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The Erdős–Moser Equation

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Posed by Paul Erdős / Leo Moser · 1956 · number theory · ~1 min read · difficulty 3/5

The problem

Question: solve \(1^k + 2^k + \cdots + (k-1)^k = k^k\) in integers \(k \geq 2\). Moser proved \(k > 10^{10^6}\) for any solution; Moree and co-authors later pushed related congruence constraints further.

History & significance

A deceptively innocent Diophantine equation circulating since the 1950s. The elementary observation that the left side is ≡ Σ \(i^k\) mod k, combined with von Staudt–Clausen structure of Bernoulli denominators, forces k to be enormous — Moser's celebrated double-exponential bound. Every refinement since has only raised the floor. Whether the answer is 'no solutions' or 'one monstrous example' is genuinely open, making this the purest tiny-question-big-mathematics specimen on the shelf.

Still open.

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