Inversion of A000257.

Terms

a(0) =1a(1) =2a(2) =6a(3) =23a(4) =103a(5) =512a(6) =2740a(7) =15485a(8) =91245a(9) =555662a(10) =3475090a(11) =22214707a(12) =144640291a(13) =956560748a(14) =6411521056a(15) =43478151737a(16) =297864793993