Number of skew Dyck paths of semilength n ending with a left step.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =15a(5) =58a(6) =232a(7) =954a(8) =4010a(9) =17156a(10) =74469a(11) =327168a(12) =1452075a(13) =6501156a(14) =29326743a(15) =133166064a(16) =608188737a(17) =2791992736a(18) =12876049123a(19) =59626721244a(20) =277150709717