Lüroth's theorem for fields of rational functions in infinitely many permuted variables

Abstract

L\"uroth's theorem describes the dominant maps from rational curves over a field. In this note we study the dominant maps from cartesian powers $X^{\Psi}$ of absolutely irreducible varieties $X$ over a field $k$ for infinite sets $\Psi$ that are equivariant with respect to all permutations of the factors $X$. At least some of such maps arise as compositions $h:X^{\Psi}\xrightarrow{f^{\Psi}}Y^{\Psi}\to H\backslash Y^{\Psi}$, where $X\xrightarrow{f}Y$ is a dominant $k$-map and $H$ is an automorphism group $H$ of $Y|k$, acting diagonally on $Y^{\Psi}$. In characteristic 0, we show that this construction, when properly modified, gives all dominant equivariant maps from $X^{\Psi}$, if $\dim X=1$. For arbitrary $X$, the results are only partial. In a subsequent paper, the `quasicoherent' equivariant sheaves on the targets of such $h$'s will be studied. Some preliminary results have already appeared in arXiv:math/2205.15144. A somewhat similar problem is to check, whether the irreducible invariant subvarieties of $X^{\Psi}$ arise as pullbacks under $f^{\Psi}$ (for appropriate $f$'s) of subvarieties of $Y$ diagonally embedded into $Y^{\Psi}$. This would be a complement to the famous theorem of D.E.Cohen on the noetherian property of the symmetric ideals. We show that this is the case if $\dim X=1$.