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A282538
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Odd integers n with the property that the largest prime factor of n^2+4 is less than n.
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2
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11, 29, 49, 59, 99, 111, 121, 127, 141, 161, 179, 199, 205, 211, 213, 219, 237, 247, 261, 283, 289, 309, 311, 335, 359, 369, 387, 393, 411, 417, 419, 433, 441, 469, 479, 485, 521, 523, 527, 535, 569, 581, 595, 603, 611, 619, 621, 633, 643, 679, 691, 705, 711, 715, 723, 729, 739, 741, 749, 759
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OFFSET
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1,1
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COMMENTS
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Every Pythagorean prime p can be uniquely written as the sum of two positive integers a and b such that ab is congruent to 1 (mod p). If a>b, then the difference a-b must be an odd number; no number on this list can be said difference, and every positive odd integer NOT on this list is the difference of exactly one pair.
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LINKS
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EXAMPLE
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Examples: 5 is not on this list, and 17-12=5 while 17+12=29 and (17)(12)==1 mod 29. 9 is not on this list, and 13-4=9 while 13+4=17 and (13)(4)==1 mod 17. 13 is not on this list, and 93-80=13 while 93+80=173 and (93)(80)==1 mod 173. Note that 5^2+4=29, 9^2+4=85=17(5), and 13^2+4=173
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MATHEMATICA
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fQ[n_] := FactorInteger[n^2 + 4][[-1, 1]] < n; Select[2 Range[380] - 1, fQ] (* Robert G. Wilson v, Feb 17 2017 *)
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PROG
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(PARI) isok(n) = (n%2) && vecmax(factor(n^2+4)[, 1]) < n; \\ Michel Marcus, Feb 18 2017
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CROSSREFS
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Cf. A256011 (generated similarly, but for n^2+1 instead of n^2+4).
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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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