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A077764
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Number of ways of pairing the even squares of the numbers 1 to n with the odd squares of the numbers n+1 to 2n such that each pair sums to a prime. a(1) is defined to be 1.
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2
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1, 1, 1, 1, 1, 1, 1, 1, 2, 4, 4, 8, 6, 14, 14, 44, 22, 30, 12, 41, 137, 667, 401, 517, 149, 286, 306, 1312
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,9
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COMMENTS
| It appears that a pairing is always possible. The Mathematica program uses backtracking to find all solutions. The Print statement can be uncommented to print all solutions. The product of this sequence and A077763 gives A077762.
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EXAMPLE
| a(5)=1 because only one pairing is possible: 4+49=53, 16+81=97
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MATHEMATICA
| try[lev_] := Module[{j}, If[lev>n, (*Print[soln]; *) cnt++, For[j=1, j<=Length[s[[lev]]], j++, If[ !MemberQ[soln, s[[lev]][[j]]], soln[[lev]]=s[[lev]][[j]]; try[lev+2]; soln[[lev]]=0]]]]; maxN=28; For[lst2={1}; n=2, n<=maxN, n++, s=Table[{}, {n}]; For[i=2, i<=n, i=i+2, For[j=n+1, j<=2n, j++, If[PrimeQ[i^2+j^2], AppendTo[s[[i]], j]]]]; soln=Table[0, {n}]; cnt=0; try[2]; AppendTo[lst2, cnt]]; lst2
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CROSSREFS
| Cf. A077762, A077763.
Sequence in context: A180444 A065608 A184396 * A110794 A117295 A093820
Adjacent sequences: A077761 A077762 A077763 * A077765 A077766 A077767
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KEYWORD
| hard,nonn
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AUTHOR
| T. D. Noe (noe(AT)sspectra.com), Nov 15 2002
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