3-nilpotent obstructions to pi_1 sections for P^1_Q - {0,1,infty}

We study which rational points of the Jacobian of P^1_K -{0,1,infty} can be lifted to sections of geometrically 3 nilpotent quotients of etale pi_1 over the absolute Galois group. This is equivalent to evaluating certain triple Massey products of elements of H^1(G_K). For K=Q_p or R, we give a complete mod 2 calculation. This permits some mod 2 calculations for K = Q. These are computations of obstructions of Jordan Ellenberg.Comment: 49 page

2-Nilpotent Real Section Conjecture

We show a 2-nilpotent section conjecture over R: for a geometrically connected curve X over R such that each irreducible component of its normalization has R-points, pi_0(X(R)) is determined by the maximal 2-nilpotent quotient of the fundamental group with its Galois action, as the kernel of an obstruction of Jordan Ellenberg. This implies that for X smooth and proper, X(R)^{+/-} is determined by the maximal 2-nilpotent quotient of Gal(C(X)) with its Gal(R)-action, where X(R)^{+/-} denotes the set of real points equipped with a real tangent direction, showing a 2-nilpotent birational real section conjecture.Comment: 24 pages, 1 figure. To appear Mathematiche Annale

Desuspensions of S^1 /\ P^1_Q-{0,1,infty}

We use the Galois action on $\pi_1^{\textrm{et}}(\mathbb{P}_{\overline{\mathbb{Q}}}^1 - \{0,1,\infty \})$ to show that the homotopy equivalence $S^1 \wedge (\mathbb{G}_{m,\mathbb{Q}} \vee \mathbb{G}_{m,\mathbb{Q}}) \cong S^1 \wedge (\mathbb{P}_{\mathbb{Q}}^1 - \{0,1,\infty \})$ coming from purity does not desuspend to a map $\mathbb{G}_{m,\mathbb{Q}} \vee \mathbb{G}_{m,\mathbb{Q}} \to \mathbb{P}_{\mathbb{Q}}^1 - \{0,1,\infty \}$

Massey products <y,x,x,...,x,x,y> in Galois cohomology via rational points

For $x$ an element of a field other than $0$ or $1$, we compute the order $n$ Massey products $$\langle (1-x)^{-1}, x^{-1}, \ldots, x^{-1}, (1-x)^{-1} \rangle$$ of $n-2$ factors of $x^{-1}$ and two factors of $(1-x)^{-1}$ by embedding $\mathbb{P}^1 - \{0,1,\infty\}$ into its Picard variety and constructing $\operatorname{Gal}(k^s/k)$ equivariant maps from $\pi_1$ applied to this embedding to unipotent matrix groups. This method produces obstructions to $\pi_1$-sections of $\mathbb{P}^1 - \{0,1,\infty\}$, partial computations of obstructions of Jordan Ellenberg, and also computes the Massey products $$\langle x^{-1} , (-x)^{-1}, \ldots, (-x)^{-1}, x^{-1} \rangle.$$Comment: To appear in the Journal of Pure and Applied Algebra's special issue in honor of Chuck Weibel, edited by Guillermo Corti\~nas, Eric Friedlander, and Christian Haesemeyer. 26 pages. This paper continues arXiv:1011.0655 and arXiv:1107.179

Cartier's first theorem for Witt vectors on Z_{>= 0}^n - 0

We show that the dual of the Witt vectors on Z_{\geq 0}^n - 0 as defined by Angeltveit, Gerhardt, Hill, and Lindenstrauss represent the functor taking a commutative formal group G to the maps of formal schemes Ahat^n -> G, and that the Witt vectors are self-dual for Q-algebras or when n=1.Comment: Corrected an error in the previous version of the main theorem, which is now stated in terms of the dual of the Witt vectors. The original statement holds under the additional hypothesis that we are working over a Q-algebra. To appear in Algebraic Topology: Applications and New Directions, Proceedings of the Stanford Symposium 2012. 7 pages, AIM preprint number AIM 2012-8

Unstable Motivic Homotopy Theory

We give an introduction to unstable motivic homotopy theory of Morel and Voevodsky, and survey some results.Comment: Prepared for the Handbook of Homotopy Theory, edited by Haynes Mille

The Simplicial EHP Sequence in A1-Algebraic Topology

We give a tool for understanding simplicial desuspension in A1-algebraic topology: we show that $X \to \Omega (S^1 \wedge X)\to \Omega (S^1 \wedge X \wedge X)$ is a fiber sequence up to homotopy in 2-localized A1-algebraic topology for $X = (S^1)^m \wedge G_m^{\wedge q}$ with $m>1$. It follows that there is an EHP sequence spectral sequence $$Z_{(2)} \otimes \pi_{n+1+i}^{A1} (S^{2n+2m+1} \wedge (G_m)^{\wedge 2q}) \Rightarrow Z_{(2)} \otimes \pi_{i}^{A1, s} (S^{m} \wedge (G_m)^{\wedge q}).$$Comment: Revisions made at the suggestion of an anonymous refere

Splitting Varieties for Triple Massey Products

We construct splitting varieties for triple Massey products. For a,b,c in F^* the triple Massey product of the corresponding elements of H^1(F, mu_2) contains 0 if and only if there is x in F^* and y in F[\sqrt{a}, \sqrt{c}]^* such that b x^2 = N_{F[\sqrt{a}, \sqrt{c}]/F}(y), where N_{F[\sqrt{a}, \sqrt{c}]/F} denotes the norm, and F is a field of characteristic different from 2. These varieties satisfy the Hasse principle by a result of D.B. Lee and A.R. Wadsworth. This shows that triple Massey products for global fields of characteristic different from 2 always contain 0.Comment: 18 pages. To appear in Journal of Pure and Applied Algebr

An \'etale realization which does not exist

For a global field, local field, or finite field $k$ with infinite Galois group, we show that there can not exist a functor from the Morel--Voevodsky $\mathbb{A}^1$-homotopy category of schemes over $k$ to a genuine Galois equivariant homotopy category satisfying a list of hypotheses one might expect from a genuine equivariant category and an \'etale realization functor. For example, these hypotheses are satisfied by genuine $\mathbb{Z}/2$-spaces and the $\mathbb{R}$-realization functor constructed by Morel--Voevodsky. This result does not contradict the existence of \'etale realization functors to (pro-)spaces, (pro-)spectra or complexes of modules with actions of the absolute Galois group when the endomorphisms of the unit is not enriched in a certain sense. It does restrict enrichments to representation rings of Galois groups.Comment: 18 pages. To appear in the New Directions in Homotopy Theory volume of Contemporary Mathematics. V2 has a slightly generalized main theorem, and stable realization functors are explicitly assumed to be additive, which is necessary. V3 minor modification

An Arithmetic Count of the Lines on a Smooth Cubic Surface

We give an arithmetic count of the lines on a smooth cubic surface over an arbitrary field $k$, generalizing the counts that over $\mathbb{C}$ there are $27$ lines, and over $\mathbb{R}$ the number of hyperbolic lines minus the number of elliptic lines is $3$. In general, the lines are defined over a field extension $L$ and have an associated arithmetic type $\alpha$ in $L^*/(L^*)^2$. There is an equality in the Grothendieck-Witt group $\operatorname{GW}(k)$ of $k$ $$\sum_{\text{lines}} \operatorname{Tr}_{L/k} \langle \alpha \rangle = 15 \cdot \langle 1 \rangle + 12 \cdot \langle -1 \rangle,$$ where $\operatorname{Tr}_{L/k}$ denotes the trace $\operatorname{GW}(L) \to \operatorname{GW}(k)$. Taking the rank and signature recovers the results over $\mathbb{C}$ and $\mathbb{R}$. To do this, we develop an elementary theory of the Euler number in $\mathbb{A}^1$-homotopy theory for algebraic vector bundles. We expect that further arithmetic counts generalizing enumerative results in complex and real algebraic geometry can be obtained with similar methods.Comment: 34 pages. Accepted for publication in Compositio Mathematic