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 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,... 17
 1, 1, 2, 3, 27, 165, 676, 3584, 19108, 80754, 386776, 1807342, 8218582, 114618650, 1410831012, 12144300991, 126350575684 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified May 27 15:55 EDT 2024. Contains 372880 sequences. (Running on oeis4.)