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 A055555 a(n) = n!*(n!+1)/2. 2
 1, 1, 3, 21, 300, 7260, 259560, 12703320, 812871360, 65841128640, 6584096534400, 796675481078400, 114721266640780800, 19387894024929830400, 3800027228319587865600, 855006126362753549184000, 218881568348707987666944000, 63256773252773762936322048000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS a(n) is the number of unordered pairs (not necessarily distinct) of elements in S_n (the symmetric group on n letters).  That is, a(n) = binomial(n!,2) + n!. - Geoffrey Critzer, Jan 09 2016 LINKS FORMULA a(n) + (-n^2-n-3)*a(n-1) + (n-1)*(n^2+2*n-1)*a(n-2) - 2*(n-1)*(n-2)^2*a(n-3) = 0. - R. J. Mathar, Mar 21 2013 a(n) = Sum_{k=1..n!} k. - Pedro Caceres, Mar 10 2018 a(n) = A000217(A000142(n)). - Michel Marcus, Mar 11 2018 MATHEMATICA Table[n!*(n! + 1)/2, {n, 0, 20}] (* Vladimir Joseph Stephan Orlovsky, Jul 07 2011 *) PROG (MAGMA) [Factorial(n)*(Factorial(n)+1)/2: n in [0..20]]; // Vincenzo Librandi, Jan 10 2016 (PARI) a(n) = n!*(n!+1)/2; \\ Altug Alkan, Jan 10 2015 CROSSREFS Cf. A000142, A000217. Sequence in context: A222035 A171201 A193206 * A208731 A158888 A331583 Adjacent sequences:  A055552 A055553 A055554 * A055556 A055557 A055558 KEYWORD nonn AUTHOR N. J. A. Sloane, Jul 19 2000 STATUS approved

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Last modified May 31 22:21 EDT 2020. Contains 334756 sequences. (Running on oeis4.)