Number of subwords of the form DDDD in nondecreasing Dyck paths of length 2n.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =7a(6) =33a(7) =131a(8) =473a(9) =1608a(10) =5242a(11) =16567a(12) =51123a(13) =154793a(14) =461525a(15) =1358646a(16) =3957088a(17) =11420995a(18) =32707809a(19) =93040751a(20) =263113505a(21) =740238852a(22) =2073098086a(23) =5782387855a(24) =16070206191a(25) =44516728277a(26) =122956408493a(27) =338707969266a(28) =930787894348