OFFSET
0,3
FORMULA
a(n) = A087221(3n).
EXAMPLE
Given f(x) = 1 + x + x^5 + x^21 + x^85 + x^341 + ...
so that f(x)^3 = 1 + 3x + 3x^2 + x^3 + 3x^5 + 6x^6 + 3x^7 + 3x^10 + ...
then A(x) = 1 + x*A(x)*(1 + 3x + 3x^2 + x^3 + 3x^5 + 6x^6 + ...)
= 1 + x + 4x^2 + 10x^3 + 26x^4 + 69x^5 + 184x^6 + ...
MATHEMATICA
nmax = 30; CoefficientList[Series[1/(1 - Sum[x^((4^k - 1)/3), {k, 0, nmax}]^3*x), {x, 0, nmax}], x] (* Vaclav Kotesovec, Oct 11 2020 *)
PROG
(PARI) a(n)=local(A, m); if(n<1, n==0, m=1; A=1+O(x); while(m<=3*n+3, m*=4; A=1/(1/subst(A, x, x^4)-x)); polcoeff(A, 3*n))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Aug 27 2003
STATUS
approved