G.f.: x^((k^2+k)/2)/(mul(1-x^i,i=1..k)*mul(1+x^r,r=1..oo)) with k = 3.

Terms

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