OFFSET
0,2
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..386
Thomas Wieder, The number of certain k-combinations of an n-set, Applied Mathematics Electronic Notes, vol. 8 (2008).
FORMULA
a(n) = Sum_{k=0..n} k^k * (n-k)^(n-k). - Tilman Neumann, Dec 13 2008
a(n) ~ 2 * n^n. - Vaclav Kotesovec, Dec 03 2021
EXAMPLE
(1 + x + 4x^2 + 27x^3 + 256x^4 +...)^2 = 1 + 2x + 9x^2 + 62x^3 +...
MATHEMATICA
nn=17; f[x_]=1+Sum[n^n x^n, {n, 1, nn}]; CoefficientList[Series[f[x]^2, {x, 0, nn}], x] (* Geoffrey Critzer, Nov 05 2013 *)
PROG
(PARI) a(n) = sum(k=0, n, k^k*(n-k)^(n-k)); \\ Seiichi Manyama, Dec 03 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, Nov 20 2004
STATUS
approved