A simple context-free grammar: convolution square of A049140.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =5a(5) =16a(6) =56a(7) =204a(8) =768a(9) =2970a(10) =11726a(11) =47060a(12) =191412a(13) =787304a(14) =3269100a(15) =13684864a(16) =57691353a(17) =244713654a(18) =1043684478a(19) =4472828400a(20) =19252045120a(21) =83188965420a(22) =360734837280