a(n) = A000041(n)*A000070(n-1), n >= 1.

Terms

a(0) =1a(1) =4a(2) =12a(3) =35a(4) =84a(5) =209a(6) =450a(7) =990a(8) =2010a(9) =4074a(10) =7784a(11) =15015a(12) =27472a(13) =50355a(14) =89408a(15) =158004a(16) =271755a(17) =466620a(18) =782530a(19) =1308549a(20) =2149488a(21) =3513012a(22) =5657540a(23) =9076725a(24) =14367804a(25) =22645056a(26) =35313320a(27) =54810756a(28) =84269900a(29) =129032100