Erdős problem 403
Does the equation with have only finitely many solutions?
Sources
FormalConjectures/ErdosProblems/
403.lean
Does the equation with 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 In fact Lin showed that the largest power of which can divide a sum of distinct factorials containing is , and that there are only 5 solutions to (when ).
See also [404].
A solution is encoded below as a pair where is the finite set of positive integers, so the distinctness of the is given by set membership. The empty set contributes no solutions since .
The linked proof gives more than finiteness: it classifies the solutions outright, as , , , and , so the set below has exactly five elements.
True ↔ {p | (∀ a ∈ p.2, 0 < a) ∧ 2 ^ p.1 = ∑ a ∈ p.2, a.factorial}.Finite