A000780 a(n) = (n+1)!/2 + (n-1)(n-1)!.

1, 4, 16, 78, 456, 3120, 24480, 216720, 2136960, 23224320, 275788800, 3552595200, 49337164800, 734788454400, 11681891020800, 197458829568000, 3535951491072000, 66869236482048000, 1331693730791424000, 27856727993622528000, 610658404052336640000

Offset

1,2

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..200

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 392

J. R. Stembridge, Some combinatorial aspects of reduced words in finite Coxeter groups, Trans. Amer. Math. Soc. 349 (1997), no. 4, 1285-1332.

Formula

a(n) = (n-1)!* (n^2+3*n-2)/2. - Gary Detlefs, May 22 2010

Maple

seq((n-1)!* (n^2+3*n-2)/2, n = 1..19); # Gary Detlefs, May 22 2010

Mathematica

Table[(n + 1)!/2 + (n - 1)*(n - 1)!, {n, 40}] (* Vladimir Joseph Stephan Orlovsky, Feb 23 2012 *)

Prog

(Magma) [Factorial(n+1)/2+(n-1)*Factorial(n-1): n in [1..25]]; // Vincenzo Librandi, Jun 07 2013

Crossrefs

Sequence in context: A204208 A138294 A014514 * A002713 A221763 A362750

Adjacent sequences: A000777 A000778 A000779 * A000781 A000782 A000783

Keyword

nonn

Author

N. J. A. Sloane

Status

approved