Timeline for What is the number of self-inverse permutations on a set of cardinality $N$?
Current License: CC BY-SA 3.0
10 events
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| Apr 13, 2017 at 12:19 | history | edited | CommunityBot |
replaced http://math.stackexchange.com/ with https://math.stackexchange.com/
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| Apr 29, 2016 at 16:46 | comment | added | Juan Sebastian Lozano | @IraGessel Oh, you're 100% right, I didn't even know OEIS had a formula section until you pointed it out. That's also probably what Alex Degtyarev was referring to too. | |
| Apr 29, 2016 at 16:30 | comment | added | Ira Gessel | The formula a(n)=Sum_{k=0..[ n/2 ]} n!/((n-2*k)!*2^k*k!) is the fourth line of the OEIS "Formula" section. | |
| Apr 27, 2016 at 12:34 | comment | added | Juan Sebastian Lozano | @ToddLeason I stand corrected, it is both trivial and well known. Thank you for linking to those places. | |
| Apr 27, 2016 at 12:26 | comment | added | Todd Leason | See for example math.stackexchange.com/questions/795541/involutions-of-s-n or google for "number of involutions in symmetric group". BTW: According to the acctepted SE answer the product in your formular can be resolved. | |
| Apr 27, 2016 at 11:42 | comment | added | Juan Sebastian Lozano | @AlexDegtyarev Although my idea of counting the number of graphs isn't novel (it's listed as Don Knuth's contribution on OIES), and there are a few other ones in terms of sums of other sequences, I couldn't find one with an explicit form like this. | |
| Apr 27, 2016 at 11:34 | comment | added | Alex Degtyarev | There are several similar "closed" formulas (in terms of sums) in OEIS. | |
| Apr 27, 2016 at 11:32 | comment | added | Juan Sebastian Lozano | Two reasons: 1) I didn't know if this was known, since I couldn't find it anywhere, and I wanted to see if it was even correct, and 2) as a reference for anyone (like me) who might be looking for the answer. I think that's pretty much why SE lets you answer your own questions. | |
| Apr 27, 2016 at 11:23 | comment | added | Todd Leason | You posted this answer along with your question in the same minute. So I wonder why you actually asked the question if you already had the answer ? | |
| Apr 27, 2016 at 10:34 | history | answered | Juan Sebastian Lozano | CC BY-SA 3.0 |