

A227050


Number of essentially different ways of arranging numbers 1 through 2n around a circle so that the sum and absolute difference of each pair of adjacent numbers are prime.


5



0, 0, 0, 0, 0, 2, 1, 4, 88, 0, 976, 22277, 22365, 376002, 3172018, 5821944, 10222624, 424452210, 6129894510
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OFFSET

1,6


COMMENTS

See a similar problem, but for the set of numbers {0 through (n1)}.  Stanislav Sykora, May 30 2014


LINKS

Table of n, a(n) for n=1..19.
Gary Antonick, Numberplay: Bernardo Recamán’s Primes in a Circle Puzzle, Jun 17 2013
Stanislav Sykora, On NeighborProperty Cycles, Stan's Library, Volume V, 2014; Table III.


EXAMPLE

For n = 6 the a(6) = 2 solutions are (1, 4, 9, 2, 5, 12, 7, 10, 3, 8, 11, 6) and (1, 6, 11, 8, 3, 10, 7, 4, 9, 2, 5, 12) because abs(1  4) = 3 and 1 + 4 = 5 are prime, etc.


MATHEMATICA

A227050[n_] :=
Count[Map[lpf, Map[j1f, Permutations[Range[2, 2 n]]]], 0]/2;
j1f[x_] := Join[{1}, x, {1}];
lpf[x_] := Length[
Join[Select[asf[x], ! PrimeQ[#] &],
Select[Differences[x], ! PrimeQ[#] &]]];
asf[x_] := Module[{i}, Table[x[[i]] + x[[i + 1]], {i, Length[x]  1}]];
Table[A227050[n], {n, 1, 6}]
(* OR, a less simple, but more efficient implementation. *)
A227050[n_, perm_, remain_] := Module[{opt, lr, i, new},
If[remain == {},
If[PrimeQ[First[perm]  Last[perm]] &&
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] && PrimeQ[Last[perm] + new]),
Continue[]];
A227050[n, Join[perm, {new}],
Complement[Range[2 n], perm, {new}]];
];
Return[ct];
];
];
Table[ct = 0; A227050[n, {1}, Range[2, 2 n]]/2, {n, 1, 10}]
(* Robert Price, Oct 22 2018 *)


PROG

(C++) // Listed in the Sykora link.


CROSSREFS

Cf. similar sequences: A051252 (with sums of neighbors prime), A242527 (with sums of neighbors prime), A228626 (with differences of neighbors prime), A242528 (with sums and differences of neighbors prime).
Sequence in context: A061655 A009830 A053374 * A093876 A322334 A198371
Adjacent sequences: A227047 A227048 A227049 * A227051 A227052 A227053


KEYWORD

nonn,more,hard


AUTHOR

Tim Cieplowski, Jun 29 2013


EXTENSIONS

a(15)a(18) added by Tim Cieplowski, Jan 04 2015
a(19) from Fausto A. C. Cariboni, Jun 06 2017


STATUS

approved



