OFFSET
0,4
COMMENTS
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Michael Somos, Introduction to Ramanujan theta functions
N. J. A. Sloane and B. K. Teo, Theta series and magic numbers for close-packed spherical clusters, J. Chem. Phys. 83 (1985) 6520-6534.
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of 4 * q^3 * psi^3(q^8) + (phi^3(q^4) - phi^3(-q^4)) / 2 in powers of q where phi(), psi() are Ramanujan theta functions. - Michael Somos, Aug 17 2009
a(8*n + 0) = a(8*n + 1) = a(8*n + 2) = a(8*n + 5) = a(8*n + 6) = a(8*n + 7) = 0. - Michael Somos, Aug 17 2009
EXAMPLE
4*q^3 + 6*q^4 + 12*q^11 + 8*q^12 + 12*q^19 + 24*q^20 + 16*q^27 + ... - Michael Somos, Aug 17 2009
MATHEMATICA
a[n_]:= SeriesCoefficient[4*q^3*QPochhammer[-q^8, q^8]^3* QPochhammer[q^16, q^16]^3 + (EllipticTheta[3, 0, q^4]^3 - EllipticTheta[3, 0, -q^4]^3)/2, {q, 0, n}]; Table[a[n], {n, 0, 50}] (* G. C. Greubel, Apr 01 2018 *)
PROG
(PARI) {a(n) = if( n<0, 0, if( n%8 == 3, n \= 8; polcoeff( 4 * sum(k=0, (sqrtint(8*n+1)-1)\2, x^((k^2+k)/2), x*O(x^n))^3, n), if( n%8 == 4, n /= 4; polcoeff( sum(k=1, sqrtint(n), 2*x^k^2, 1 + x*O(x^n))^3, n), 0 )))} /* Michael Somos, Aug 17 2009 */
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved