a(n) is the position of A138534(n) in A025487.

Terms

a(0) =1a(1) =2a(2) =6a(3) =17a(4) =67a(5) =166a(6) =676a(7) =1373a(8) =4475a(9) =10446a(10) =30036a(11) =51032a(12) =196386a(13) =315302a(14) =737515a(15) =1654229a(16) =4227565a(17) =6301902a(18) =17975187a(19) =26010425a(20) =70085244a(21) =133337963