Which lattices occur as intervals in subgroup lattices of finite groups?
Conjecture
There exists a finite lattice that is not an interval in the subgroup lattice of a finite group.
This would settle the Finite Lattice Representation Problem.
Bibliography
[P5] Palfy and Pudlak. Congruence lattices of finite algebras and intervals in subgroup lattices of finite groups. Algebra Universalis, 11 (1980), 22–27.
* indicates original appearance(s) of problem.