Partial sums of A000272.

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =22a(5) =147a(6) =1443a(7) =18250a(8) =280394a(9) =5063363a(10) =105063363a(11) =2463011054a(12) =64380375278