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Is \[\sum_{n\geq 2}\frac{1}{n!-1}\] irrational?
The decimal expansion is A331373 in the OEIS. Weisenberg has observed that this sum can also be written as \[\sum_{k\geq 1}\sum_{n\geq 2}\frac{1}{(n!)^k}.\]
Additional thanks to: Desmond Weisenberg