This is an open problem, see Palfy and Pudlak's result here:http://www.math.hawaii.edu/~williamdemeo/latticetheory/Palfy-IntervalsInSubgroupLattices-GStA-1993.pdf&ved=0CCcQFjAC&usg=AFQjCNGhWHFLz7L7Eb9GN2ZJDMyVeCvtOA&sig2=xUfsyNg1Eqv2d8LhhGqr7Q
Bjørn Kjos-Hanssen
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