a(n) is the number of arches for semi-meanders with n top arches that have exactly two covering arches.

A322507

a(n) is the number of arches for semi-meanders with n top arches that have exactly two covering arches.

Terms

    a(0) =0a(1) =0a(2) =0a(3) =2a(4) =6a(5) =28a(6) =82a(7) =310a(8) =918a(9) =3246a(10) =9762a(11) =33586a(12) =102490a(13) =348220a(14) =1076238a(15) =3635050a(16) =11358178a(17) =38261430a(18) =120676406a(19) =406137648a(20) =1291309912a(21) =4345989212a(22) =13914717064a(23) =46856195890

External references