This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A069660 #9 Jul 12 2025 08:34:39
%S A069660 6,8,120,72,5040,1152,362880,28800,39916800,1036800,6227020800,
%T A069660 50803200,1307674368000,3251404800,355687428096000,263363788800,
%U A069660 121645100408832000,26336378880000,51090942171709440000,3186701844480000,25852016738884976640000,458885065605120000
%N A069660 Order of the subgroup of the symmetric group S_n generated by the cycles (1,3) and (1,2,3,...,n).
%F A069660 If n is odd a(n) = n!, if n is even a(n) = 2 * ((n/2)!)^2 = 2 * n! / A000984(n/2) = 2 * A001044(n/2).
%F A069660 Sum_{n>=3} 1/a(n) = BesselI(0, 2)/2 + sinh(1) - 2. - _Amiram Eldar_, Jul 12 2025
%t A069660 a[n_] := If[OddQ[n], n!, 2 * ((n/2)!)^2]; Array[a, 20, 3] (* _Amiram Eldar_, Jul 12 2025 *)
%o A069660 (PARI) a(n) = if(n % 2, n!, 2 * ((n/2)!)^2); \\ _Amiram Eldar_, Jul 12 2025
%Y A069660 Cf. A000984, A070734, A000142, A009445, A001044.
%K A069660 nonn,easy
%O A069660 3,1
%A A069660 Sharon Sela (sharonsela(AT)hotmail.com), May 16 2002
%E A069660 More terms from _Benoit Cloitre_, May 20 2002