a(n) is the number of permutations w of 1,2,...,n such that both w and w^{-1} are alternating.
A007999
a(n) is the number of permutations w of 1,2,...,n such that both w and w^{-1} are alternating.
Terms
- a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =3a(6) =8a(7) =19a(8) =64a(9) =213a(10) =880a(11) =3717a(12) =18288a(13) =92935a(14) =531440a(15) =3147495a(16) =20525168a(17) =138638825a(18) =1015694832a(19) =7700244745a(20) =62623847536a(21) =526317901451
External references
- oeis: A007999