Triangle of ballot numbers.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =2a(8) =2a(9) =1a(10) =1a(11) =3a(12) =4a(13) =4a(14) =1a(15) =1a(16) =4a(17) =6a(18) =9a(19) =9a(20) =1a(21) =1a(22) =5a(23) =8a(24) =15a(25) =21a(26) =21a(27) =1a(28) =1a(29) =6