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 A051252 Number of essentially different ways of arranging numbers 1 through 2n around a circle so that sum of each pair of adjacent numbers is prime. 26
 1, 1, 1, 2, 48, 512, 1440, 40512, 385072, 3154650, 106906168, 3197817022, 82924866213, 4025168862425, 127854811616691 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Jud McCranie reports that he was able to find a solution for each n <= 225 (2n <= 450) in just a few seconds. - Jul 05 2002 Is there a proof that this can always be done? The Mathematica program for this sequence uses backtracking to find all solutions for a given n. To verify that at least one solution exists for a given n, the backtracking function be made to stop when the first solution is found. Solutions have been found for n <= 48. - T. D. Noe, Jun 19 2002 This sequence is from the prime circle problem. There is no known proof that a(n) > 0 for all n. However, for many n (see A072618 and A072676), we can prove that a(n) > 0. Also, the sequence A072616 seems to imply that there are always solutions in which the odd (or even) numbers are in order around the circle. - T. D. Noe, Jul 01 2002 Prime circles can apparently be generated for any n using the Mathematica programs given in A072676 and A072184. - T. D. Noe, Jul 08 2002 The following seems to always produce a solution: Work around the circle starting with 1 but after that always choosing the largest remaining number that fits. For example, if n = 4 this gives 1, 6, 7, 4, 3, 8, 5, 2. See A088643 for a sequence on a related idea. - Paul Boddington, Oct 30 2007 See A228917 for a similar conjecture on twin primes. - Zhi-Wei Sun, Sep 08 2013 See A242527 for a similar problem on the set of numbers {0 through (n-1)}. - Stanislav Sykora, May 30 2014 James Tilley and Stan Wagon report that all terms up to n = 10^6 are nonzero. Charles R Greathouse IV, Feb 05 2016 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, second edition, Springer, 1994. See section C1. LINKS S. Sykora, On Neighbor-Property Cycles, Stan's Library, Volume V, 2014. Eric Weisstein's World of Mathematics, Prime Circle. EXAMPLE One arrangement for 2n=6 is 1,4,3,2,5,6 and this is essentially unique, so a(3)=1. MATHEMATICA \$RecursionLimit=500; try[lev_] := Module[{t, j}, If[lev>2n, (*then make sure the sum of the first and last is prime*) If[PrimeQ[soln[]+soln[[2n]]]&&soln[]<=soln[[2n]], (*Print[soln]; *) cnt++ ], (*else append another number to the soln list*) t=soln[[lev-1]]; For[j=1, j<=Length[s[[t]]], j++, If[ !MemberQ[soln, s[[t]][[j]]], soln[[lev]]=s[[t]][[j]]; try[lev+1]; soln[[lev]]=0]]]]; For[lst={}; n=1, n<=7, n++, s=Table[{}, {2n}]; For[i=1, i<=2n, i++, For[j=1, j<=2n, j++, If[i!=j&&PrimeQ[i+j], AppendTo[s[[i]], j]]]]; soln=Table[0, {2n}]; soln[]=1; cnt=0; try; AppendTo[lst, cnt]]; lst (* T. D. Noe *) PROG (C++) listed in the link (S. Sykora) CROSSREFS Cf. A000341, A070897, A072616, A072617, A072618, A072676, A072184, A103839, A227050, A228917, A242527, A242528. Sequence in context: A101362 A215186 A058090 * A231654 A005429 A035606 Adjacent sequences:  A051249 A051250 A051251 * A051253 A051254 A051255 KEYWORD nonn,nice AUTHOR EXTENSIONS a(14)-a(15) from Max Alekseyev, Sep 19 2013 STATUS approved

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Last modified August 3 10:49 EDT 2020. Contains 336198 sequences. (Running on oeis4.)