Number of permutations p of [n] such that p(p(i)) = i for all i or p(n+1-p(i)) = n+1-i for all i.
A297708
Number of permutations p of [n] such that p(p(i)) = i for all i or p(n+1-p(i)) = n+1-i for all i.
Terms
- a(0) =1a(1) =1a(2) =2a(3) =6a(4) =14a(5) =46a(6) =132a(7) =444a(8) =1452a(9) =5164a(10) =18680a(11) =71080a(12) =278920a(13) =1135624a(14) =4774448a(15) =20692560a(16) =92381072a(17) =423566224a(18) =1994458656a(19) =9619233888a(20) =47516407008a(21) =239904464608
External references
- oeis: A297708