OFFSET
0,6
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Euler transform of period 42 sequence [ 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, ...].
G.f.: Product_{k>0} (1 + x^k) * (1 + x^(21*k)) / ((1 + x^(3*k)) * (1 + x^(7*k))) = Product_{k>0} P42(x^k) where P42 is the 42nd cyclotomic polynomial.
Expansion of chi(-q^3) * chi(-q^7) / (chi(-q) * chi(-q^21)) in powers of q where chi() is a Ramanujan theta function.
Expansion of q^(-1/2) * eta(q^2) * eta(q^3) * eta(q^7) * eta(q^42) / (eta(q) * eta(q^6) * eta(q^14) * eta(q^21)) in powers of q.
Given g.f. A(x), then B(x) = x * A(x^2) satisfies 0 = f(B(x), B(x^2), B(x^4)) where f(u, v, w) = (1 + v) * (v^2 - u^2*w^2) - (v - v^2) * (u^2 + w^2).
G.f. is a period 1 Fourier series which satisfies f(-1 / (168 t)) = 1 / f(t) where q = exp(2 Pi i t).
EXAMPLE
1 + x + x^2 + x^3 + x^4 + 2*x^5 + 2*x^6 + 2*x^7 + 2*x^8 + 2*x^9 + ...
q + q^3 + q^5 + q^7 + q^9 + 2*q^11 + 2*q^13 + 2*q^15 + 2*q^17 + 2*q^19 + ...
a(10) = 3 since 5 + 5 = 5 + 1 + 1 + 1 + 1 + 1 = 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 in 3 ways.
MATHEMATICA
eta[q_]:= q^(1/24)*QPochhammer[q]; CoefficientList[Series[q^(-1/2)* eta[q^2]* eta[q^3]*eta[q^7]*eta[q^42]/(eta[q]*eta[q^6]*eta[q^14] *eta[q^21]), {q, 0, 50}], q] (* G. C. Greubel, Apr 18 2018 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A) * eta(x^3 + A) * eta(x^7 + A) * eta(x^42 + A) / (eta(x + A) * eta(x^6 + A) * eta(x^14 + A) * eta(x^21 + A)), n))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael Somos, Jun 26 2005, Jan 12 2009
STATUS
approved