Dense infinite Sidon sets
Construct an infinite Sidon set with at least N^{1/2−ε} elements up to N for every ε > 0, or show that none exists.
Open, $500 prizeper Erdős Problems, 2026-10-20$500 prize
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Problems
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The de Bruijn–Newman constant
Narrow the de Bruijn–Newman constant, which equals 0 exactly when the Riemann hypothesis holds; it is known that 0 ≤ Λ ≤ 0.2.
Openper Optimization Constants in Mathematics, 2026-10-20
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Composite numbers in sumsets of sparse sets
Show that if A, B ⊆ {1,…,N} each have at least N^0.49 elements, then the sumset A + B contains a composite number.
Openper Ben Green's 100 open problems, 2026-10-20
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Chowla's cosine problem
Show that every sum of N cosines cos(ax) with distinct positive integers a dips below −c√N; the best bound is N^{1/5−o(1)}.
The Brocard–Ramanujan equation n! + 1 = m²
Prove that n! + 1 is a perfect square only for n = 4, 5 and 7, or find a fourth solution.
A composite number that passes the Baillie–PSW test
Find an odd composite number passing both the strong base-2 Fermat test and the strong Lucas test of Baillie–PSW; none is known below 2^64.
Unsolvedper FrontierMath: Open Problems, 2026-10-20
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