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2/3 A Local Construction of Stable Motivic Homotopy Theory

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2/3 A Local Construction of Stable Motivic Homotopy Theory
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V. Voevodsky [6] invented the category of framed correspondences with the hope to give a new construction of stable motivic homotopy theory SH(k) which will be more friendly for computational purposes. Joint with G. Garkusha we used framed correspondences to develop the theory of framed motives in [4]. This theory led us in [5] to a genuinely local construction of SH(k). In particular, we get rid of motivic equivalences completely. In my lectures I will recall the definition of framed correspondences and describe the genuinely local model for SH(k) (assuming that the base field k is infinite and perfect). I will also discuss several applications. Let Fr(Y,X) be the pointed set of stable framed correspondences between smooth algebraic varieties Y and X. For the first two applications I choose k = ℂ for simplicity. For further two applications k is any infinite and perfect field. (1) The simplicial space Fr(∆alg,S^1) has the homotopy type of the topological space Ω∞Σ∞(S^1_top). So the topological space Ω^∞S1Σ^∞_S1(S^1_top) is recovered as the simplicial set Fr(∆alg,S^1), which is described in terms of algebraic varieties only. This is one of the computational miracles of framed correspondences. (2) The assignment X ↦ π(Fr(∆alg,X⨂S^1)) is a homology theory on complex algebraic varieties. Moreover, this homology theory regarded with ℤ/n-coefficients coincides with the stable homotopies X ↦ π ^S_(X+^S^1_top;ℤ/n) with ℤ/n-coefficients. The latter result is an extension of the celebrated Suslin–Voevodsky theorem on motivic homology of weight zero to the stable motivic homotopy context. (3) Another application of the theory is as follows. It turns out that π^s_0,0(X+) = H0(ℤF(∆,X)), where (ℤF(∆,X)) is the chain complex of stable linear framed correspondences introduced in [4]. For X = G_m^^n this homology group was computed by A. Neshitov as the nth Milnor–Witt group K_n^MW (k) of the base field k recovering the celebrated theorem of Morel. (4) As a consequence of the theory of framed motives, the canonical morphism of motivic spaces can: C_Fr(X) → Ω^∞ℙ^1 Σ^∞_ℙ^1 (X+) is Nisnich locally a group completion for any smooth simplicial scheme X. In particular, if CFr(X) is Nisnevich locally connected, then the morphism can is a Nisnevich local weak equivalence. Thus in this case C_Fr(X) is an infinite motivic loop space and π_n(C_FR(X)(K)) = π^A1_n,0 (Σ^∞_ℙ^1 (X+))(K). In my lectures I will adhere to the following references: [1] A. Ananyevskiy, G. Garkusha, I. Panin, Cancellation theorem for framed motives of algebraic varieties, arXiv:1601.06642 [2] G. Garkusha, A. Neshitov, I. Panin, Framed motives of relative motivic spheres, arXiv:1604.02732v3. [3] G. Garkusha, I. Panin, Homotopy invariant presheaves with framed transfers, Cambridge J. Math. 8(1) (2020), 1-94. [4] G. Garkusha, I. Panin, Framed motives of algebraic varieties (after V. Voevodsky), J. Amer. Math. Soc., to appear. [5] G. Garkusha, I. Panin, The triangulated categories of framed bispectra and framed motives, arXiv:1809.08006. [6] V. Voevodsky, Notes on framed correspondences, unpublished, 2001, www.math.ias.edu/vladimir/publications
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Transcript: English(auto-generated)
the title of my second lecture a local approach to sh of k the purpose of this lecture is to give
sorry is to give another approach to a stable motivic uh mahometopy category precisely we define a translated category of framed by spectra sh frame
means and prove and prove that it recovers the translated category sh of k the main feature of
this new category sh frame is of k is that its construction is genuinely local in the sense that it doesn't use any kind of material equivalencies in other words we get rid of material equivalencies completely making sh frame miss
uh of k more amenable to fpc calculation than sh k of maravlovsky so let me start with some details consider the category the category of a spectra
uh in the category m dot where m dot is the category of motivic spaces motivic spaces means pointed motivic spaces means pointed simplicial nimshviz on smooth where and here gm smash one is the mapping core of the one section spec plus
of the maps the class to gm plus in
in the pointed motivic spaces so this map takes the point plus to the point plus and take non-distinguished point in the spec of k to the point one in gm more precisely gm smash one is the push out of the exact diagram of this form
in which i is the point of simplicial set delta of one so the interval the simplicial interval is the base point one so one put draw a picture how does it look like but let me
skip this because this is not quite essential the category uh of bispectra comes equipped with a stable projective local model structure
defined as follows firstly by a work apologize uh it is well known that the criteria of pointed motivic spaces come execute with the projective
local monoidal model structure in which weak equivalence is and just so a map of motivic spaces is called the weak equivalence
uh in this structure provided that uh it is stock wise uh weak equivalences of uh stock wise uh weak equivalence stabilizing the model structure in s1 direction we get the category spectra s1 of k of material s1 spectra which is equipped with the stable
projective local monoidal structure where weak equivalences are maps of spectra inducing
isomorphism on shifts on both stable homotopy groups or in different fields uh induces induced unstable equivalences on stocks of the motivic s1 spectra sorry now stabilizing
the model structure uh on uh spectra s1 of a uh in the gm smash one direction we arrive
at the stable projective local model structure uh on the category of bispectra uh it's triangulated category if triangulated the homotopy category
is denoted by sh needs of pain and that is this category is triangulated uh homotopy category is the very basis for all our further definitions
so the nearest aim uh is to define this category sh frame needs of pain
as a false category of sh needs it will be a false category of
sh needs so but to do that i need to recall few definitions concerning
uh frame shifts uh frame shifts uh and so on namely definition uh a pointed frame
pretty shift f on smooth of k is a contravening factor from frame for the frame plus of k to the category of point it says a frankness navy shift on smooth k is a frame per shift
such that its restriction to the smooth of k even is new issue a frame for five billion groups on smooth k is a contravening factor from frame parts of k to the
category of five billion groups to the category of five billion groups uh it is called additive uh if f evaluated on the empty seam uh is the zero group and f of is joint union uh of small seams is the product uh f of x1 cross f of x2
a really new point with frame pressures uh is the following one a frame friendship f of a billion group is called stable if for each uh smooth variety x the morphism uh from f of x
to f of x induced by the suspension morphism sigma x from x to x i recall that there was
suspension morphism uh defined in the previous lecture uh that the induced morphism uh between
f of x and f of x uh induced by sigma x is the identity a frankness navy shift of a billion groups uh on smooth of k uh is a frankness shift of a billion groups f such that its
restriction to smooth of k is a misnomer chief finally a frame pre-shift is thematic invariant or even a a1 invariant uh equaling x and projection uh a1 cross x to x the induced map
uh is bi-direction uh or uh it is an isomorphism of groups provided that uh the chief uh is appreciable groups a frankness name is chief uh is a one invariant if it is
uh a one a one invariant is a frame pressure so these are all standard definitions about three uh pre-shifts uh and as as i stressed stressed the only non-standard and mu
is uh this one that uh a frame pre-shift f of a billion groups is called stable provided that uh this induced map uh sigma uh up a star x from f of x to itself is the identity map
now the very key definition namely the definition of the category sh-frame is of k we define sh-frame is of k as a full subcategory
in sh news of k consisting of those bias paper e satisfying the following conditions firstly first uh each motivic space e i j of e either space is frame correspondences
that is a point it's simply initial shift defined on the category of frame correspondences frame plus of k the structure maps this is the second condition in both direction
in this one direction and uh in the j m smash one direction uh respect uh or preserve in different way respect a frame correspondences the third condition uh is this uh for every
j into j which is non-negative we can consider the s1 spectrum e star comma j
and the requirement is this the frame pre-shift pre-shifts of stable stable homotopy groups pi over star uh of this s1 uh spectrum of this material response spectrum uh stable radiative and a1 a1 and the final condition the final requirement uh is uh called
cancellation theorem uh and uh it sounds like this for every integer j which is at least zero the structure map between the j's uh s1 spectrum and
uh in the home from gm naive in the home from gm slash one to uh j was plus one once
my key is one spectra is a stable local experience in the stable local experience objects of the category frame miss of k are called framed by by spectra we should stress
that the definition of this category sh frame is of k is local in the sense that its molecules are computed in the category sh needs of k and has nothing to do with the
category sh of p here is uh the main theorem the there is the nature of fanta f
which is the identity on objects so we should go to sh needs of k using the full embedding and then project to project project to
sh of k which is the identity of an object is an equivalent of categories
it's quasi quasi inverse is given by the big frame motif uh you know it uh like this um which is described uh in the uh paper four but uh also another is quasi uh another uh
quasi inverse fanta to the front f and is given by a fountain m bar big frame uh constructed
below so in the lecture today i will use i will construct this fanta and i will use this fanta as a quasi index for the fanta f so i will prove that it will be
minutes two from that so before uh proving this theorem uh let me show some immediate
immediate advantages of the category sh very nice of p of the category sh for a minute of p um the field given by spectrum e we shall also write e in the following form
e of naught e of one and so on and so on where e of j is the motivic s1 spectrum with spacious e of j with the subscript i is equal to the space e i j for all i equal to zero
and we also call the s1 spectrum u of j the j's weight of p so here is the first very nice
really very nice property of framed by spectra if e and f are full framed by spectra then a more amorphism f between e and f
f q d f q d e and q d f is a stable motivic evidence in the sense of maryland valyubovsky if and only if uh the induced morphism
of nystini which is of stable homotopy groups the induced morphism of this form are isomorphisms in each weight q therefore stable motivic equivalences
between frames by spectra inside with the naive level wise local equivalences between them
so next what i would like to stress is the following property usually it's very difficult to compute a1 homotopy shifts of a motivic spectra or a motivic bispectra so the next property
the next property says that the material homotopy shifts of a framed bispectra purely e are computed in terms of ordinary nystini shifts of stable homotopy groups
for weighted response spectra of e namely let e be written in the form e of naught e of one and so on b a frame by spectrum so by spectrum in s h frames and p q v two
uh in touches then the a1 homotopy shifts of the q d e are computed as follows firstly if q
is at more zero then the a1 homotopy shift is by index pi q of q e coincide with the pi minus q nystini shift of the spectrum e of let me write e of q
with this stuka where q with this stuka is the modulus of q i just know don't know the name
of this vertical box so and the if q is greater than zero then the stable the a1
amount of p and shift is indexed in the ts pi q coincide with the following one firstly should take the
s1 sphere term e of zero then compute the nystini shift of homotopy groups with index p minus q and then use the operation take the operation minus q
v and the separation means the qth contraction of the shift p of the shift pi nystini p minus q of u of naught the third property
uh is as follows let e be e of naught e of one e of two and so on be a by spectrum
in sh3 means of k and let e f be e naught uh subscribe f e1 subscribe f and so on be a by spectrum obtained from e by by taking
a local stable fiber replacement of each weight e or j in the stable local projective model structure on my tv is one spectra then the spectrum e f is already
my tv qualified here i would like to stress that this kind of property is very often happen happen for many uh vice paper we uh have seen we have seen say uh this kind of property
was used yesterday uh in the lecture by uh stash initiative oh sorry so the nearest my aim
uh is to describe this uh fanta m bar b frame uh between sh of k and sh three means of p
and which is supposed to be a quasi inverse uh to the fanta f which i which is identity
on the objects which i described above this is the nearest my name so some notation
for some scheme x write t star frame of x four three four frame delta dot cross bar of x and i recall that frame delta bar cross bar of x
uh are the mycivics species i used um i'll see i advertised in uh in the first uh my lecture uh saying that this is uh a kind of the very key construction uh in this table
uh so uh it is appointed uh and so in different terminology it is appointed framed material space
frank material space replacing x uh with a superficial objects in frame not of k uh and taking the diagonal we get appointed frame mycivics space c star frame of x dot
finally if a is a filter at the limit of k smooth simplicial schemes ais then define c star frame of a as the filter at the limit of c star frame of ais and here is
first over to say a key construction suppose the bispectrum e suppose the bispectrum a consists of mycivics species a i j which are filter limits
of simplicial smooth a schemes we then take c star frame of a i j at every nd
and get a bispectrum which we call c star frame of a let me skip uh how to write down the start maps they're written down in an obvious way so let me skip this
finally take this bispectrum and stabilize it in the gm direction in the standard way the stabilization stabilization is hooked to the infinity gm smash one of the star frame of a
but typically i will call it i mean i will pronounce uh t the infinite c star frame of a because i will not use stabilization in s1 direction i will use only stabilization in gm
smash one direction so it can be checked that the bispectra c star frame of a and tip infinity c star frame of a are subjected conditions one two three of definition two two so since uh most
probably you have forgotten already about those conditions let me show you take back and show you those conditions the conditions uh are these ones so i will think back so yeah so this uh
bispectra has subjected conditions uh one two three uh or definition to two so therefore
we still cannot say that they are framed bispectra but nevertheless the significance of this general construction is reflected in the following examples for each small
scene x we can take the bispectrum the bisuspension spectrum of x plus and then apply the construction t staff right and it turns out that this bispectrum will be already a frame
bispectrum so it will be in sh frames much earlier so what i should maybe stress here that particularly to prove this property we need the cancellation theorem for
um a1 invariant uh relative uh and sigma stable sorry sorry we need a cancellation theorem for frame modus but i didn't speak about maybe i should stress yeah so maybe i
should stress that this spectrum uh is in the paper four is denoted as m g frame of x
okay uh so more general there uh let a be an so then take firstly the gm suspension spectrum of a
and then apply the construction sista frame to this suspension spectrum
gm suspension spectrum of a and the result is a frame by spectrum so and third example
is the more general one so you could take a by spectrum a such that every entry a i j of a is a physical limit of smooth superficial skin then this spectrum is the infinite
of sista frame okay uh is again a frame a frame by spectrum moreover the canonical morphing of y spectra is a stable material equipment this will be used
and uh it turns out that each ring by spectrum each ring by spectrum is isomorphic to the one of this form maybe i should say also that if you take the eilinger marketing spectrum
uh by spectrum h of z hz the will be in this category as well
and many other uh spectra if you take i don't remember exactly the notation maybe like this so the spectrum
yeah like this is also is an uh frame miss okay and many other interesting spectra are in this category say the best spectrum which was described yes by uh sasha is in this category as well sorry now i am ready to
define to construct this phantom m bar big frame from s h of k to s h three minutes of k
namely choose a functorio co-file on replacement uh in the project model structure uh on vice paper then uh each uh e c
vice paper e c consists of material expresses e c i j which are built at the limits of basically therefore by the example three one has the property
this by spectrum if infinity staff frame of e c
is a frame by spectrum so we define m bar big frame uh of e uh s the t to infinity star frame of e c so this construction in is factorial
in by spectra and uh let me show that it takes magnetic frequencies uh to stable uh to
um local equivalencies to naive level wise stable local occurrences of by spectra for that consider a stable material equivalencies of uh by spectra then uh the induced morphism
between curly c and curly uh curly e curly e c curly f c is again a stable
material equivalence of vice paper then by the example three these two morphemes are stable material equivalencies this is because uh each this uh
that this spectra has this property that its entries uh uh filter the limits of officials and the same for f c so so these two morphemes are stable material
equivalencies by the properties three and we could write down a simple diagram and conclude that the induced morphism between
big frame motif of e and big frame motif is again a stable material equivalence so maybe let me uh write down the diagram because otherwise uh so uh i think here is it
okay so these maps are of the type hyper and they are stable magnetic frequencies this is a
since that i am from use therefore this is a stable magnetic event this is a stable
magnetic event but at least by spectra this by spectra yeah this is the spectra
are framed one so they are in as a string is okay therefore the property one of by spectra shows that the morphism m big frame is in fact a naive level was wise stable local appearance and thus this panther m big m bar big frame converts
so maybe i should put here so m bar big frame can be stable
material equivalences to naive level wise local stable equivalences and in this way we could eventually this panther so now we are ready to prove
theorem with some technical problem we are ready to prove hearing saying that the functor f that the functor f which goes this way and which has identity
on objects is an equivalent of fattie base and m bar big frame is in fact in quasi inverse
so firstly let me prove that the functor f is fully quasars winning so for that
take two by spectra e and p prime which are framed and compute and compute in sh for a miss between e and e prime
this home is computed in fact by the very definition in sa miss of k so we need to
replace e by its profile replacement ec and we need to replace e prime by its local level wise
fiber replacement e prime f then compute home of by spectra from ec to e prime f and no doubt by sorry by the naive come out of here by the naive come out of the equivalence
and from other side from other side so here is the computation of
the homes in sh frame miss but from other side if it is sent to sh and compute home we'll try to compute home in between e and e prime is in shk
sh of k then we have to do the pooling we must we have to do the pooling we have to replace e again by a co-frame is co-fibrant replacement
but as we know by the property three
metallic fiber replacement of e prime can be computed as e prime f which is used here uh therefore we can cancel this m and here's just if prime f was the computer for our computation
and we see uh that uh these two formulas are identically the same so this formula
and this formula sorry these are identically the same formula therefore this map is an isomorphism
so we proved that the front of f is fully created what is uh remained to prove the theorem uh it's remained to prove
uh that the functor that the essential image of the functor f coincide with s of sh of k so for that consider the functor m bar big tree constructed above
uh and consider also the zigzag equivalent the zigzag this is the co-fibrant replacement
and this is the morphism alpha both arrows are stable material frequencies
so the fact that the left arrow is stable material it is even level wise uh but the right hand side uh level uh uh is a material a stable material equivalence due to the example three so we have proved that uh each uh my tv by spectrum e uh is uh
isomorphic in the category usage uh to uh the spectrum uh of the form m bar big frame of e
and also due to example three uh we know yeah as i uh told uh above that this functor uh in this category so we have proved that
e is isomorphic to the functor to m bar big frame of e and hence the functor m big frame is quasi inverse to the functor so the theorem is proved so i have
several more minutes and uh okay so i will uh continue let me describe one more nice property
uh of uh the category uh sh frame 10 is of k namely denote by sh1 of the stable
category of s1 spectra there is a canonical pair of drawing functions the suspension functor
and the uh omega loop functor in the dream direction so we have the following results which is the consequence of theorem 2 3 and
uh theorem 2 1 from the work for namely the it's very easy i will say in the following way
it's very easy to describe uh the functor the omega look uh infinite omega loop functor in terms of the particular sh frame mist namely one should take a frame by spectrum e
and send it to the zero weight e of zero and this is the functor this very very convenient in turn the composite functor which is the
uh so the suspension functor we would like to describe the suspension functor but again in terms of this category and it takes uh a motivic s1 spectrum e
to the frame by spectrum of this form where you see uh is the um co-fibrant replacement of e so we need to take uh the co-fibrant replacement
uh ec of e then we need to take the gm suspension uh by spectrum of that and eventually apply the construction sister frame and what we will get we will get the
suspension interpretation of the suspension functor using this category so what i would like to say a little bit more uh is as follows uh we would like to compute for each motivic
by spectrum so for each motivic s spectrum s1 spectrum b the
omega infinity loops so the composition of these two functions omega infinity loops of the infinite suspension of the infinite suspension spectrum of b and it has a very nice description
namely we need to replace b with the co-fibrant replacement bc and apply the functor sister frame sister frame
uh so sister frame of bc this is uh again uh an s1 um a motivic s1 spectrum and this canonical morphism which is uh written here uh will be a stable local appearance of material this one so this means that this computation of omega
uh is given locally in a very easy way a simple way so i have
five more minutes so i will maybe say what the next lecture is supposed to be about and let me uh make uh it begins with the following remark
that the category sh frame is okay is in fact the triangulated category in natural way namely the shift functor one is the shift functor in the category sh units of k
so and distinguished triangles in sh-frame needs of k are those which are distinguished in sh units of k and k him to three states came to three states the the functor f and m bar b-frame are usually inverse equivalencies of recognize
sh-frames and sh of k but one could say a little bit more namely the following simple lemma is through the functor's f and m bar b-frame are usually inverse
equivalencies of kind of wicked categories of sh-frames and sh of k and so very final point i would like to stress uh is uh this is uh something for the future my last lecture uh uh i would like to stress that at the very first glance the category
sh frame is okay uh looks a little bit artificially constructed
but in fact i will show in the next lecture that this category is not at all artificially constructed it is a category of local objects for a localization functor
which will be called a-frame motivic this is a localization functor uh in the category sorry in the category sh needs but let me stop
on let me stop and say that this is the end of my lecture today okay many thanks indeed and let's thank uh ivan uh for the lecture
uh any questions or comments it seems that there are no questions and let's thank uh the lecturer again and we'll meet at half past three p.m time for the lecture of gonzalo