How many permutations of $1, 2,\dots , 7$ are NOT derangements?
To start, do I do an inclusion exclusion starting with the total number of permutations minus derangements with 1, then with 2, etc?
2026-02-23 03:01:34.1771815694
Derangements and permutations
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$5040-\lfloor\frac{5040}{e}\rfloor=5040-1854=3186$ if you follow Numberphile's video on the topic. https://oeis.org/A000166 is subtracted from https://oeis.org/A000142