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A218654
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Number of ways to write n as x+y with 0<x<=y and x^2+3xy+y^2 prime.
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12
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0, 1, 1, 1, 2, 1, 2, 2, 2, 1, 4, 1, 4, 2, 4, 2, 3, 2, 6, 3, 4, 3, 6, 2, 6, 3, 4, 3, 8, 3, 8, 2, 5, 5, 8, 4, 8, 6, 5, 4, 8, 2, 10, 6, 6, 3, 11, 4, 9, 6, 9, 7, 10, 4, 14, 6, 9, 3, 11, 4, 12, 7, 9, 10, 10, 4, 11, 5, 10, 9, 15, 4, 15, 9, 9, 8, 14, 6, 12, 8, 9, 8, 18, 4, 17, 11, 9, 11, 20, 5, 14, 10, 13, 7, 16, 9, 17, 6, 16, 10
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OFFSET
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1,5
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COMMENTS
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Conjecture: a(n)>0 for all n=2,3,4,...
It is known that any prime p = 1 or -1 (mod 5) can be written uniquely in the form x(p)^2+3x(p)y(p)+y(p)^2 with x(p)>y(p)>0.
Zhi-Wei Sun also conjectured that
(sum_{p<N, p=1,-1(mod 5)}x(p))
/(sum_{p<N, p=1,-1(mod 5)}y(p))
has the limit 1+sqrt(5) as N tends to the infinity.
These conjectures are similar to the ones mentioned in the comments in A218585.
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LINKS
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EXAMPLE
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For n=12 we have a(12)=1 since x^2+3x(12-x)+(12-x)^2 with 0<x<=6 is prime only when x=5.
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MATHEMATICA
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a[n_]:=a[n]=Sum[If[PrimeQ[x^2+3x(n-x)+(n-x)^2]==True, 1, 0], {x, 1, n/2}]; Do[Print[n, " ", a[n]], {n, 1, 20000}]
Table[Count[IntegerPartitions[n, {2}], _?(PrimeQ[#[[1]]^2+3Times@@#+ #[[2]]^2]&)], {n, 110}] (* Harvey P. Dale, Feb 28 2018 *)
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PROG
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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