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$G$ is an abelian group, $A$ and $B$ are non empty finite subsets of $G$. Set $A+B := \{a+b\mid a\in A, b\in B\}$ and $H := \mathrm{stab}(A+B)=\{g\in G \mid g+A+B = A+B\}$.

Prove that $$ |A+B|\ge|A+H|+|B+H|-|H| $$ Please help me in finding answer.Thank you in advance

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Welcome to maths.SE. May I encourage you to edit your question to describe where this problem has come from and where exactly you are stuck? – Douglas S. Stones May 6 '13 at 17:52
    
You should look up Kneser's Theorem and realize that there's a good reason as to why you couldn't solve this problem right away. – Nonoffensive name Aug 1 '13 at 21:55
    
Here is the shortest proof I've found arxiv.org/pdf/1303.3539v1.pdf – Nonoffensive name Aug 1 '13 at 22:00

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