The weak commutativity construction for Lie algebras

We study the analogue of Sidki's weak commutativity construction, defined originally for groups, in the category of Lie algebras. This is the quotient $\chi(\mathfrak{g})$ of the Lie algebra freely generated by two isomorphic copies $\mathfrak{g}$ and $\mathfrak{g}^{\psi}$ of a fixed Lie algebra by the ideal generated by the brackets $[x,x^{\psi}]$, for all $x$. We exhibit an abelian ideal of $\chi(\mathfrak{g})$ whose associated quotient is a subdirect sum in $\mathfrak{g} \oplus \mathfrak{g} \oplus \mathfrak{g}$ and we give conditions for this ideal to be finite dimensional. We show that $\chi(\mathfrak{g})$ has a subquotient that is isomorphic to the Schur multiplier of $\mathfrak{g}$. We prove that $\chi(\mathfrak{g})$ is finitely presentable or of homological type $FP_2$ if and only if $\mathfrak{g}$ has the same property, but $\chi(\mathfrak{f})$ is not of type $FP_3$ if $\mathfrak{f}$ is a non-abelian free Lie algebra.Comment: Incorporated referee's suggestions, results unchange

Galois subcovers of the Hermitian curve in characteristic pp with respect to subgroups of order p2p^2

A (projective, geometrically irreducible, non-singular) curve $\mathcal{X}$ defined over a finite field $\mathbb{F}_{q^2}$ is maximal if the number $N_{q^2}$ of its $\mathbb{F}_{q^2}$-rational points attains the Hasse-Weil upper bound, that is $N_{q^2}=q^2+2\mathfrak{g}q+1$ where $\mathfrak{g}$ is the genus of $\mathcal{X}$. An important question, also motivated by applications to algebraic-geometry codes, is to find explicit equations for maximal curves. For a few curves which are Galois covered of the Hermitian curve, this has been done so far ad hoc, in particular in the cases where the Galois group has prime order. In this paper we obtain explicit equations of all Galois covers of the Hermitian curve with Galois group of order $p^2$ where $p$ is the characteristic of $\mathbb{F}_{q^2}$. Doing so we also determine the $\mathbb{F}_{q^2}$-isomorphism classes of such curves and describe their full $\mathbb{F}_{q^2}$-automorphism groups

Ramification filtration by moduli in higher-dimensional global class field theory

In their approach to higher-dimensional global class field theory, Kato and Saito define the class group of a proper arithmetic scheme \bar{X} as an inverse limit C_{KS}(\bar{X}) = \varprojlim_{\mathcal{I}} C_{\mathcal{I}}(\bar{X}) of certain Nisnevich cohomology groups C_{\mathcal{I}}(\bar{X}) taken over all non-zero coherent ideal sheaves \mathcal{I} of \mathcal{O}_{\bar{X}}. The ideal sheaves \mathcal{I} should be regarded as higher-dimensional analogues of the classical moduli \mathfrak{m} on a global field K, which induce a filtration of the idele class group C_K by the ray class groups C_K/C_K^{\mathfrak{m}}. In higher dimensions however, it is not clear how the induced filtration of the abelian fundamental group can be interpreted in terms of ramification. In view of Wiesend's class field theory, we define an easier notion of moduli in higher dimensions only involving curves on the scheme. We then show that both notions agree for moduli that correspond to tame ramification