OFFSET
1,7
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
a(n) is multiplicative with a(2^e) = a(5^e) = (-1)^e, a(p^e) = (1 + (-1)^e)/2 if p == 3, 17, 21, 27, 29, 31, 33, 39 (mod 40), a(p^e) = e+1 if p == 1, 9, 11, 19 (mod 40), a(p^e) = (-1)^e * (e+1) if p == 7, 13, 23, 37 (mod 40).
|a(n)| = A035180(n).
EXAMPLE
G.f. = q - q^2 + q^4 - q^5 - 2*q^7 - q^8 + q^9 + q^10 + 2*q^11 - 2*q^13 + ...
MATHEMATICA
f[x_, y_] := QPochhammer[-x, x*y]*QPochhammer[-y, x*y]*QPochhammer[x*y, x*y]; phi[x_] := f[x, x]; A163819[n_] := SeriesCoefficient[ (phi[q]*phi[q^10] - phi[q^2]*phi[q^5])/2, {q, 0, n}]; Table[A163819[n], {n, 1, 50}] (* G. C. Greubel, Aug 05 2017 *)
PROG
(PARI) {a(n) = if( n<1, 0, (qfrep([1, 0; 0, 10], n)[n] - qfrep([2, 0; 0, 5], n)[n]))};
(PARI) {a(n) = if( n<1, 0, sumdiv(n, d, kronecker(-10, d)) * if( n%5, kronecker(5, n), (-1)^(0 != sum(k=0, sqrtint(n \ 50), issquare( n/5 - 10*k^2 )))))};
CROSSREFS
KEYWORD
sign,mult
AUTHOR
Michael Somos, Aug 04 2009
STATUS
approved