$K$theory of division algebras over local fields
Derived algebraic geometry and its applications March 25, 2019  March 29, 2019
Location: MSRI: Simons Auditorium
Algebraic $K$theory
division rings
topological cyclic homology
16Hesselholt
This is joint work with Michael Larsen and Ayelet Lindenstrauss. Let $K$ be a complete discrete valuation field with finite residue field of characteristic $p$, and let $D$ be a central division algebra over $K$ of finite index $d$. Thirty years ago, Suslin and Yufryakov showed that for all prime numbers $\ell \neq p$ and integers $j \geq 1$, there exists a canonical "reduced norm" isomorphism of $\ell$adic $K$groups $\operatorname{Nrd}_{D/K} \colon K_j(D,\mathbb{Z}_{\ell}) \to K_j(K,\mathbb{Z}_{\ell})$ such that $d \cdot \operatorname{Nrd}_{D/K}$ is equal to the norm homomorphism $N_{D/K}$. We prove the analogous statement for the $p$adic $K$groups. To do so, we employ the cyclotomic trace map to topological cyclic homology and show that there exists a "reduced trace" equivalence $\operatorname{Trd}_{A/S} \colon \operatorname{THH}(A\,\,D,\mathbb{Z}_p) \to \operatorname{THH}(S\,\,K,\mathbb{Z}_p)$ between two $p$complete cyclotomic spectra associated with $D$ and $K$, respectively, from which the statement for the $p$adic $K$groups ensues. Interestingly, we show that if $p$ divides $d$, then it is not possible to choose said equivalence such that, as maps of cyclotomic spectra, $d \cdot \operatorname{Trd}_{A/S}$ agrees with the trace $\operatorname{Tr}_{A/S}$, although this is possible as maps of spectra with $\mathbb{T}$action.
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