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 A072129 Number of distinct ways of arranging the squares {1,4,9,...,(2n)^2} in a circle so that the sum of each two adjacent entries is a prime. 1
 1, 0, 0, 0, 6, 0, 96, 272, 1408, 61622, 33736, 356606, 86520774, 192570133, 1560696233 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 LINKS Table of n, a(n) for n=1..15. EXAMPLE a(5)=6 because there are 6 essentially different ways: {1, 4, 9, 64, 49, 100, 81, 16, 25, 36}, {1, 4, 49, 64, 9, 100, 81, 16, 25, 36}, {1, 16, 81, 100, 9, 4, 49, 64, 25, 36}, {1, 16, 81, 100, 9, 64, 49, 4, 25, 36}, {1, 16, 81, 100, 49, 4, 9, 64, 25, 36} and {1, 16, 81, 100, 49, 64, 9, 4, 25, 36} 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[[1]]^2+soln[[2n]]^2]&&soln[[2]]<=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^2+j^2], AppendTo[s[[i]], j]]]]; soln=Table[0, {2n}]; soln[[1]]=1; cnt=0; try[2]; AppendTo[lst, cnt]]; lst CROSSREFS Cf. A051252, A073451. Sequence in context: A145223 A365979 A219948 * A085511 A187525 A187696 Adjacent sequences: A072126 A072127 A072128 * A072130 A072131 A072132 KEYWORD nonn,more,changed AUTHOR Santi Spadaro, Jun 25 2002 EXTENSIONS Corrected and extended by T. D. Noe, Jul 03 2002 a(13) corrected and a(14)-a(15) from Sean A. Irvine, Sep 03 2024 STATUS approved

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Last modified September 16 22:04 EDT 2024. Contains 375979 sequences. (Running on oeis4.)