Left restriction monoids from left $E$-completions

Abstract

Given a monoid $S$ with $E$ any non-empty subset of its idempotents, we present a novel one-sided version of idempotent completion we call left $E$-completion. In general, the construction yields a one-sided variant of a small category called a constellation by Gould and Hollings. Under certain conditions, this constellation is inductive, meaning that its partial multiplication may be extended to give a left restriction semigroup, a type of unary semigroup whose unary operation models domain. We study the properties of those pairs $S,E$ for which this happens, and characterise those left restriction semigroups that arise as such left $E$-completions of their submonoid of elements having domain $1$. As first applications, we decompose the left restriction semigroup of partial functions on the set $X$ and the right restriction semigroup of left total partitions on $X$ as left and right $E$-completions respectively of the transformation semigroup $T_X$ on $X$, and decompose the left restriction semigroup of binary relations on $X$ under demonic composition as a left $E$-completion of the left-total binary relations. In many cases, including these three examples, the construction embeds in a semigroup Zappa-Sz\'{e}p product.