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
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
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
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 19 2000
STATUS
approved