The commutative inverse semigroup of partial abelian extensions
This paper is a new contribution to the partial Galois theory of groups. First, given a unital partial action $α_G$ of a finite group $G$ on an algebra $S$ such that $S$ is an $α_G$-partial Galois extension of $S^{α_G}$ and a normal subgroup $H$ of $G$, we prove that $α_G$ induces a unital partial action $α_{G/H}$ of $G/H$ on the subalgebra of invariants $S^{α_H}$ of $S$ such that $S^{α_H}$ is an $α_{G/H}$-partial Galois extension of $S^{α_G}$. Second, assuming that $G$ is abelian, we construct a commutative inverse semigroup $T_{par}(G,R)$, whose elements are equivalence classes of $α_G$-partial abelian extensions of a commutative algebra $R$. We also prove that there exists a group isomorphism between $T_{par}(G,R)/ρ$ and $T(G,A)$, where $ρ$ is a congruence on $T_{par}(G,R)$ and $T(G,A)$ is the classical Harrison group of the $G$-isomorphism classes of the abelian extensions of a commutative ring $A$. It is shown that the study of $T_{par}(G,R)$ reduces to the case where $G$ is cyclic. The set of idempotents of $T_{par}(G,R)$ is also investigated.