OFFSET
0,3
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.18.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..200
Richard Stanley and Albert Nijenhuis, Problem 5932, Amer. Math. Monthly, 82 (1975), 86-87.
FORMULA
E.g.f.: exp ( Sum_{j>=1} x^j / j*phi(j) ), where phi = Euler phi function (A000010).
MAPLE
with(numtheory); exp(add(x^n/(n*phi(n)), n=1..31));
MATHEMATICA
max = 22; f[x_] := Exp[ Sum[ x^k/(k*EulerPhi[k]), {k, 1, max}]]; CoefficientList[ Series[ f[x], {x, 0, max}], x]*Range[0, max]! (* Jean-François Alcover, Oct 12 2011 *)
CROSSREFS
KEYWORD
nonn,easy,nice
AUTHOR
STATUS
approved