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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =0a(9) =1a(10) =-1a(11) =2a(12) =-3a(13) =5a(14) =-7a(15) =11a(16) =-15a(17) =21a(18) =-29a(19) =39a(20) =-52a(21) =69a(22) =-90a(23) =116a(24) =-150a(25) =190a(26) =-241a(27) =303a(28) =-379a(29) =470