a(n) = [x^n] Product_{k=1..n} (1+x^k)^3 / x^(2*k).

Terms

a(0) =1a(1) =1a(2) =3a(3) =13a(4) =61a(5) =324a(6) =1800a(7) =10340a(8) =60969a(9) =366486a(10) =2237120a(11) =13829487a(12) =86394782a(13) =544547651a(14) =3458637273a(15) =22113504345a(16) =142212705879a(17) =919294844898