A032107 Number of reversible strings with n labeled beads of 2 colors.

2, 4, 24, 192, 1920, 23040, 322560, 5160960, 92897280, 1857945600, 40874803200, 980995276800, 25505877196800, 714164561510400, 21424936845312000, 685597979049984000, 23310331287699456000

Offset: 1

Author

Christian G. Bower

Keywords

• nonn

Links

• C. G. Bower, Transforms (2)

Programs

Mathematica

Join[{2},Table[2^(n-1) n!,{n,2,20}]] (* Harvey P. Dale, May 11 2025 *)

Formula

2^(n-1)*n! (n>1). E.g.f.: (1-2*x^2)/(1-2*x). Also "BIJ" (reversible, indistinct, labeled) transform of 2, 0, 0, 0...

Extensions

Essentially same as A002866.