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Ricardo Andrade
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verret
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2-group with two isomorphic normal subgroups of index 4 with non-isomorphic quotients

Hypothesis: Let P be a finite 2-group with two isomorphic normal subgroups M and N such that P/MC4 (the cyclic group of order 4) while P/NC22. By the lattice theorem, there are exactly three groups G with N<G<P.

Question: Is it possible that these three groups are all isomorphic?

Comments: There are plenty of examples satisfying the hypothesis. The smallest is P=C4×C2, with M=1×C2 and N=C2×1. In this case, of the three groups between N and P, two are isomorphic to C4 and one is isomorphic to C22. Using magma, I have checked all groups P of order at most 128 and it seems that the answer to the question is "no" for these (barring a mistake in my code, which is entirely possible).