A087906 a(n) = Sum_{d|n} (n-1)!/(d-1)!.

1, 2, 3, 13, 25, 301, 721, 10921, 60481, 740881, 3628801, 106777441, 479001601, 12462690241, 134399865601, 2833553923201, 20922789888001, 892191453753601, 6402373705728001, 268633265290790401, 3652732042831872001, 102181898422712908801, 1124000727777607680001

Offset: 1

Author

Vladeta Jovovic, Oct 15 2003

Keywords

• nonn

Links

• G. C. Greubel, Table of n, a(n) for n = 1..450

Crossrefs

Cf. A057625, A061095, A038041, A038048, A005225, A209902 (exp).

Programs

Mathematica

Array[n \[Function] DivisorSum[n, (n - 1)!/(# - 1)! &], 25] (* J. Mulder (jasper.mulder(AT)planet.nl), Jan 25 2010 *)

PARI

a(n)=sumdiv(n,d,(n-1)!/(d-1)!); \\ Joerg Arndt, May 21 2013

Formula

E.g.f.: Sum_{k>0} (exp(x^k)-1)/k = -Sum_{k>0} log(1-x^k)/k!.

Extensions

More terms from J. Mulder (jasper.mulder(AT)planet.nl), Jan 25 2010