OFFSET
0,13
COMMENTS
H(n) is usually called the Hurwitz class number.
a(n) = 0 if n = 1 or 2 (mod 4).
REFERENCES
D. Zagier, The Eichler-Selberg Trace Formula on SL_2(Z), Appendix to S. Lang, Introduction to Modular Forms, Springer, 1976.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 0..10000
N. Lygeros, O. Rozier, A new solution to the equation tau(rho) == 0 (mod p), J. Int. Seq. 13 (2010) # 10.7.4.
Wikipedia, Hurwitz class number.
FORMULA
H(n) = A259825(n) / 12. - Michael Somos, Jul 05 2015
EXAMPLE
-1/12, 0, 0, 1/3, 1/2, 0, 0, 1, 1, ...
MATHEMATICA
terms = 100; gf[m_] := With[{r = Range[-m, m]}, -2 Sum[(-1)^k*x^(k^2 + k)/(1 + (-x)^k)^2, {k, r}]/EllipticTheta[3, 0, x] - 2 Sum[(-1)^k*x^(k^2 + 2 k)/(1 + x^(2k))^2, {k, r}]/EllipticTheta[3, 0, -x]]; CoefficientList[ gf[terms // Sqrt // Ceiling] + O[x]^terms, x]/12 // Numerator (* Jean-François Alcover, Apr 02 2017, after Michael Somos *)
PROG
(PARI) H(n)=sumdiv(core(n, 1)[2], d, my(D=-n/d^2); if(D%4<2, qfbclassno(D)/max(1, D+6)))
a(n)=if(n, numerator(H(n)), -1) \\ Charles R Greathouse IV, Apr 25 2013
(PARI) {a(n) = numerator( qfbhclassno( n))}; /* Michael Somos, Jul 06 2015 */
CROSSREFS
KEYWORD
sign,frac
AUTHOR
N. J. A. Sloane, Dec 09 2000
STATUS
approved