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A214518
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Record differences between the numbers n such that 4*n^2 + 1 is prime.
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2
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1, 2, 5, 7, 8, 10, 17, 20, 23, 44, 50, 56, 65, 76, 106, 144, 165, 173
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OFFSET
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1,2
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LINKS
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EXAMPLE
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a(1) = 1 because 4*1^2 + 1 = 5 and 4*2^2 + 1 = 17 are primes.
a(2) = 2 because 4*3^2 + 1 = 37 is prime, 4*4^2 + 1 = 65 is composite, and 4*5^2 + 1 = 101 is prime.
a(3) = 5 because 4*13^2 + 1 is prime, 4*n^2 + 1 is composite for n = 14..17, and 4*18^2 + 1 is prime.
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MATHEMATICA
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n = 1; last = 1; t = {1}; While[Length[t] < 15, n++; p = 1 + 4*n^2; If[PrimeQ[p], If[n - last > t[[-1]], AppendTo[t, n - last]]; last = n]]; t
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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STATUS
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approved
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