Wikipedia:Reference desk/Archives/Mathematics/2021 July 20

= July 20 =

(!n)
The (!n) math notation seems missing in the Glossary of mathematical symbols and the ! articles. WolframAlpha shows the alternate form (Γ(1 + n, -1)/e) which appears to be the definition. I don't feel sufficiently competent in Math notation to add this symbol or even if it should, for whatever reason, be added be added at all. I'm sure that one of you Math guys can fix or explain, thanks hydnjo (talk) 14:29, 20 July 2021 (UTC)
 * The subfactorial $$!n$$ is discussed in our derangement article. I have added an entry to Glossary of mathematical symbols with a link to that article. CodeTalker (talk) 16:06, 20 July 2021 (UTC)
 * A subfactorial is the number of ways to arrange the natural numbers 1 to n so that the kth number never occurs in the kth position, for clarification. It's easy to see that the subfactorial of 1 is 0. The subfactorial of 2 is 1 because (2,1) is the only way you can arrange 2 items to meet the above criterion. Then we have (3,1,2) and (2,3,1) so the subfactorial of 3 is 2. But after that subfactorials get large. For n = 4, we have 3 ways to arrange 4 items that meet this criterion where the first is 2; these are (2,3,4,1) (2,1,4,3) and (2,4,1,3) then there are likewise 3 where the first is 3 and these are (3,4,1,2) (3,1,4,2) and (3,4,2,1) and then we have 3 more where the first is 4; these are (4,1,2,3) (4,3,1,2) and (4,3,2,1); so this means 4 subfactorial must be 9. The first few subfactorials are 0, 1, 2, 9, 44, 265, 1854, and 14833. For all positive integers n, exactly one of n and n subfactorial is odd and the other even. The subfactorial of n is always divisible by n-1. Georgia guy (talk) 16:34, 20 July 2021 (UTC)
 * The subfactorial, or derangement, series is listed in the OEIS and is entry number A000166. There is a note there saying that the sequence was described as early as Leonhard Euler, who not only listed its first several members, but also derived formulas to generate it.  Also, as it has an N-number, it dates to the original 1973 Handbook of Integer Sequences, one of Neil Sloane's originals.  -- Jayron 32 18:04, 20 July 2021 (UTC)
 * It has to be said that it was missing because it is by no means a common nor a standard notation in mathematics (certainly not comparable with the factorial). In the very few occasions where one needs a special notation for the number of derangements, this or other notations have to be introduced and explained every time. pm a 19:27, 23 July 2021 (UTC)

Which simple challenge you can ask to someone over the phone using common math in order to ask him to prove he as a specific computing power?
Such type of benchmarking without using a full program is something which usually requires generating large numbers from small numbers. I was thinking about a challenge that takes less than 2 minutes to complete using GMP and ask to provide the first or last 5 digits of the result, but I was unable to find something which can t be heavily simplified to the point a full computer isn t required to compute the result. Or I found formulas that have ambiguous writing when dictated over the phone.

I prefer to prove ram amount over speed if possible. 2A01:E0A:401:A7C0:5CA2:C19F:2FCA:369E (talk) 18:51, 20 July 2021 (UTC)


 * This question was also posted at WP:RD/Computing (Added: and an answer has been posted). Please pick one reference desk and delete the other question. --184.147.181.169 (talk) 19:33, 20 July 2021 (UTC)