OFFSET
0,2
COMMENTS
The cube-root of g.f. A(x) is an integer series (A112281).
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
FORMULA
EXAMPLE
A(x) = 1 + 6*x + 5*x^3 + 2*x^6 + 0*x^10 + 7*x^15 + 4*x^21 +... = (1 - 3*x + 5*x^3 - 7*x^6 + 9*x^10 - 11*x^15 +...) (mod 9).
A(x)^(1/3) = 1 + 2*x - 4*x^2 + 15*x^3 - 60*x^4 + 268*x^5 -+...
Notation: q-series (q;q)_oo = Product_{n>=1} (1-q^n) = 1 + Sum_{n>=1} (-1)^n*[q^(n*(3*n-1)/2) + q^(n*(3*n+1)/2)].
MAPLE
seq(coeff(series( add(`mod`((-1)^n*(2*n+1), 9)*x^(n*(n+1)/2), n = 0 .. 140), x, n+1), x, n), n = 0 .. 120); # G. C. Greubel, Nov 05 2019
MATHEMATICA
CoefficientList[Series[ Sum[Mod[(-1)^n*(2*n+1), 9]* x^(n(n+1)/2), {n, 0, 140}] , {x, 0, 120}], x] (* G. C. Greubel, Nov 05 2019 *)
PROG
(PARI) {a(n)=polcoeff(sum(k=0, sqrtint(2*n+1), (((-1)^k*(2*k+1))%9)*x^(k*(k+1)/2)+x*O(x^n)), n)}
(Magma) R<x>:=PowerSeriesRing(Integers(), 120); Coefficients(R!( (&+[((-1)^n*(2*n+1) mod 9)*x^Binomial(n+1, 2): n in [0..140]]) )); // G. C. Greubel, Nov 05 2019
(Sage) [ (sum(((-1)^n*(2*n+1)%9) *x^(n*(n+1)/2) for n in (0..140)) ).series(x, n+1).list()[n] for n in (0..120)] # G. C. Greubel, Nov 05 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Sep 01 2005
STATUS
approved