Number of ballot sequences of length n having exactly two descents.

Terms

a(0) =3a(1) =21a(2) =101a(3) =373a(4) =1174a(5) =3337a(6) =8642a(7) =21148a(8) =48713a(9) =108147a(10) =229984a(11) =476430a(12) =955667a(13) =1879169a(14) =3603751a(15) =6801106a(16) =12584048a(17) =22974152a(18) =41268291a(19) =73301654a(20) =128441712a(21) =222871187a(22) =382322022a(23) =650209758a(24) =1094918922a(25) =1829777544a(26) =3031447986a(27) =4987910871a(28) =8144753275