A finite poset is said to have the Sperner property if at least one of its maximum antichains is formed from elements of the same height. A polygonal graph is a directed acyclic graph derived from a circuit by some orientation of its edges. The reachability relation of a polygonal graph is a partial order. A criterion is presented for posets associated with polygonal graphs to have the Sperner property.
It is proven that principal ideals generated by congruences of a path having the same type are isomorphic lattices. The number of elements, atoms and coatoms is found for the principal ideal generated by a given congruence of a path.
