G.f.: x^2*(3+3*x+x^2) / ( (2*x+1) * (1+x) * (1+x+x^2) * (x^2-x+1) ) .

Terms

a(0) =0a(1) =0a(2) =3a(3) =-6a(4) =10a(5) =-21a(6) =42a(7) =-84a(8) =171a(9) =-342a(10) =682a(11) =-1365a(12) =2730a(13) =-5460a(14) =10923a(15) =-21846a(16) =43690a(17) =-87381a(18) =174762a(19) =-349524a(20) =699051a(21) =-1398102a(22) =2796202a(23) =-5592405a(24) =11184810a(25) =-22369620a(26) =44739243a(27) =-89478486a(28) =178956970a(29) =-357913941