a(n) is the total number of top arches with exactly one covering arch for semi-meanders with n top arches.

A301620

a(n) is the total number of top arches with exactly one covering arch for semi-meanders with n top arches.

Terms

    a(0) =0a(1) =0a(2) =2a(3) =4a(4) =18a(5) =42a(6) =156a(7) =398a(8) =1398a(9) =3778a(10) =12982a(11) =36522a(12) =124290a(13) =360182a(14) =1220440a(15) =3618090a(16) =12237698a(17) =36938158a(18) =124880222a(19) =382471606a(20) =1293363816a(21) =4009185912a(22) =13565790984a(23) =42478788432a(24) =143851766298a(25) =454339269482

External references