Catafusenes (see reference for precise definition).

Terms

a(0) =1a(1) =4a(2) =18a(3) =80a(4) =355a(5) =1580a(6) =7066a(7) =31772a(8) =143645a(9) =652860a(10) =2981910a(11) =13682328a(12) =63046776a(13) =291646860a(14) =1353967250a(15) =6306552800a(16) =29464361530a(17) =138045441260a(18) =648449195350