A242520 Number of cyclic arrangements of S={1,2,...,2n} such that the difference between any two neighbors is 3^k for some k=0,1,2,...

1, 1, 2, 3, 27, 165, 676, 3584, 19108, 80754, 386776, 1807342, 8218582, 114618650, 1410831012, 12144300991, 126350575684

Offset

1,3

Comments

a(n)=NPC(2n;S;P) is the count of all neighbor-property cycles for a specific set S of 2n elements and a specific pair-property P. For more details, see the link and A242519.

In this particular instance of NPC(n;S;P), all the terms with odd cycle lengths are necessarily zero.

Links

Table of n, a(n) for n=1..17.

S. Sykora, On Neighbor-Property Cycles, Stan's Library, Volume V, 2014.

Example

The two such cycles of length n=6 are:
C_1={1,2,3,6,5,4}, C_2={1,2,5,6,3,4}.
The first and last of the 27 such cycles of length n=10 are:
C_1={1,2,3,4,5,6,7,8,9,10}, C_27={1,4,7,8,5,2,3,6,9,10}.

Mathematica

A242520[n_] := Count[Map[lpf, Map[j1f, Permutations[Range[2, 2 n]]]], 0]/2;
j1f[x_] := Join[{1}, x, {1}];
lpf[x_] := Length[Select[Abs[Differences[x]], ! MemberQ[t, #] &]];
t = Table[3^k, {k, 0, 10}];
Join[{1}, Table[A242520[n], {n, 2, 5}]]
(* OR, a less simple, but more efficient implementation. *)
A242520[n_, perm_, remain_] := Module[{opt, lr, i, new},
   If[remain == {},
     If[MemberQ[t, Abs[First[perm] - Last[perm]]], ct++];
     Return[ct],
     opt = remain; lr = Length[remain];
     For[i = 1, i <= lr, i++,
      new = First[opt]; opt = Rest[opt];
      If[! MemberQ[t, Abs[Last[perm] - new]], Continue[]];
      A242520[n, Join[perm, {new}],
       Complement[Range[2, 2 n], perm, {new}]];
      ];
     Return[ct];
     ];
   ];
t = Table[3^k, {k, 0, 10}];
Join[{1}, Table[ct = 0; A242520[n, {1}, Range[2, 2 n]]/2, {n, 2, 8}]] (* Robert Price, Oct 22 2018 *)

Prog

(C++) See the link.

Crossrefs

Cf. A242519, A242521, A242522, A242523, A242524, A242525, A242526, A242527, A242528, A242529, A242530, A242531, A242532, A242533, A242534.

Sequence in context: A184506 A126203 A126655 * A132533 A059089 A098812

Adjacent sequences: A242517 A242518 A242519 * A242521 A242522 A242523

Keyword

nonn,hard,more

Author

Stanislav Sykora, May 27 2014

Extensions

a(14)-a(17) from Andrew Howroyd, Apr 05 2016

Status

approved