a(n) = A002720(n) - 1.

Terms

a(0) =0a(1) =1a(2) =6a(3) =33a(4) =208a(5) =1545a(6) =13326a(7) =130921a(8) =1441728a(9) =17572113a(10) =234662230a(11) =3405357681a(12) =53334454416a(13) =896324308633