a(n) = [x^n] Product_{k>=1} ((1 + x^k)/(1 + x^(4*k)))^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =13a(4) =47a(5) =181a(6) =729a(7) =2948a(8) =12031a(9) =49540a(10) =205153a(11) =853546a(12) =3565505a(13) =14943839a(14) =62810786a(15) =264650683a(16) =1117486463a(17) =4727486583a(18) =20032950744a(19) =85017558081a(20) =361289789377