Number of trails between opposite vertices in a triangle strip.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =23a(6) =62a(7) =174a(8) =497a(9) =1433a(10) =4150a(11) =12044a(12) =34989a(13) =101695a(14) =295642a(15) =859566a(16) =2499277a(17) =7267081a(18) =21130538a(19) =61441732a(20) =178655937a(21) =519483767a(22) =1510520966a(23) =4392195390a(24) =12771343961a(25) =37135696841