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Let $A$ be a finite set of integers with $|A \hat{+} A| \leq K|A|$, where the $\hat{+}$ denotes restricted sumset: the set of all $a_1 + a_2$ with $a_1, a_2 \in A$ and $a_1 \neq a_2$.

Claim: $|A + A| \leq (K + o(1))|A|$, where $o(1)$ denotes a quantity tending to 0 as $|A|$ tends to $\infty$.

Sketch proof: Let $A'$ be the set of all $a \in A$ for which $2a$ cannot be represented as $a_1 + a_2$ with $a_1, a_2 \in A$, $a_1 \neq a_2$. Note that $2A' = (A + A) \setminus (A \hat{+} A)$, so it suffices to show that $A'$ is small. But $A'$ has no 3-term arithmetic progressions. But by standard results (Freiman, Ruzsa...) a set $A'$ with no 3-term progressions has $|A' + A'| \gg |A'| \log^c |A'|$. Thus indeed $|A'| = o(|A|)$.

I know I've seen this argument in the literature, and my question is simply: where?

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up vote 5 down vote accepted

For a somewhat similar argument, see Proposition 2.5 from Alon's 1987 paper ``Subset sums", available here.

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Thanks Seva - but I'm sure I've seen this exact argument somewhere. I guess not in one of your papers then? –  Ben Green Apr 19 '13 at 11:30
    
Not that I could recall it, at least... –  Seva Apr 19 '13 at 11:50
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I finally dug out the reference myself using a MathSciNet search. MR1931192 (2003f:11016) Reviewed Schoen, Tomasz(PL-POZN) The cardinality of restricted sumsets. (English summary) J. Number Theory 96 (2002), no. 1, 48–54. –  Ben Green Apr 19 '13 at 16:02

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