Hilbert's 13th problem for algebraic groups

The algebraic form of Hilbert's 13th problem asks for the resolvent degree \operatorname{RD}(n) of the general polynomial f(x) = x^n + a_1 x^{n-1} + \cdots + a_n of degree n , where a_1, \ldots, a_n are independent variables. The resolvent degree is the minimal integer d such that every root of f(x) can be obtained in a finite number of steps, starting with \mathbb{C}(a_1, \ldots, a_n) and adjoining algebraic functions in \leqslant\nobreak d variables at each step. Recently Farb and Wolfson defined the resolvent degree \operatorname{RD}_k(G) for every finite group G and any base field k of characteristic 0 . In this setting \operatorname{RD}(n) = \operatorname{RD}_{\mathbb{C}}(\operatorname{S}_n) , where \operatorname{S}_n denotes the symmetric group. In this paper we extend their definition of \operatorname{RD}_k(G) to an arbitrary algebraic {group} G over an arbitrary field k . We investigate the dependency of this quantity on k and show that \operatorname{RD}_k(G) \leqslant 5 for any field k and any connected group G . The question whether \operatorname{RD}_k(G) can be bigger than 1 for any field k and any algebraic group G over k (not necessarily connected) remains open.

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