G.f.: x^((k^2 + k)/2) / (Product_{i=1..k} (1 - x^i) * Product_{r>=1} (1 + x^r)) with k = 2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =0a(5) =1a(6) =-1a(7) =1a(8) =-2a(9) =1a(10) =-3a(11) =2a(12) =-4a(13) =3a(14) =-5a(15) =5a(16) =-6a(17) =7a(18) =-8a(19) =10a(20) =-10a(21) =13a(22) =-13a(23) =17a(24) =-17a(25) =21a(26) =-22a(27) =27a(28) =-28a(29) =33