A325791 Number of necklace permutations of {1..n} such that every positive integer from 1 to n * (n + 1)/2 is the sum of some circular subsequence.

1, 1, 1, 2, 4, 20, 82, 252, 1074, 7912, 39552, 152680, 776094, 5550310, 30026848, 108376910

Offset

0,4

Comments

A necklace permutation is a permutation that is either empty or whose first part is the minimum. A circular subsequence is a sequence of consecutive terms where the last and first parts are also considered consecutive. The only circular subsequence of maximum length is the sequence itself, not any rotation of it. For example, the circular subsequences of (1,3,2) are: (), (1), (2), (3), (1,3), (2,1), (3,2), (1,3,2).

Links

Table of n, a(n) for n=0..15.

Example

The a(1) = 1 through a(5) = 20 permutations:
  (1)  (1,2)  (1,2,3)  (1,2,3,4)  (1,2,3,4,5)
              (1,3,2)  (1,3,2,4)  (1,2,3,5,4)
                       (1,4,2,3)  (1,2,4,3,5)
                       (1,4,3,2)  (1,2,4,5,3)
                                  (1,2,5,4,3)
                                  (1,3,2,5,4)
                                  (1,3,4,2,5)
                                  (1,3,4,5,2)
                                  (1,3,5,2,4)
                                  (1,3,5,4,2)
                                  (1,4,2,3,5)
                                  (1,4,2,5,3)
                                  (1,4,3,2,5)
                                  (1,4,5,2,3)
                                  (1,4,5,3,2)
                                  (1,5,2,3,4)
                                  (1,5,2,4,3)
                                  (1,5,3,2,4)
                                  (1,5,3,4,2)
                                  (1,5,4,3,2)

Mathematica

subalt[q_]:=Union[ReplaceList[q, {___, s__, ___}:>{s}], DeleteCases[ReplaceList[q, {t___, __, u___}:>{u, t}], {}]];
Table[Length[Select[Permutations[Range[n]], #=={}||First[#]==1&&Range[n*(n+1)/2]==Union[Total/@subalt[#]]&]], {n, 0, 5}]

Crossrefs

Cf. A002033, A008965, A032153, A103295, A126796, A304793, A325684, A325781, A325786, A325788, A325789, A325790.

Sequence in context: A009291 A081440 A204438 * A188326 A158094 A108879

Adjacent sequences: A325788 A325789 A325790 * A325792 A325793 A325794

Keyword

nonn,more

Author

Gus Wiseman, May 23 2019

Extensions

a(11)-a(15) from Bert Dobbelaere, Nov 01 2020

Status

approved