On the subsemilattices of first-order definable and openly first-order definable congruences of the congruence lattice of a universal algebra.
The variety of lattices generated by lattices of all convex sublattices of distributive lattices is investigated.
This paper grew out from attempts to determine which modular lattices of finite height are locally order affine complete. A surprising discovery was that one can go quite far without assuming the modularity itself. The only thing which matters is that the congruence lattice is finite Boolean. The local order affine completeness problem of such lattices easily reduces to the case when is a subdirect product of two simple lattices and . Our main result claims that such a lattice is locally...