The number of permutations p of {1,...,n} such that p(1)=1, p(n)=n, and |p(i)-p(i+1)| is in {1,2,3} for all i from 1 to n-1.

A249665

The number of permutations p of {1,...,n} such that p(1)=1, p(n)=n, and |p(i)-p(i+1)| is in {1,2,3} for all i from 1 to n-1.

Terms

    a(0) =1a(1) =1a(2) =1a(3) =2a(4) =6a(5) =14a(6) =28a(7) =56a(8) =118a(9) =254a(10) =541a(11) =1140a(12) =2401a(13) =5074a(14) =10738a(15) =22711a(16) =48001a(17) =101447a(18) =214446a(19) =453355a(20) =958395a(21) =2025963a(22) =4282685a(23) =9053286a(24) =19138115a(25) =40456779a(26) =85522862a(27) =180789396a(28) =382176531a(29) =807895636

External references