A contribution to the characterization of finite minimal automorphic posets of width three

Abstract

The characterization of the finite minimal automorphic posets of width three is still an open problem. Niederle has shown that this task can be reduced to the characterization of the nice sections of width three having a non-trivial tower of nice sections as retract. We solve this problem for a sub-class $\mathfrak{N}_2$ of the finite nice sections of width three. On the one hand, we characterize the posets in $\mathfrak{N}_2$ having a retract of width three being a non-trivial tower of nice sections, and on the other hand we characterize the posets in $\mathfrak{N}_2$ having a 4-crown stack as retract. The latter result yields a recursive approach for the determination of posets in $\mathfrak{N}_2$ having a 4-crown stack as retract. With this approach, we determine all posets in $\mathfrak{N}_2$ with height up to six having such a retract. For each integer $n \geq 2$, the class $\mathfrak{N}_2$ contains $2^{n-2}$ different isomorphism types of posets of height $n$.