A327940 Expansion of e.g.f. exp(Sum_{i>=1} Sum_{j=1..i-1} x^(i*j) / (i*j)).
1, 0, 1, 2, 9, 44, 385, 1854, 23233, 153656, 2151441, 18787130, 338487721, 3165541092, 60609811249, 835202858294, 14913805143105, 228441779869424, 5319673396479073, 81040768940877426, 2153026504862728201, 39759334398324543260, 988919906784578473761
Offset: 0
Keywords
Programs
-
Mathematica
nmax = 22; CoefficientList[Series[Exp[Sum[Floor[DivisorSigma[0, k]/2] x^k/k, {k, 1, nmax}]], {x, 0, nmax}], x] Range[0, nmax]! a[n_] := a[n] = If[n == 0, 1, Sum[Floor[DivisorSigma[0, k]/2] a[n - k], {k, 1, n}]/n]; Table[n! a[n], {n, 0, 22}]