OFFSET
0,2
COMMENTS
REFERENCES
Bruce C. Berndt, Ramanujan's Notebooks Part III, Springer-Verlag, 1991, see p. 249, Entry 8(iv).
LINKS
Michael Somos, Introduction to Ramanujan theta functions, 2019.
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions.
FORMULA
G.f.: 1 + 5*( Sum_{k>0} (k*x^k / (1 + x^k)) * Kronecker(5, k) ).
a(n) = 5 * A113260(n) if n>0.
Sum_{k=1..n} abs(a(k)) ~ c * n^2, where c = Pi^2/(2*sqrt(5)) = 2.2069106... . - Amiram Eldar, Jan 28 2024
EXAMPLE
G.f. = 1 + 5*x - 15*x^2 - 10*x^3 + 25*x^4 + 5*x^5 + 30*x^6 - 30*x^7 - 55*x^8 + ...
MATHEMATICA
a[ n_] := If[ n < 1, Boole[n == 0], -5 DivisorSum[ n, # KroneckerSymbol[ 5, #] (-1)^(n/#) &]]; (* Michael Somos, Sep 07 2018 *)
PROG
(PARI) {a(n) = if( n<1, n==0, -5 * sumdiv(n, d, d * kronecker(5, d) * (-1)^(n/d)))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 20 2005
STATUS
approved