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Rudin's conjecture is a mathematical conjecture in additive combinatorics and elementary number theory about an upper bound for the number of squares in finite arithmetic progressions. The conjecture, which has applications in the theory of trigonometric series, was first stated by Walter Rudin in his 1960 paper Trigonometric series with gaps.

For positive integers ${\displaystyle N,q,a}$ define the expression ${\displaystyle Q(N;q,a)}$ to be the number of perfect squares in the arithmetic progression ${\displaystyle qn+a}$, for ${\displaystyle n=0,1,\ldots ,N-1}$, and define ${\displaystyle Q(N)}$ to be the maximum of the set {Q(N; q, a) : q, a ≥ 1} . The conjecture asserts (in big O notation) that ${\displaystyle Q(N)=O({\sqrt {N}})}$ and in its stronger form that, if ${\displaystyle N>6}$, ${\displaystyle Q(N)=Q(N;24,1)}$.

References

1. Cilleruelo, Javier; Granville, Andrew (2007). "Lattice points on circles, squares in arithmetic progressions and sumsets of squares". In Granville, Andrew; Nathanson, Melvyn Bernard; Solymosi, József (eds.). Additive combinatorics. CRM Proceedings & Lecture Notes, vol. 43. American Mathematical Society. pp. 241–262. ISBN 978-0-8218-7039-6. arXiv.org preprint
2. Rudin, Walter (1960). "Trigonometric series with gaps". Journal of Mathematics and Mechanics. 9 (2): 203–227. JSTOR 24900534.
3. ^ González-Jiménez, Enrique; Xarles, Xavier (2014). "On a conjecture of Rudin on squares in arithmetic progressions". LMS Journal of Computation and Mathematics. 17 (1): 58–76. arXiv:1301.5122. doi:10.1112/S1461157013000259.

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