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Erdős problem 403

Does the equation 2m=a1!++ak!2^m=a_1!+\cdots+a_k! with a1<a2<<aka_1<a_2<\cdots <a_k have only finitely many solutions?

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403.lean

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Does the equation 2m=a1!++ak!2^m=a_1!+\cdots+a_k! with a1<a2<<aka_1<a_2<\cdots <a_k have only finitely many solutions?

Asked by Burr and Erdős. Frankl and Lin [Li76] independently showed that the answer is yes, and the largest solution is 27=2!+3!+5!.2^7=2!+3!+5!. In fact Lin showed that the largest power of 22 which can divide a sum of distinct factorials containing 22 is 22542^{254}, and that there are only 5 solutions to 3m=a1!++ak!3^m=a_1!+\cdots+a_k! (when m=0,1,2,3,6m=0,1,2,3,6).

See also [404].

A solution is encoded below as a pair (m,s)(m, s) where ss is the finite set {a1<a2<<ak}\{a_1 < a_2 < \cdots < a_k\} of positive integers, so the distinctness of the aia_i is given by set membership. The empty set contributes no solutions since 2m1>02^m \geq 1 > 0.

The linked proof gives more than finiteness: it classifies the solutions outright, as (0,{1})(0,\{1\}), (1,{2})(1,\{2\}), (3,{2,3})(3,\{2,3\}), (5,{2,3,4})(5,\{2,3,4\}) and (7,{2,3,5})(7,\{2,3,5\}), so the set below has exactly five elements.

FormalConjectures/ErdosProblems/403.leanErdos403.erdos_4031 lineExact file
True ↔ {p | (∀ ap.2, 0 < a) ∧ 2 ^ p.1 = ∑ ap.2, a.factorial}.Finite
SolvedProof has a holelean4external proof

The proof uses `sorry`: part of the argument is written but not proved. Lean accepts the file; it does not accept the theorem.

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