Expansion of solution of functional equation.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =21a(5) =79a(6) =312a(7) =1277a(8) =5369a(9) =23049a(10) =100612a(11) =445214a(12) =1992606a(13) =9004260a(14) =41025315a(15) =188259072a(16) =869305315a(17) =4036286518a(18) =18832973733a(19) =88259024068a(20) =415252542641