Date of Award
Spring 2021
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mathematics
First Advisor
Goncharov, Alexander
Abstract
Denote by $B\Gamma$ the classifying space of an algebraic group $\Gamma(\mathbb{C})$. In this thesis we will take Milnor's realization of it -- the classifying simplicial space $B_{\Gamma\bullet}$. Consider the weight two motivic complex $ \mathbb{Z}_{\mathcal{M}}(2)$ for a regular algebraic variety $X$, placed in degrees $[1,4]$: $$ B_2(\mathbb{C}(X)) \to \bigwedge^2 \mathbb{C}(X)^* \to \bigoplus\limits_{D\in div(X)}\mathbb{C}(D)^* \to \bigoplus\limits_{D\in cod_2( X)}\mathbb{Z}.$$ Let $G$ be a split semisimple simply connected algebraic group over $\mathbb{Q}$. Then the degree 4 cohomology $H^4(B_{G\bullet}, \mathbb{Z}_{\mathcal{M}}(2))$ is known to be isomorphic to $\mathbb{Z}$. This can be deduced, for example, from the main result of Brylinski and Deligne \cite{BD}, although they did not consider the weight two motivic complex. The goal of this thesis is to explicitly construct a cocycle generating $H^4(B_{\Gamma\bullet}, \mathbb{Z}_{\mathcal{M}}(2))$. Such a cocycle was constructed in the special case $G = SL_n$ by Goncharov in \cite{ECCC}. In the case of general $G$ our construction uses the existence of a cluster $K_2$-variety $\mathcal{A}_{G,S}$ as well as a cluster-Poisson variety $\mathcal{P}_{G,S}$ -- the moduli spaces of local $G$-bundles on a surface $S$ with punctures and marked points on its boundary. The existence of such cluster structures was proven by Goncharov and Shen in \cite{QLS} for any split semi-simple algebraic group $G$. In the case of $G = SL_n$ these cluster varieties were discovered by Fock and Goncharov in \cite{MSLS}. The cluster $K_2$-variety structure on the moduli space of a triples of decorated flags (here surface $S$ is a disk with 3 special points on the boundary) provides us with an element $\omega \in \bigwedge^2 \mathbb{C}(G^3 / G)^*$, which gives rise to an invariant 2-form $\Omega$. Our main construction will extend this element to a nontrivial cocycle. As a part of our construction of an element of $H^4(BG, \mathbb{Z}_{\mathcal{M}}(2))$ we will present an algebraic geometric construction of a generator in the 3-dimensional cohomology for the group $G$$$H^3(G(\mathbb{C}),\mathbb{Z}(2)) \simeq H^4(BG,\mathbb{Z}(2))\text{, where } \mathbb{Z}(k) = (2\pi i)^k \mathbb{Z}.$$ The latter group is isomorphic to $\mathbb{Z}$, as $H^*(BG,\mathbb{C}) \simeq \mathbb{C}[\mathfrak{g}]^G$, and there is a unique up to a scalar degree 2 polynomial invariant under the $G$ action, coming from the Killing form on the root lattices. One of the main results of the thesis is that the element we constructed generates $H^3(G(\mathbb{C}), \mathbb{Z}(2))$. This implies that the constructed cocycle of $H^4(BG, \mathbb{Z}_{\mathcal{M}}(2))$ produces the second motivic Chern class for the universal $G$-bundle over the classifying space $BG$. This motivic class gives rise to the topological Chern class, using the Lie-exponential complex of sheaves, as explained in \cite{EC}. This thesis is based on joint work with Alexander Goncharov.
Recommended Citation
Kislinskyi, Oleksii, "Construction of the Universal Second Motivic Chern Class using Cluster Varieties" (2021). Yale Graduate School of Arts and Sciences Dissertations. 66.
https://elischolar.library.yale.edu/gsas_dissertations/66