Main diagonal of array A083857.

Terms

a(0) =0a(1) =1a(2) =3a(3) =10a(4) =39a(5) =171a(6) =819a(7) =4229a(8) =23247a(9) =135088a(10) =824163a(11) =5255361a(12) =34876647a(13) =240130801a(14) =1710323667a(15) =12572548582a(16) =95183606559a(17) =740843916471