Coefficient of x^(2*n) in (-1 + Product_{k>=1} 1 / (1 + x^k))^n.

Terms

a(0) =1a(1) =0a(2) =2a(3) =3a(4) =10a(5) =25a(6) =71a(7) =203a(8) =562a(9) =1650a(10) =4667a(11) =13673a(12) =39427a(13) =115440a(14) =336639a(15) =987628a(16) =2898658a(17) =8529257a(18) =25134200a(19) =74173606a(20) =219207815a(21) =648546314a(22) =1921045953a(23) =5695642513a(24) =16902924883a(25) =50203798050a(26) =149229323544a(27) =443895849894