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A078587
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Largest prime p such that p<n and 2n-p is also prime.
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9
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3, 3, 5, 3, 5, 7, 7, 5, 11, 7, 11, 13, 13, 11, 17, 7, 17, 19, 13, 17, 19, 19, 23, 23, 19, 17, 29, 19, 23, 29, 31, 29, 31, 31, 29, 37, 37, 29, 41, 19, 41, 43, 31, 41, 43, 37, 47, 43, 43, 47, 47, 43, 53, 53, 43, 47, 59, 43, 53, 59, 61, 59, 61, 61, 53, 67, 67, 59, 71, 67, 59, 71
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OFFSET
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4,1
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COMMENTS
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Suggested by Goldbach Conjecture.
Also, values of p from A143697. This follows from the factorization n^2-k^2 = (n-k)(n+k). - T. D. Noe, Jan 22 2009
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LINKS
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FORMULA
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MATHEMATICA
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Table[p=n+1; q=2n-p; While[q>0&&!(PrimeQ[p]&&PrimeQ[q]), p++; q-- ]; q, {n, 4, 100}]
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PROG
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(PARI) a(n) = {my(p = precprime(n-1)); while(!isprime(2*n-p), p = precprime(p-1)); p; } \\ Michel Marcus, Oct 22 2016
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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