On Proximity Subgroups of Groups

Abstract

By a $\text{proximity subgroup}$ of a group $G$ we mean a norm $\nu$ on $G$ valued in a complete lattice $L$ equipped with a binary sum (an additive counterpart of a multiplicative structure). The idea is that the smaller the value $\nu (y)$ is, the closer $y\in G$ is to the subgroup of $G$ defined by the equation $\nu (x)=\perp$, where $\perp$ is the bottom element of $L$. We test whether this notion is a suitable substitute for the usual notion of a subgroup (recovered by taking $L=\{\perp ,\top \}$ with join as the sum), by showing that the form of proximity subgroups is noetherian. Intuitively, this means that proximity subgroups are as well suited for formulating and proving isomorphism theorems and homological diagram lemmas for groups as the usual subgroups are. We study several questions in relation to this, including what makes the noetherian form of usual subgroups special among all of these noetherian forms, and in fact, all noetherian forms of the category of groups.