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A204388
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Numbers k such that 4^k + 25 is prime.
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4
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1, 2, 3, 4, 5, 10, 11, 17, 35, 46, 56, 59, 118, 125, 189, 219, 285, 327, 400, 818, 1424, 2474, 2835, 3386, 3747, 4003, 4528, 5519, 8134, 10708, 10869, 16685, 39353, 56065, 63223, 82023, 109625, 118216, 184024, 262077, 265405, 320427, 349870, 373151, 377019, 377188, 465988, 494781
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OFFSET
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1,2
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COMMENTS
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Since 4^(6k) + 25 = 4096^k + 25 == (1^k + 25) mod 13 = 26 mod 13 == 0 mod 13, no multiple of 6 will be in this sequence. - Timothy L. Tiffin, Aug 07 2016
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LINKS
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FORMULA
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EXAMPLE
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a(1) = 1, since 4^1 + 25 = 4 + 25 = 29, which is prime.
a(2) = 2, since 4^2 + 25 = 16 + 25 = 41, which is prime.
a(3) = 3, since 4^3 + 25 = 64 + 25 = 89, which is prime.
a(4) = 4, since 4^4 + 25 = 256 + 25 = 281, which is prime.
a(5) = 5, since 4^5 + 25 = 1024 + 25 = 1049, which is prime.
a(6) = 10, since 4^10 + 25 = 1048576 + 25 = 1048601, which is prime. (End)
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MATHEMATICA
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Select[Range[2000], PrimeQ[4^# + 25] &] (* T. D. Noe, Feb 03 2012 *)
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PROG
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(PARI) for(n=1, 1e5, if(isprime(4^n + 25), print1(n", "))) \\ Altug Alkan, Oct 17 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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EXTENSIONS
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STATUS
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approved
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