Number of 2-alternating permutations of 1,2,...,n, that is, a(n) is the number of down/up permutations (A000111) of 1,2,...,n such that any two consecutive terms differ by at least two.
A245377
Number of 2-alternating permutations of 1,2,...,n, that is, a(n) is the number of down/up permutations (A000111) of 1,2,...,n such that any two consecutive terms differ by at least two.
Terms
- a(0) =1a(1) =1a(2) =0a(3) =0a(4) =1a(5) =4a(6) =17a(7) =80a(8) =422a(9) =2480a(10) =16095a(11) =114432a(12) =884969a(13) =7398464a(14) =66502048a(15) =639653632a(16) =6556170841a(17) =71340409600a(18) =821408397105
External references
- oeis: A245377