OFFSET
0,4
COMMENTS
Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700).
Case k=5,i=3 of Gordon/Goellnitz/Andrews Theorem.
Number of partitions in which no odd part is repeated, with at most 2 parts of size less than or equal to 2 and where differences between parts at distance 4 are greater than 1 when the smaller part is odd and greater than 2 when the smaller part is even.
REFERENCES
G. E. Andrews, The Theory of Partitions, Addison-Wesley, 1976, p. 114.
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
Expansion of psi(-x^5) / psi(-x) in powers of x where psi() is a Ramanujan theta function. - Michael Somos, Oct 27 2008
Expansion of q^(-1/2) * eta(q^2) * eta(q^5) * eta(q^20) / (eta(q) * eta(q^4) * eta(q^10)) in powers of q. - Michael Somos, Oct 27 2008
Euler transform of period 20 sequence [ 1, 0, 1, 1, 0, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, ...]. - Michael Somos, Oct 27 2008
a(n) ~ exp(Pi*sqrt(2*n/5)) / (2^(5/4) * 5^(3/4) * n^(3/4)). - Vaclav Kotesovec, May 10 2018
EXAMPLE
1 + x + x^2 + 2*x^3 + 3*x^4 + 3*x^5 + 4*x^6 + 6*x^7 + 8*x^8 + 10*x^9 + ...
q + q^3 + q^5 + 2*q^7 + 3*q^9 + 3*q^11 + 4*q^13 + 6*q^15 + 8*q^17 + ...
MATHEMATICA
a[n_] := SeriesCoefficient[EllipticTheta[2, 0, I*q^(5/2)]/( q^(1/2) * EllipticTheta[2, 0, I*q^(1/2)]), {q, 0, n}]; Table[a[n], {n, 0, 50}] (* G. C. Greubel, Apr 16 2017 *)
nmax = 60; CoefficientList[Series[Product[(1 - x^(4*k - 2))*(1 - x^(20*k))*(1 - x^(20*k+5-20))*(1 - x^(20*k- 5))/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, May 10 2018 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A) * eta(x^5 + A) * eta(x^20 + A) / (eta(x + A) * eta(x^4 + A) * eta(x^10 + A)), n))} /* Michael Somos, Oct 27 2008 */
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved