G.f.: Sum((1-x)^(2*l)*Sum(x^((2*l-1)*k)/(1-2*x+x^k)^(2*l),k=1..infinity),l=1..infinity).

Terms

a(0) =1a(1) =1a(2) =4a(3) =6a(4) =13a(5) =24a(6) =51a(7) =101a(8) =205a(9) =407a(10) =814a(11) =1624a(12) =3248a(13) =6490a(14) =12979a(15) =25950a(16) =51898a(17) =103798a(18) =207619a(19) =415288a(20) =830690a(21) =1661590a(22) =3323566a(23) =6647779a(24) =13296602a(25) =26594769a(26) =53191708a(27) =106386020a(28) =212774300a(29) =425548246