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A121259
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Numbers k such that (3*k^2 + 1)/4 is prime.
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6
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3, 5, 7, 9, 13, 19, 21, 23, 27, 29, 35, 47, 49, 51, 55, 57, 61, 65, 69, 75, 77, 83, 85, 91, 97, 99, 105, 111, 117, 125, 127, 133, 135, 149, 161, 163, 173, 177, 181, 183, 187, 191, 211, 217, 239, 247, 251, 257, 259, 273, 281, 285, 295, 307, 313, 315, 317, 329, 331, 341
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OFFSET
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1,1
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LINKS
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FORMULA
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EXAMPLE
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(3*5^2 + 1)/4 = 19 is the 2nd prime of this form, so a(2) = 5.
(3*13^2 + 1)/4 = 127 is the 5th prime of this form, so a(5) = 13.
(3*19^2 + 1)/4 = 271 is the 6th prime of this form, so a(6) = 19.
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MATHEMATICA
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Select[Range[400], PrimeQ[(3#^2+1)/4]&] (* Harvey P. Dale, Mar 24 2011 *)
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PROG
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CROSSREFS
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Cf. comment by Michael Somos in A002407.
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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