Staircase on Pascal's triangle.

Terms

a(0) =1a(1) =3a(2) =6a(3) =20a(4) =35a(5) =126a(6) =210a(7) =792a(8) =1287a(9) =5005a(10) =8008a(11) =31824a(12) =50388a(13) =203490a(14) =319770a(15) =1307504a(16) =2042975a(17) =8436285a(18) =13123110a(19) =54627300a(20) =84672315a(21) =354817320a(22) =548354040a(23) =2310789600a(24) =3562467300a(25) =15084504396a(26) =23206929840a(27) =98672427616a(28) =151532656696