Catafusenes (see reference for precise definition).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =12a(6) =56a(7) =282a(8) =1365a(9) =6666a(10) =32002a(11) =153350a(12) =731156a(13) =3480594a(14) =16546884a(15) =78644579a(16) =373813930a(17) =1777680136a(18) =8459543997a(19) =40290717546a(20) =192072529566a(21) =916541991433