A161520 - OEIS

%I #5 Dec 04 2018 07:41:41

%S 1,0,-1,-1,0,0,0,0,1,2,0,-1,0,0,0,-1,0,0,-2,0,0,0,0,0,3,0,-1,0,0,2,0,

%T 0,0,0,0,-1,0,0,0,-2,0,0,-2,0,0,2,0,-1,1,0,0,0,0,0,0,0,2,2,0,0,0,0,0,

%U -1,0,0,-2,0,0,0,0,0,2,0,-3,-2,0,0,0,0,1,0,0,0,0,0,0,0,0,2,0,0,0,2,0,0,2,0,0

%N Expansion of Sum_{n >= 1} q^n(1-q)(1-q^2)...(1-q^(2n-1))/((1+q^2)(1+q^4)...(1+q^(2n))).

%C a(n) is -(-1)^(n(n+1)/2) times the number of inequivalent elements of norm n in Z[sqrt{6}].

%H Jeremy Lovejoy, <a href="https://doi.org/10.1016/j.jnt.2003.12.014">Overpartitions and real quadratic fields</a>, J. Number Theory, 106 (2004), 178-186.

%K sign

%O 1,10

%A _Jeremy Lovejoy_, Jun 12 2009