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A179882
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a(n) is the corresponding value of contraharmonic mean B(h) of numbers k such that gcd(k, h) = 1 (k < h) for numbers h from A179877(n) and A179878(n).
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21
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1, 7, 15, 31, 39, 55, 71, 111, 119, 151, 175, 177, 231, 239, 255, 303, 311, 313, 319, 329, 335, 337, 345, 375, 391, 393, 479, 521, 559, 575, 591, 593, 601, 607, 623, 655, 657, 679, 777, 785, 791, 823, 855, 863, 871, 879, 889, 905, 911, 929, 937, 959, 961, 991
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OFFSET
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1,2
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COMMENTS
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This happens for this subsequence of A179877: 10, 22, 46, 58, 82, 106, 166, 178, ... apparently "Semiprimes of form prime - 1" >= 10 (see A077065). - Michel Marcus, Jul 27 2018
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LINKS
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FORMULA
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MATHEMATICA
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{1}~Join~Select[Partition[Table[ContraharmonicMean@ Select[Range[n - 1], GCD[#, n] == 1 &], {n, 2, 1500}], 2, 1], And[IntegerQ@ First@ #, SameQ @@ #] &][[All, 1]] (* Michael De Vlieger, Jul 30 2018 *)
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PROG
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(PARI) lista(nn) = {vch = vector(nn, k, ch(k)); for (i=1, nn-1, if ((vch[i] == vch[i+1]) && !frac(vch[i]), print1(vch[i], ", ")); ); } \\ Michel Marcus, Jul 27 2018
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CROSSREFS
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Cf. A179871, A179872, A179873, A179874, A179875, A179876, A179877, A179878, A179879, A179880, A179883, A179884, A179885, A179886, A179887, A179890, A179891.
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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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