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A073467 a(n) is the number of essentially different ways in which the integers 1,2,3,...,2n can be arranged in a circle such that (1) all pairs of adjacent integers sum to a prime number and (2) all pairs of integers opposite each other on the circle sum to a prime.

%I A073467 #11 Jun 24 2019 03:37:24
%S A073467 1,0,0,0,4,0,8,0,556,0,16156,0,4545745,0,1697587998,0
%N A073467 a(n) is the number of essentially different ways in which the integers 1,2,3,...,2n can be arranged in a circle such that (1) all pairs of adjacent integers sum to a prime number and (2) all pairs of integers opposite each other on the circle sum to a prime.
%C A073467 Note that a(n) = 0 for all even n because opposite numbers sum to an even number. The Mathematica program uses a backtracking algorithm to count the arrangements. To print the unique arrangements, remove the comments from around the print statement.
%H A073467 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_176.htm">Puzzle 176: Primes in a Circle</a>
%e A073467 a(5)=4 because there are four essential different arrangements: {1,2,3,4,7,6,5,8,9,10}, {1,2,3,10,7,6,5,8,9,4}, {1,2,9,4,7,6,5,8,3,10} and {1,2,9,10,7,6,5,8,3,4}.
%t A073467 maxN=9; $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]]+soln[[2n]]]&&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]]], If[lev<=n||MemberQ[s[[soln[[lev-n]]]], s[[t]][[j]]], soln[[lev]]=s[[t]][[j]]; try[lev+1]; soln[[lev]]=0]]]]]; For[lst={}; n=1, n<=maxN, 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]]]]; Delete[s[[1]], -1]; (* these will all be duplicates *) soln=Table[0, {2n}]; soln[[1]]=1; cnt=0; try[2]; AppendTo[lst, cnt]]; lst
%Y A073467 Cf. A051252.
%K A073467 hard,more,nice,nonn
%O A073467 1,5
%A A073467 _T. D. Noe_, Aug 02 2002
%E A073467 a(14)-a(16) from _Bert Dobbelaere_, Jun 24 2019