a(n) = A004061(n) - 1.

Terms

a(0) =2a(1) =6a(2) =10a(3) =12a(4) =46a(5) =126a(6) =148a(7) =180a(8) =618a(9) =928a(10) =3406a(11) =10948a(12) =13240a(13) =13872a(14) =16518a(15) =201358a(16) =396412a(17) =1888278a(18) =3300592