Post date: Apr 4, 2019 7:58:37 PM
The only number that is the sum of the subfactorials of its digits.
148,349 = !1 + !4 + !8 + !3 + !4 + !9
!n = n! (1 - 1/1! + 1/2! - 1/3! + 1/4! - . . . + 1/n!)
If n letters are written to n different addresses and there are n matching envelopes, how many ways can the letters be placed in the envelopes so that every letter is in the wrong envelope? The answer is !n.
Therefore, as n approaches infinity, the probability that all the letters are in the wrong envelope is 1/e.