vertices. And we denote by Pj the weight vectors at every Vj.
In other words, just the derivatives. So these purple vectors are Pj. And we denote the whole order Pj. We put them as a tuple, bold P. So here we have five
one-valent vertices, V1, V2, V3, V4, V5. And V5 shoots up vertically. Which means that the leg V5 is mapped to a point. The map h can be constant on the whole leg.
And in this case, this P5 is zero, the derivative. So recall that we said that the essential skeleton of M0n is homomorphic to the moduli space of
tropical curves, rational tropical curves with n legs. And in fact, this holds also after taking closure. So the closed essential skeleton of M0n is actually isomorphic,
homomorphic to the moduli space of stable extended nodal rational tropical curves with n legs. That's gamma, the nodal. Oh, sorry. Yeah. Oh, I have a question on this.
So the trop bar is just a naive closure of the tropicalization. Trop bar is a compactification of the moduli space of tropical curves. So you all know internal legs of infinite lengths. Yeah.
The legs, yes, I allow some edges to have infinite length. Yeah, because I don't know like the, I think the Jonathan Wise and the Melody Chen, they have, they defined the trop bar, like, which is, yeah, I don't know, is it coinciding
with the Jonathan Wise and the Melody Chen's definition of the trop bar? So this trop bar 0n, I think it was first considered in the paper by Abramovich, Caparazzo and Payne. Okay. Called the tropicalization of moduli space of stable curves, probably. And with Sean, we show
that here the essential skeleton is just the skeleton given by the classical Delingman for the compactification. And then we apply a result in the paper of Abramovich,
Caparazzo and Payne, which identifies the skeleton associated to the Delingman for the compactification with this moduli space of extended tropical curves.
Okay, thanks. Yeah, so they are really natural objects when we consider compactification. Yeah, so we have our nodal metric tree, and it's just a point in this moduli space
of tropical curves. So by this homomorphism, we obtain a point in the skeleton of M0n. And recall, we have our natural map phi i from the moduli space of analytic curves to the moduli space of domain times our local BL. Inside, we have a product of skeletons,
and then we have a point gamma in the skeleton of the first factor, and we also have the point hv i. So in this picture, it's just this point, h of v5. We also have this point
in the skeleton of u, so the pair together gives a point in the target. And now we just take the pre-image by phi i of this point in the target.
And by the skeletal curve theorem, the pre-image is just a finite set and consists of only skeletal curves. But now we have a finite set,
and not all curves inside this finite set are good. So we further restrict to a subset f i h beta consisting of stable maps whose spine is equal to h.
So this subset, this set phi i inverse, it just says that our curve has the correct domain, and the internal marked point i p i maps to the correct place. That's all. It doesn't
say anything about the spine. That's why we consider a subset with the right spine, and then the count n i h beta that we want, that was our goal, we want to define,
we just let it be the length of this subset, considered as a zero-dimensional analytic space, because probably we have some neopotents
or multiplicities. If we pass to a bigger enough algebraic if we pass to an algebraic closure, then it's enough to take just the cardinality of this set.
So we define the count n i h beta to be this length, and n i h beta just means the number of skeletal curves associated to the spine h,
curve class beta, and by evaluating at the i-th marked point. So, intuitively, this number counts these purple rational curves, closed rational curves, with the given spine,
given red spine. And more generally, we consider also non-extended spines. Sometimes we call it truncated spines. In other words, we allow some one-valent vertices, vj, to be finite.
So the idea is to use toric tail condition to define the counts associated to truncated spines as in the first lecture. Here is the picture. We have a skeleton of our log-calabial,
and we consider a truncated spine. So here, the vertices v1, v3, v4, they are finite vertices, and v2 and v5, they remain infinite vertices.
And in order to count the skeletal curves associated to such spines, we recall that we have torus inside u with co-character lattice m, and this implies that the essential skeleton of u is equal to the essential skeleton of the torus and
is homomorphic just to m tensor with r, rn. And now we can extend the truncated spine. This truncated spine H together with curve classes, and we obtain an extended spine H hat
and an extended curve class beta hat. So I wrote things regarding curve classes in blue
just to mean that you can ignore it if you are not familiar with the theory. They are more auxiliary. So we can just focus on the spine.
So we apply the constructions above. We apply the constructions above to this extended spine H hat and extended curve class beta hat. We obtain a finite set, fi H hat beta hat as above, consisting of closed curves with spine H hat.
And now we consider a subset, a further subset satisfying the toric tail condition. We ask each punctured tail disk to lie inside our torus. So then
we are ready finally to define our count associated to such a truncated spine to be simply the length of this subset considered as a zero-dimensional analytic space.
So intuitively, this number just counts this kind of open curves with given spine. By open, we mean curves with boundaries. So that's the definition of our naive counts.
And we have the following theorem concerning this number, this counting number.
So assume the spine H is in the general position. More precisely, we assume H is transverse to walls inside the skeleton of U. So I will introduce the notion of walls in the next lecture.
Here, let's just imagine that H is in some general position, and then the count H i n i H beta, meaning the number of skeletal curves associated to the spine H and the curve class
beta by evaluating at i-th marked point is independent of the choice of the internal marked point i and nor of the choice of the torus inside.
So remark, the independence on i used to be called the symmetry theorem and had a tricky proof via deformation invariance. Now we have a much more conceptual proof via skeletal curves, and let me sketch below. So that shows another application of
skeletal curves. We can get a conceptual understanding of this independence of the
choice of the marked point where we evaluate. If there are any questions, you can ask. Otherwise, I'll just go to the proof of the symmetry theorem.
Let me explain the symmetry theorem via skeletal curves. The symmetry theorem is just the independence of our count on the choice of the place,
the point where we evaluate. I mean, if you think why this is true, it's not really obvious because we evaluate at an internal marked point i, and we want to show that it doesn't depend
on the choice of this internal marked point. So maybe we want to move if we have two different places where we evaluate, maybe we want to move from one place to another. But the trouble is that when we move from one place to one place
to another, at some point we will cross some walls and the spine is no longer transverse. So this kind of deformation invariance no longer holds if we move across the walls.
In general, we will have some wall crossing formula if we move across some walls. And here,
the way we want to show via skeletal curves is that we can actually move through the walls if it is a skeletal curve. So let me give more details. So the idea is to move from one place to another in the skeletal curve setting,
and in that setting, we can go through the walls without some complicated wall crossing formula. So let's recall the setting from the proof of the skeletal curve theorem.
We have a home scheme, parameterizing maps from domain curve C to the target Y, and we consider a subspace consisting of maps with given intersection pattern with the boundary
and also some given class beta. And we also had these natural maps. We have universal curve, two projections, universal curve is just a product, and we have the natural map phi.
First factor is just projection to C. Second factor is the universal map. And on Y,
we have volume from omega, and on H, by deformation theory, we produced a volume from omega H. For any point F in H, we denote CF, the fiber of the universal curve at H, and
we denote the map, induced map again by F, because that's what F means. So recall from the proof of the skeletal curve theorem, F being skeletal is equivalent to F lies in the skeleton of H associated to the volume form,
and phi X lies in the skeleton of the target if and only if F lies in the skeleton of H,
and X lies in the skeleton of the fiber. So that is what we have shown. The main point in the proof of the skeletal curve theorem, if you're confused about the rest, just...
Yeah, no, you can't formally write F lies in the skeleton. F of curve lies here. F is a map. No, F is a map, but F is also a point of the space of maps. Ah, H. Ah, yes, yeah, sure, sorry. You're right. Ah, sorry.
Yeah, so F is a map, but it's also a point in the space of maps. Oh, okay, sorry, yeah. Yeah, so here we really showed that F as a map is a skeletal if and only if F as a point lies
in the skeleton. So that's what the theorem says. Yeah, so now we assume F to be skeletal. In other words, we assume the point associated to the map lies in the skeleton. So we have a
map to the skeleton of the fiber, which is the same as skeleton of the curve. So it maps to
the skeleton of U. So that's all what we have done in the proof of the skeletal curve theorem. And now let delta denote the graph of the spine H. And here we make a claim. Assume
that the spine H is in general position. In other words, assume it is transverse to walls. Then the skeleton of the fiber CF inside the pre-image by phi of delta
is a connected component. So recall from this equivalence, or recall from just from the fact that the whole skeleton of the curve lies in the skeleton of U,
the skeleton of this fiber just lies in the pre-image. And we claim that this subset is a connected component. So I drew a picture for your understanding.
Recall that our natural map phi goes from the universal curve C times H to C times Y. And we have the graph of the spine delta inside this target C times Y. And we have phi
going from C times H. So this is C times H. H is the base, the space of maps, and every fiber, and this total space is product C times H, every fiber is the curve C.
So if we take pre-image by phi of delta, by the finiteness of phi, we obtain some graph inside the product C times H.
We obtain some graph. So this fiber, the skeleton of this fiber CF, it lives inside the pre-image because this goes to the skeleton, as we have a skeletal curve. But we also have some other pieces.
And the claim says that this fiber is actually a connected component. They do not, it doesn't touch with other fibers. So it's not difficult to see that, to show the claim.
First, by the equivalence star, we see that this fiber, the skeleton of fiber, is equal to the fiber of the pre-image
to the fiber of the pre-image over F. So this implies that since it's a fiber and the fiber is always closed, so this implies that the inclusion is closed. And we are left to prove
that the inclusion is open. And we, suppose the contrary, we pick some, so suppose the contrary, we pick a germ of a path, like this green germ, starting from the fiber,
the skeleton, this skeleton of a fiber. We pick a germ of path, zero epsilon, this fiber, and then goes out. And we can, so since the image
by phi of this alpha lies in this product, we can write it, write alpha as
two components, QTFT. Maybe I should say that since, so since we have shown that
the pre-image of phi, so alpha is a germ of path in the pre-image by phi, but we have shown that the pre-image of phi is just the product of skeleton, skeleton of C and skeleton of H. So we can write alpha as two components, QTFT.
QT is some point on the curve, and FT is some points in the modular space of maps. And we denote, since everything is skeletal here, we denote by HT the span of FT.
And now observe that the condition that alpha lies in the pre-image by phi of delta, or phi alpha lies in delta, and the delta being the graph of H0, this just implies
that HT of QT is equal to H0 of QT. So we have QT fixed, a fixed point on our curve, and it implies that for this small deformation of our map H, the image of this point doesn't
move. And then by the continuity of tropicalization from FT to HT and the rigidity of transverse spines, this I will give more details in the next lecture, we deduce immediately that
this equality must imply that HT is constant. In other words, there is no way to perturb HT, no way to perturb the spine while keeping this equality.
So intuitively, it's very simple. We have a spine and we have a fixed point QT, and we fix the image of that point. Then if this spine is transverse to walls,
we cannot move this spine. It's just fixed at that point. In other words, this HT is constant. And if HT is constant, it means that QTFT lives in the pre-image of this fixed point,
Q0, H0, Q0, for any T. And that is a contradiction to the quasi finiteness of the map phi. So I said that by deformation theory, generically over the target,
phi is finitely tau. So in particular, it's quasi finite. But here we just produced infinitely many. We just produced a germ in the pre-image by phi of some point. And
that's a contradiction. So that completes the proof of the claim. And the claim produces us this nice connected component. And so let's just, I just explained the proof of the claim, but let us recapitulate what is
the statement of the claim. So we have our natural map phi from C times H to C times Yn. And we have a skeletal curve. We have a skeletal curve F from C to Yn. And we assume that the
associated spine is transverse. Then the claim says that the skeleton of the fiber CF inside the pre-image by phi of the graph of H is a connected component.
And now, observe the following. First, observe that the first factor of phi decides exactly where we evaluate for the second factor. The second factor of phi is a universal map.
And the first factor of phi is the projection to C. So the first factor determines where we are evaluating for the second map. And furthermore, observe that if we take
sum of degree of our map of phi restricted to this skeleton fiber. And here,
the degree makes sense exactly by the claim, because we know that the map of phi is finite et al generically over the target. So the degree makes sense. But if we restrict to a subset,
the degree may no longer make sense. And here, it still makes sense because we restrict this finite et al map to some connected component. And then the degree is still well-defined because the map remains to be finite et al over some thickening of this
connected component, some neighborhood. So the degree makes sense. And we take the degree, and we take sum of such degrees over all skeletal curves whose associated span is equal to h.
And that is exactly the counts, the following count, nw hw beta, where we count the number of skeletal curves associated to the span hw, which is just
the span h, but we add an internal marked point at w, meaning that we add some internal leg at w, which is contracted, the leg. And then we consider the count of skeletal curves
associated to this augmented span and the curve class beta by evaluating at the added marked point w. And the left hand side is equal to the right hand side by the definition of this count.
So now we can conclude the symmetry theorem. For transverse span, the count, now we see that the count nh beta is independent of the choice of the internal marked point,
because here we see that the left hand doesn't depend on the choice of w. And the right hand side is the count of skeletal curves where we evaluate at this point w, and the w is allowed to move everywhere. So the count is invariant when we move w anywhere along the span.
And this shows the symmetry theorem. Furthermore, we can show that adding or removing internal marked points does not affect the counts at all. So this is an illustration of
how we use skeletal curves for establishing important properties of our counts. And we
will see further examples of that in later parts of the lectures. So here for the symmetry property, symmetry theorem actually we can have different proofs without passing through
skeletal curves. But for other properties, we must use skeletal curves. And here it's nice to see that using skeletal curves, we really have the freedom of moving the point w everywhere
without using, without, if the curve is not a skeletal, there's no way to cross a wall while keeping the invariance. As a proof of the symmetry theorem, we don't move across the wall if we don't use the skeletal curve. So that's what I want to explain today. And for the next
lecture, I will talk about deformation invariance, and also many other properties of the counts that finally leads to the proof of
the associativity of the mirror algebra. And for deformation invariance, as we said, usually it only holds outside the walls. When we cross a wall, we are supposed to have
wall crossing formula. We no longer expect deformation invariance. But for skeletal curves, actually there is some trickier deformation invariance that somehow similar to this situation
about moving around this marked point across walls. For skeletal curves, we can actually move across walls a little bit as long as it's sufficiently transverse, but not really
transverse. For non-skeletal curves, it must be transverse in order to have deformation invariance. But for skeletal curves, we can relax a little bit the transversality condition, and that's actually important in the proof of associativity and also in the proof of
wall crossing formula. Because in associativity, I mean the definition of structure constants, if you remember from the last lecture,
the place we evaluate, we ask the point to go to Q. And the Q, although it's a very generic point at the level of analytic geometry, it's a very special
point at the level of tropical geometry. So all the spans that appear in the definition of structure constants, as in the previous lecture, they are all very special. They are not
transverse at all. And of course we can make them transverse if we don't ask the marked point to go to Q, but to go to some place, to go to some point sufficiently close to Q.
But then we will have the choice of asking it to go to either the left of the wall or the right of the wall. Or if there are many more walls, then we have even many more choices of chambers. But in general, we have the choice of asking it to go to the left or go to the right.
And it's not clear at all whether the structure constants for the marked point going to the left is equal to the structure constants for the marked point going to the right.
And this going from left to right across in the wall, we have to use the theory of skeletal curves again. So I will explain more about that in the next lecture, the next month. Thank you very much for your attention.
Okay, thank you very much. And maybe we don't have time for questions. Actually, I have a very simple question. You have this variety H here, which has the same dimension as Y. It also has logarithmic volume form here.
Yeah. But is it? Yeah, so it means that you can start to reproduce from some log Calabi or another log Calabi in a sense. Yes. Yes. And this is does this H contains a torus again, if you assume that Y contains a torus.
This H. Yeah. H is a cover of the torus. Probably itself is not torus. It could be more.
H is really the modular space. And also, we don't really have a good compactification of H. Ah, so it's not log Calabi or more. It's got the chromified cover, yeah. It's not clear whether it's log Calabi or not, because
we only considered the essential skeleton of H associated to this particular volume form. Yes, yes. Maybe there are other volume forms. Yeah, or maybe this volume form has zeros. It can have zeros or yes. Okay. Okay, so thank you.