Coefficient of x^n in Product_{k>=1} (1+x^k)^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =13a(4) =51a(5) =206a(6) =855a(7) =3585a(8) =15155a(9) =64525a(10) =276278a(11) =1188353a(12) =5130999a(13) =22226049a(14) =96544003a(15) =420368858a(16) =1834203955a(17) =8018057345a(18) =35107961175a(19) =153950675585a(20) =675978772326