Subdiagrams equal in number to their duals (Q1088695)

A subdiagram S of an ordered set P is a cover-preserving ordered subset of P. If S is finite, \(\ell (S)\neq 2\) and S is, as a down set, embedded in a selfdual lattice of subspaces of a projective geometry then the authors prove that S is a ''dual subdiagram invariant'' which means: For any modular lattice M, the number of subdiagrams of M isomorphic to S equals the number of subdiagrams of M dually isomorphic to S.