18668849760domain: NAppears in sequencesa(n) = (2n+3)!/(n!*(n+2)!).at n=15A000917Number of valleys in all left factors of Dyck paths of length n. A valley is a (1,-1)-step followed by a (1,1)-step.at n=34A191522a(n) = (n!*m)/(m!*(m+1)!) where m = floor(n/2).at n=33A237884