Forbidden configurations for coherency

Abstract: Right (and left) coherency and right (and left) weak coherency are natural finitary conditions for monoids, however, determining whether a given monoid is right, or dually left, coherent turns out to be hard and disparate techniques have been developed to show that certain classes of monoids are (or are not) right coherent. We exhibit a particular configuration of elements in a monoid subsemigroup of a right (respectively, left) $E$-Ehresmann monoid, relative to the Ehresmann structure of the overmonoid, that prohibits left (respectively, right) coherence. We apply this technique to show that the free left Ehresmann monoid of rank at least $2$ is not left coherent and the free Ehresmann monoid of rank at least $2$ is neither left nor right coherent. We show that our technique is particularly useful in the case where the overmonoid is an $E$-unitary inverse monoid, and apply this to recover the following results of Gould and Hartmann: the free inverse monoids and free ample monoids of rank at least 2 are neither left nor right coherent, and the free left ample monoid of rank at least $2$ is not left coherent. Viewed as a unary monoid, a free left Ehresmann monoid does not embed into an inverse monoid. We show that a free left Ehresmann monoid viewed as a monoid (the standpoint of this paper) does embed into an $E$-unitary inverse monoid. In another positive direction, we demonstrate that the free left Ehresmann monoid is weakly coherent.