## Essential dimension in mixed characteristic. (arXiv:1801.02245v2 [math.AG] UPDATED)

Suppose $G$ is a finite group and $p$ is either a prime number or $0$. For
$p$ positive, we say that $G$ is weakly tame at $p$ if $G$ has no non-trivial
normal $p$-subgroups. By convention we say that every finite group is weakly
tame at $0$. Now suppose that $G$ is a finite group which is weakly tame at the
residue characteristic of a discrete valuation ring $R$. Our main result shows
that the essential dimension of $G$ over the fraction field $K$ of $R$ is at
least as large as the essential dimension of $G$ over the residue field $k$. We
also prove a more general statement of this type for a class of \'etale gerbes
over $R$.
As a corollary, we show that, if $G$ is weakly tame at $p$ and $k$ is any
field of characteristic $p >0$ containing the algebraic closure of
$\mathbb{F}_p$, then the essential dimension of $G$ over $k$ is less than or
equal to the essential dimension of $G$ over any characteristic $0$ field. A
conjecture of A. Ledet asserts that the essential dimension,
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