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NEAR OPTIMAL BOUNDS IN FREIMAN'S THEOREM

NEAR OPTIMAL BOUNDS IN FREIMAN'S THEOREM

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128 MARGULIS and MOHAMMADI<br />

α ∈ R 2 be given, and define β as above. Suppose that at least one of the following<br />

holds.<br />

(i) The vector β = (β1,β2) is Diophantine.<br />

(ii) There exists N,C > 0 such that, for all triples of integers (p1,p2,q) with<br />

q ≥ 2, we have<br />

<br />

<br />

max Ai −<br />

i=1,2<br />

pi<br />

<br />

<br />

><br />

q<br />

C<br />

q<br />

Then for any interval (a,b) with 0 /∈ (a,b), we have<br />

lim<br />

T →∞ Rh,α(a,b,T ) = c 2<br />

h (b − a). (22)<br />

Hence the spectrum satisfies the Berry-Tabor conjecture for the pair correlation<br />

function.<br />

The case of h = Z 2 was proved by Marklof [Mark]. His approach utilizes results from<br />

the theory of unipotent flows combined with an application of theta sums. We also use<br />

the theory of unipotent flows in our proof; however, our strategy to control the integral<br />

of unbounded functions over certain orbits is dynamical and rests heavily on [EMM1]<br />

and [EMM2].<br />

Outline of the proof. Let Qξ be a quadratic form of signature (p, q),andletn = p+q.<br />

Fix an interval (a,b), andletU⊂Rnbe a “suitably chosen" compact set such that<br />

a

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