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A113541
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Numbers k such that 18*k^2+1 is multiple of 19.
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3
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1, 18, 20, 37, 39, 56, 58, 75, 77, 94, 96, 113, 115, 132, 134, 151, 153, 170, 172, 189, 191, 208, 210, 227, 229, 246, 248, 265, 267, 284, 286, 303, 305, 322, 324, 341, 343, 360, 362, 379, 381, 398, 400, 417, 419, 436, 438, 455, 457, 474, 476, 493, 495, 512
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OFFSET
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1,2
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COMMENTS
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Numbers congruent to {1, 18} mod 19. - Amiram Eldar, Feb 27 2023
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LINKS
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FORMULA
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a(1)=1, a(2)=18 then add alternatively 2 and 17.
Sum_{n>=1} (-1)^(n+1)/a(n) = cot(Pi/19)*Pi/19. - Amiram Eldar, Feb 27 2023
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MATHEMATICA
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s={}; Do[If[IntegerQ[(18n^2+1)/19], AppendTo[s, n]], {n, 200}]; s
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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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