Left-hand border of triangle A046937.

Terms

a(0) =1a(1) =2a(2) =3a(3) =8a(4) =24a(5) =83a(6) =324a(7) =1400a(8) =6609a(9) =33758a(10) =185136a(11) =1083233a(12) =6726366a(13) =44130128a(14) =304741623a(15) =2207682188a(16) =16729947276a(17) =132281116715