Betti Realization Research Articles

On realizations of the subalgebra 𝒜^{ℝ}(1) of the ℝ-motivic Steenrod algebra

In this paper, we show that the finite subalgebra A R ( 1 ) \mathcal {A}^\mathbb {R}(1) , generated by S q 1 \mathrm {Sq}^1 and S q 2 \mathrm {Sq}^2 , of the R \mathbb {R} -motivic Steenrod algebra A R \mathcal {A}^\mathbb {R} can be given 128 different A R \mathcal {A}^\mathbb {R} -module structures. We also show that all of these A \mathcal {A} -modules can be realized as the cohomology of a 2 2 -local finite R \mathbb {R} -motivic spectrum. The realization results are obtained using an R \mathbb {R} -motivic analogue of the Toda realization theorem. We notice that each realization of A R ( 1 ) \mathcal {A}^\mathbb {R}(1) can be expressed as a cofiber of an R \mathbb {R} -motivic v 1 v_1 -self-map. The C 2 {\mathrm {C}_2} -equivariant analogue of the above results then follows because of the Betti realization functor. We identify a relationship between the R O ( C 2 ) \mathrm {RO}({\mathrm {C}_2}) -graded Steenrod operations on a C 2 {\mathrm {C}_2} -equivariant space and the classical Steenrod operations on both its underlying space and its fixed-points. This technique is then used to identify the geometric fixed-point spectra of the C 2 {\mathrm {C}_2} -equivariant realizations of A C 2 ( 1 ) \mathcal {A}^{\mathrm {C}_2}(1) . We find another application of the R \mathbb {R} -motivic Toda realization theorem: we produce an R \mathbb {R} -motivic, and consequently a C 2 {\mathrm {C}_2} -equivariant, analogue of the Bhattacharya-Egger spectrum Z \mathcal {Z} , which could be of independent interest.

Norms in motivic homotopy theory

If $f : S' \to S$ is a finite locally free morphism of schemes, we construct a symmetric monoidal "norm" functor $f_\otimes : \mathcal{H}_{\bullet}(S')\to \mathcal{H}_{\bullet}(S)$, where $\mathcal{H}_\bullet(S)$ is the pointed unstable motivic homotopy category over $S$. If $f$ is finite étale, we show that it stabilizes to a functor $f_\otimes : \mathcal{S}\mathcal{H}(S') \to \mathcal{S}\mathcal{H}(S)$, where $\mathcal{S}\mathcal{H}(S)$ is the $\mathbb{P}^1$-stable motivic homotopy category over $S$. Using these norm functors, we define the notion of a normed motivic spectrum, which is an enhancement of a motivic $E_\infty$-ring spectrum. The main content of this text is a detailed study of the norm functors and of normed motivic spectra, and the construction of examples. In particular: we investigate the interaction of norms with Grothendieck's Galois theory, with Betti realization, and with Voevodsky's slice filtration; we prove that the norm functors categorify Rost's multiplicative transfers on Grothendieck-Witt rings; and we construct normed spectrum structures on the motivic cohomology spectrum $H\mathbb{Z}$, the homotopy $K$-theory spectrum $KGL$, and the algebraic cobordism spectrum $MGL$. The normed spectrum structure on $H\mathbb{Z}$ is a common refinement of Fulton and MacPherson's mutliplicative transfers on Chow groups and of Voevodsky's power operations in motivic cohomology.

Betti realization of varieties defined by formal Laurent series

We give two constructions of functorial topological realizations for schemes of finite type over the field $\mathbb{C}(\!(t)\!)$ of formal Laurent series with complex coefficients, with values in the homotopy category of spaces over the circle. The problem of constructing such a realization was stated by D. Treumann, motivated by certain questions in mirror symmetry. The first construction uses spreading out and the usual Betti realization over $\mathbb{C}$. The second uses generalized semistable models and log Betti realization defined by Kato and Nakayama, and applies to smooth rigid analytic spaces as well. We provide comparison theorems between the two constructions and relate them to the \'etale homotopy type and de Rham cohomology. As an illustration of the second construction, we treat two examples, the Tate curve and the non-archimedean Hopf surface.

C2-equivariant stable homotopy from real motivic stable homotopy

We give a method for computing the C_2-equivariant homotopy groups of the Betti realization of a p-complete cellular motivic spectrum over R in terms of its motivic homotopy groups. More generally, we show that Betti realization presents the C_2-equivariant p-complete stable homotopy category as a localization of the p-complete cellular real motivic stable homotopy category.

Monodromy and log geometry

A now classical construction due to Kato and Nakayama attaches a topological space (the "Betti realization") to a log scheme over $\mathbf{C}$. We show that in the case of a log smooth degeneration over the standard log disc, this construction allows one to recover the topology of the germ of the family from the log special fiber alone. We go on to give combinatorial formulas for the monodromy and the $d^2$ differentials acting on the nearby cycle complex in terms of the log structures. We also provide variants of these results for the Kummer etale topology. In the case of curves, these data are essentially equivalent to those encoded by the dual graph of a semistable degeneration, including the monodromy pairing and the Picard-Lefschetz formula.

On equivariant and motivic slices

Let $k$ be a field with a real embedding. We compare the motivic slice filtration of a motivic spectrum over $Spec(k)$ with the $C_2$-equivariant slice filtration of its equivariant Betti realization, giving conditions under which realization induces an equivalence between the associated slice towers. In particular, we show that, up to reindexing, the towers agree for all spectra obtained from localized quotients of $MGL$ and $MR$, and for motivic Landweber exact spectra and their realizations. As a consequence, we deduce that equivariant spectra obtained from localized quotients of $MR$ are even in the sense of Hill--Meier, and give a computation of the slice spectral sequence converging to $\pi_{*,*}BP\langle n \rangle/2$ for $1 \le n \le \infty$.

On very effective hermitian K-theory

We argue that the very effective cover of hermitian K-theory in the sense of motivic homotopy theory is a convenient algebro-geometric generalization of the connective real topological K-theory spectrum. This means the very effective cover acquires the correct Betti realization, its motivic cohomology has the desired structure as a module over the motivic Steenrod algebra, and that its motivic Adams and slice spectral sequences are amenable to calculations.

Recognizing Galois representations of K3 surfaces

Under the assumption of the Hodge, Tate and Fontaine–Mazur conjectures we give a criterion for a compatible system of $$\ell $$ -adic representations of the absolute Galois group of a number field to be isomorphic to the second cohomology of a K3 surface. This is achieved by producing a motive M realizing the compatible system, using a local to global argument for quadratic forms to produce a K3 lattice in the Betti realization of M and then applying surjectivity of the period map for K3 surfaces to obtain a complex K3 surface. Finally we use a very general descent argument to show that the complex K3 surface admits a model over a number field.

The Motivic Cofiber of $\tau$

Consider the Tate twist \tau \in H^{0,1}(S^{0,0}) in the mod 2 cohomology of the motivic sphere. After 2-completion, the motivic Adams spectral sequence realizes this element as a map \tau:S^{0,-1} \to S^{0,0} , with cofiber C\tau . We show that this motivic 2-cell complex can be endowed with a unique E_\infty ring structure. Moreover, this promotes the known isomorphism \pi_{\ast,\ast} C\tau \cong \mathrm{Ext}^{\ast,\ast}_{BP_{\ast}BP}(BP_{\ast},BP_{\ast}) to an isomorphism of rings which also preserves higher products. We then consider the closed symmetric monoidal category of C\tau -modules ( _{C\tau} Mod, -\wedge_{C\tau}-) which lives in the kernel of Betti realization. Given a motivic spectrum X , the C\tau -induced spectrum X \wedge C\tau is usually better behaved and easier to understand than X itself. We specifically illustrate this concept in the examples of the mod 2 Eilenberg-Maclane spectrum H\Bbb F_2 , the mod 2 Moore spectrum S^{0,0}/2 and the connective hermitian K -theory spectrum kq .

ON THE RESIDUE OF EISENSTEIN CLASSES OF SIEGEL VARIETIES

AbstractEisenstein classes of Siegel varieties are motivic cohomology classes defined as pull-backs by torsion sections of the polylogarithm prosheaf on the universal abelian scheme. By reduction to the Hilbert–Blumenthal case, we prove that the Betti realization of these classes on Siegel varieties of arbitrary genus have non-trivial residue on zero-dimensional strata of the Baily–Borel–Satake compactification. A direct corollary is the non-vanishing of a higher regulator map.

Galois equivariance and stable motivic homotopy theory

For a finite Galois extension of fields L/k with Galois group G, we study a functor from the G-equivariant stable homotopy category to the stable motivic homotopy category over k induced by the classical Galois correspondence. We show that after completing at a prime and eta (the motivic Hopf map) this results in a full and faithful embedding whenever k is real closed and L = k[i]. It is a full and faithful embedding after eta-completion if a motivic version of Serre's finiteness theorem is valid. We produce strong necessary conditions on the field extension L/k for this functor to be full and faithful. Along the way, we produce several results on the stable C_2-equivariant Betti realization functor and prove convergence theorems for the p-primary C_2-equivariant Adams spectral sequence.

The Adams–Novikov spectral sequence and Voevodsky’s slice tower

We show that the spectral sequence converging to the stable homotopy groups of spheres, induced by the Betti realization of the slice tower for the motivic sphere spectrum, agrees with the Adams-Novikov spectral sequence, after a suitable re-indexing. The proof relies on an extension of Deligne's d\'ecalage construction to the Tot-tower of a cosimplicial spectrum.

Some remarks concerning the Grothendieck period conjecture

Abstract We discuss various results and questions around the Grothendieck period conjecture, which is a counterpart, concerning the de Rham–Betti realization of algebraic varieties over number fields, of the classical conjectures of Hodge and Tate. These results give new evidence towards the conjectures of Grothendieck and Kontsevich–Zagier concerning transcendence properties of the torsors of periods of varieties over number fields. Let ℚ ¯ ${\overline{\mathbb {Q}}}$ be the algebraic closure of ℚ in ℂ, let X be a smooth projective variety over ℚ ¯ ${\overline{\mathbb {Q}}}$ and let X ℂ an $X^{\rm an}_\mathbb {C}$ denote the compact complex analytic manifold that it defines. The Grothendieck period conjecture in codimension k on X, denoted GPC k ( X ) $\mathrm {GPC}^k(X)$ , asserts that any class α in the algebraic de Rham cohomology group H dR 2 k ( X / ℚ ¯ ) $H^{2k}_{{\rm dR}}(X/{\overline{\mathbb {Q}}})$ of X over ℚ ¯ ${\overline{\mathbb {Q}}}$ such that 1 ( 2 π i ) k ∫ γ α ∈ ℚ $ \frac{1}{(2\pi i)^k} \int _\gamma \alpha \in \mathbb {Q}$ for every rational homology class γ in H 2 k ( X ℂ an , ℚ ) $H_{2k}(X^{\rm an}_\mathbb {C}, \mathbb {Q})$ is the class in algebraic de Rham cohomology of some algebraic cycle of codimension k in X, with rational coefficients. We notably establish that GPC 1 ( X ) $\mathrm {GPC}^1(X)$ holds when X is a product of curves, of abelian varieties, and of K3 surfaces, and that GPC 2 ( X ) $\mathrm {GPC}^2(X)$ holds for a smooth cubic hypersurface X in ℙ ℚ ¯ 5 $\mathbb {P}^5_{\overline{\mathbb {Q}}}$ . We also discuss the conjectural relationship of Grothendieck classes with the weight filtration on cohomology.

A comparison of motivic and classical stable homotopy theories

Let k be an algebraically closed field of characteristic zero. Let SH(k) denote the motivic stable homotopy category of T-spectra over k and SH the classical stable homotopy category. Let c:SH -> SH(k) be the functor induced by sending a space to the constant presheaf of spaces on Sm/k. We show that c is fully faithful. In particular, c induces an isomorphism {\pi}_n(E)-> {\pi}_{n,0}c(E) for all spectra E. Fix an embedding of k into the complex numbers and let Re:SH(k) -> SH be the associated Betti realization. We show that the slice tower for the motivic sphere spectrum has Betti realization which is strongly convergent. This gives a spectral sequence "of motivic origin" converging to the homotopy groups of the classical sphere spectrum; this spectral sequence at E_2 agrees with the E_2 terms in the Adams-Novikov spectral sequence.

Relative rounding in toric and logarithmic geometry

We show that the introduction of polar coordinates in toric geometry smoothes a wide class of equivariant mappings, rendering them locally trivial in the topological category. As a consequence, we show that the Betti realization of a smooth proper and exact mapping of log analytic spaces is a topological fibration, whose fibers are orientable manifolds (possibly with boundary). This turns out to be true even for certain noncoherent log structures, including some families familiar from mirror symmetry. The moment mapping plays a key role in our proof. 14D06, 14M25, 14F45, 32S30; 53D20, 14T05

Note sur les opérations de Grothendieck et la réalisation de Betti

RésuméLe but de cette note est de prouver que la réalisation de Betti des motifs est compatible avec les six opérations de Grothendieck et les foncteurs cycles proches qui, dans le monde motivique, ont été étudiés par l'auteur. On reprend d'abord la construction de la réalisation de Betti. On établit ensuite des critères abstraits qui, appliqués à la réalisation de Betti, fournissent les compatibilités souhaitées, sauf celle qui concerne les foncteurs cycles proches. Ces derniers seront traités dans une section à part.

Réalisation de Betti des motifs de Voevodsky

For any subfield k of the field of complex numbers C, we construct a Betti realization functor from the category DM − ( k ) of Voevodsky motivic complexes over k to the category of graded abelian groups. If X is a scheme of finite type over k, the image of the associated motive through this functor is the singular cohomology ⊕ p ⩾ 0 H p ( X ( C ) , Z ) of the variety X ( C ) of complex points. To cite this article: F. Lecomte, C. R. Acad. Sci. Paris, Ser. I 346 (2008).

Motivic Cohomology of Pairs of Simplices

For an arbitrary field $F$ we study the double scissors congruence groups $A_n(F)$ generated by admissible pairs of simplices in the projective $n$-space ${\mathbb P}_F^n$ over $F$. Let $(L, M)$ be an admissible pair of simplices whose faces are defined as hyperplanes in ${\mathbb P}_F^n$ such that they do not have common faces of the same dimension. When $L$ and $M$ are in general positions we define a linearly constructible motivic perverse sheaf producing a motivic cohomology whose Betti realization is the relative cohomology $H^{\ast}({\mathbb P}_F^n\setminus L, M\setminus L; {\mathbb Q})$. The motivic cohomology provides a simple explanation of why the generic part of $A_\bullet$ forms a Hopf algebra with well-defined coproduct. When $n = 2$ we define explicitly the coproduct on $A_2$ which simplifies the approach of Beilinson et al. We also complete the same task for $A_3$ and $A_4$ which enables us to develop further results in another paper connecting Aomoto trilogarithms to the classical ones.