a(n) = [x^n] Product_{k>=1} ((1 + x^k)/(1 + x^(5*k)))^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =13a(4) =51a(5) =201a(6) =819a(7) =3389a(8) =14131a(9) =59341a(10) =250703a(11) =1064207a(12) =4535091a(13) =19390229a(14) =83139955a(15) =357354213a(16) =1539272499a(17) =6642769925a(18) =28714955571a(19) =124312591469a(20) =538895612751