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A240130
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Least prime of the form prime(n)^2 + k^2, or 0 if none.
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9
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5, 13, 29, 53, 137, 173, 293, 397, 593, 857, 977, 1373, 1697, 1913, 2213, 2909, 3517, 3821, 4493, 5077, 5333, 6257, 7213, 7937, 9413, 10301, 10613, 11549, 11897, 13093, 16193, 17417, 18773, 19421, 22397, 22817, 24749, 26573, 27893, 30029
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OFFSET
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1,1
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COMMENTS
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The positive terms form a subsequence of A185086 = Fouvry-Iwaniec primes = primes of the form prime^2 + integer^2.
Is a(n) < a(n+1) for all n? (I have checked it for n <= 10^6.) Note that A240131 is far from being monotone.
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LINKS
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Étienne Fouvry and Henryk Iwaniec, Gaussian primes, Acta Arithmetica 79:3 (1997), pp. 249-287.
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FORMULA
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a(n) == 1 (mod 4) if a(n) > 0.
a(n) > 0 if Bunyakovsky's conjecture is true.
a(n) <> a(m) if n <> m and a(n) > 0, by uniqueness in Fermat's 4n+1 Theorem.
a(n) = prime(n)^2 + A240131(n)^2 if a(n) > 0.
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EXAMPLE
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Prime(2) = 3 and 3^2 + 1^2 = 10 is not prime but 3^2 + 2^2 = 13 is prime, so a(2) = 13.
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MAPLE
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g:= proc(p) local k; for k from 2 by 2 do if isprime(p^2 + k^2) then return p^2+k^2 fi od end proc:
g(2):= 5:
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MATHEMATICA
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Table[First[Select[Prime[n]^2 + Range[20]^2, PrimeQ]], {n, 40}]
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PROG
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(PARI) a(n) = {p = prime(n); k = 1 - p%2; inc = 2; while (!isprime(q=p^2+k^2), k += inc); q; } \\ Michel Marcus, Nov 04 2015
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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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