Wednesday, 18 November 2009

gr.group theory - Slight question variant on "order information enough to guarantee 1-isomorphism"

This is a very slight variant on the question order information enough to guarantee 1-isomorphism? that I asked a while back, with an answer in the negative.



Background repeated:



I define a 1-isomorphism between two groups as a bijection that restricts to an isomorphism on every cyclic subgroup on either side. There are plenty of examples of 1-isomorphisms that are not isomorphisms. For instance, the exponential map from the additive group of strictly upper triangular matrices to the multiplicative group of unipotent upper triangular matrices is a 1-isomorphism. Many generalizations of this, such as the Baer and Lazard correspondences, also involve 1-isomorphisms between a group and the additive group of a Lie algebra/Lie ring.



Consider the following function F associated to a finite group G. For divisors $d_1$, $d_2$ of G, define $F_G(d_1,d_2)$ as the number of elements of G that have order equal to $d_1$ and that can be expressed in the form $x^{d_2}$ for some $x in G$.



New question: If G is a finite abelian group and H is a finite (not necessarily abelian) group such that $F_G = F_H$, is it necessary that there is a 1-isomorphism between G and H.



For the original question, I had not insisted that one of the groups be abelian, and Tom Goodwillie provided a counterexample with both groups non-abelian of order 32.



The reason for my interest is as follows: I want to determine which groups are 1-isomorphic to abelian groups. This will help me with exploring some generalizations of the Lazard correspondence. To do this properly, I would need to construct a combinatorial structure (such as the directed power graph) that stores all the information of the group up to 1-isomorphism. However, constructing this structure and then verifying whether the graphs thus constructed for two groups are isomorphic is computationally somewhat harder. On the other hand, $F_G$ can be stored easily and we can quickly check for two groups whether their $F$s coincide.



Apart from this computational perspective, the question is also of academic interest to me.

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