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A304904
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Greatest prime p such that 2*n^2 - p is prime.
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3
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5, 13, 29, 47, 67, 79, 109, 157, 197, 239, 283, 331, 389, 443, 509, 571, 643, 719, 797, 877, 937, 1051, 1129, 1237, 1321, 1453, 1549, 1669, 1789, 1879, 2029, 2161, 2309, 2447, 2579, 2731, 2857, 3037, 3187, 3359, 3517
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OFFSET
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2,1
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COMMENTS
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Each square > 1 can be written as the average of 2 primes p1 < p2. a(n) gives the greatest prime p2 such that n^2 = (p1 + p2) / 2. The corresponding p1 is provided in A304903.
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LINKS
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FORMULA
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EXAMPLE
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a(6) = 67 because 2*6^2 - 67 = 5 is prime whereas 72 - 71 = 1 is not a prime.
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PROG
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(PARI) a304903(n) = forprime(p=3, , if(ispseudoprime(2*n^2-p), return(p)))
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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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