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A219923
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Number of ways to write n=x+y (x>0, y>0) with x-1, x+1 and 2*x*y+1 all prime
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10
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0, 0, 0, 0, 0, 1, 1, 0, 2, 0, 1, 1, 1, 1, 3, 2, 0, 1, 2, 2, 3, 2, 1, 0, 2, 2, 0, 1, 3, 2, 2, 1, 3, 4, 2, 2, 3, 0, 4, 3, 3, 1, 1, 3, 0, 3, 2, 1, 1, 3, 3, 1, 1, 5, 3, 1, 2, 1, 3, 3, 5, 3, 1, 2, 4, 3, 3, 2, 4, 3, 2, 2, 0, 3, 5, 4, 1, 3, 6, 2, 6, 2, 2, 4, 5, 5, 2, 3, 3, 4, 1, 2, 0, 1, 4, 2, 4, 1, 6, 6
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OFFSET
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1,9
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COMMENTS
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Conjecture: a(n)>0 for all n>623.
This has been verified for n up to 10^8.
Zhi-Wei Sun made the following general conjecture: For each nonnegative integer m, any sufficiently large integer n can be written as x+y (x>0, y>0) with x-m, x+m and 2*x*y+1 all prime.
For example, when m = 2, 3, 4, 5 it suffices to require that n is greater than 28, 151, 357, 199 respectively.
Sun also conjectured that for each m=0,1,2,... any sufficiently large integer n with m or n odd can be written as x+y (x>0, y>0) with x-m, x+m and x*y-1 all prime.
For example, in the case m=1 it suffices to require that n is greater than 4 and not among 40, 125, 155, 180, 470, 1275, 2185, 3875; when m=2 it suffices to require that n is odd, greater than 7, and different from 13.
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LINKS
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EXAMPLE
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a(11)=1 since 11=6+5 with 6-1, 6+1 and 2*6*5+1=61 all prime.
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MATHEMATICA
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a[n_]:=a[n]=Sum[If[PrimeQ[Prime[k]+2]==True&&PrimeQ[2(Prime[k]+1)(n-Prime[k]-1)+1]==True, 1, 0], {k, 1, PrimePi[n-1]}]
Do[Print[n, " ", a[n]], {n, 1, 10000}]
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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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