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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =7a(5) =26a(6) =105a(7) =444a(8) =1944a(9) =8734a(10) =40040a(11) =186550a(12) =880750a(13) =4204508a(14) =20260498a(15) =98419392a(16) =481442805a(17) =2369551218a(18) =11725590555a(19) =58303117680a(20) =291151523355