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A218585
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Number of ways to write n as x+y with 0<x<=y and x^2+xy+y^2 prime.
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13
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0, 1, 1, 1, 1, 1, 2, 0, 3, 1, 2, 1, 3, 2, 3, 2, 2, 1, 4, 1, 4, 3, 4, 2, 3, 3, 3, 3, 5, 2, 6, 2, 4, 4, 5, 3, 5, 2, 8, 4, 4, 4, 7, 3, 5, 2, 8, 4, 7, 2, 8, 4, 7, 5, 7, 4, 7, 3, 8, 4, 9, 3, 11, 4, 8, 5, 10, 4, 9, 5, 9, 6, 8, 5, 6, 6, 10, 5, 10, 3, 12, 7, 10, 6, 8, 6, 11, 4, 7, 4, 15, 8, 13, 6, 9, 5, 15, 9, 10
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OFFSET
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1,7
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COMMENTS
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Conjecture: a(n)>0 for all n>1 with the only exception n=8.
Note that any prime p=1(mod 3) can be written uniquely in the form x(p)^2+x(p)y(p)+y(p)^2 with x(p)>y(p)>0.
Zhi-Wei Sun also conjectured that
(sum_{p<N, p=1(mod 3)}x(p))/(sum_{p<N, p=1(mod 3)}y(p)) tends to 1+sqrt(3), and (sum_{p<N, p=1(mod 3)}x(p)^2)/(sum_{p<N, p=1(mod 3)}y(p)^2) tends to 52/9. Similar conjectures involving p=x^2+y^2 were recently formulated by Thomas Ordowski.
Or, the number of primes of the form n*x+(n-x)^2 with 0<x<n/2.
Suggestion: the number of primes of the form n*x+(n-x)^2 with 0<x<n/3 is positive for n>12. - Zak Seidov_, Sep 25 2013
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REFERENCES
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Thomas Ordowski, Personal e-mail messages, Oct. 3-4, 2012, and Nov. 3, 2012.
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LINKS
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EXAMPLE
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For n=20 we have a(20)=1 since x^2+x(20-x)+(20-x)^2 with 0<x<=10 is prime only when x=3.
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MATHEMATICA
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A[n_]:=A[n]=Sum[If[PrimeQ[x^2+x(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+#[[1]]#[[2]]+ #[[2]]^2]&)], {n, 100}] (* Harvey P. Dale, Aug 04 2020 *)
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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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