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 A242527 Number of cyclic arrangements (up to direction) of {0,1,...,n-1} such that the sum of any two neighbors is a prime. 19
 0, 0, 0, 0, 1, 1, 2, 6, 6, 22, 80, 504, 840, 6048, 3888, 37524, 72976, 961776, 661016, 11533030, 7544366, 133552142, 208815294, 5469236592, 6429567323, 153819905698, 182409170334, 4874589558919, 7508950009102, 209534365631599 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,7 COMMENTS a(n)=NPC(n;S;P) is the count of all neighbor-property cycles for a specific set S={0,1,...,n-1} of n elements and a specific pair-property P. For more details, see the link and A242519. For the same pair-property P but the set {1 through n}, see A051252. Using for pair-property the difference, rather than the sum, one obtains A228626. LINKS S. Sykora, On Neighbor-Property Cycles, Stan's Library, Volume V, 2014. EXAMPLE The first such cycle is of length n=5: {0,2,1,4,3}. The first case with 2 solutions is for cycle length n=7: C_1={0,2,3,4,1,6,5}, C_2={0,2,5,6,1,4,3}. The first and the last of the 22 such cycles of length n=10 are: C_1={0,3,2,1,4,9,8,5,6,7}, C_22={0,5,8,9,4,3,2,1,6,7}. MATHEMATICA A242527[n_] := Count[Map[lpf, Map[j0f, Permutations[Range[n - 1]]]], 0]/2; j0f[x_] := Join[{0}, x, {0}]; lpf[x_] := Length[Select[asf[x], ! PrimeQ[#] &]]; asf[x_] := Module[{i}, Table[x[[i]] + x[[i + 1]], {i, Length[x] - 1}]]; Table[A242527[n], {n, 1, 10}] (* OR, a less simple, but more efficient implementation. *) A242527[n_, perm_, remain_] := Module[{opt, lr, i, new},    If[remain == {},      If[PrimeQ[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[! PrimeQ[Last[perm] + new], Continue[]];       A242527[n, Join[perm, {new}],        Complement[Range[n - 1], perm, {new}]];       ];      Return[ct];      ];    ]; Table[ct = 0; A242527[n, {0}, Range[n - 1]]/2, {n, 1, 15}] (* Robert Price, Oct 18 2018 *) PROG (C++) See the link. CROSSREFS Cf. A051252, A228626, A242519, A242520, A242521, A242522, A242523, A242524, A242525, A242526, A242528, A242529, A242530, A242531, A242532, A242533, A242534. Sequence in context: A320916 A119551 A100634 * A304680 A325803 A130865 Adjacent sequences:  A242524 A242525 A242526 * A242528 A242529 A242530 KEYWORD nonn,hard AUTHOR Stanislav Sykora, May 30 2014 EXTENSIONS a(23)-a(30) from Max Alekseyev, Jul 09 2014 STATUS approved

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Last modified October 21 14:41 EDT 2019. Contains 328301 sequences. (Running on oeis4.)