G.f.: x^((k^2+k)/2)/(mul(1-x^i,i=1..k)*mul(1+x^r,r=1..oo)) with k = 4.

Terms

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