A simple context-free grammar: convolution cube of A001002.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =12a(6) =49a(7) =210a(8) =927a(9) =4191a(10) =19305a(11) =90285a(12) =427570a(13) =2046324a(14) =9881862a(15) =48090824a(16) =235619133a(17) =1161257580a(18) =5753365015a(19) =28638093270a(20) =143148398085a(21) =718242481770