On a generalization of a theorem by Vosper.
We prove that there is a small but fixed positive integer such that for every prime larger than a fixed integer, every subset of the integers modulo which satisfies and is contained in an arithmetic progression of length . This is the first result of this nature which places no unnecessary restrictions on the size of .
A recent result of Balandraud shows that for every subset of an abelian group there exists a non trivial subgroup such that holds only if . Notice that Kneser’s Theorem only gives . This strong form of Kneser’s theorem follows from some nice properties of a certain poset investigated by Balandraud. We consider an analogous poset for nonabelian groups and, by using classical tools from Additive Number Theory, extend some of the above results. In particular we obtain short proofs...
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