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 A261991 Indices of abundant repunits: numbers n such that sigma((10^n-1)/9) > 2*(10^n-1)/9. 1
 18, 36, 48, 54, 60, 72, 84, 90, 96, 108, 120, 126, 132, 144, 162, 168, 180, 192, 198, 210, 216, 234, 240, 252, 264, 270, 288, 300, 306, 324, 330, 336, 342, 360, 378, 384, 390, 396, 414, 420, 432, 450, 462, 468, 480, 486, 504, 522, 528, 540, 546, 558, 576, 588 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The primitive indices are 18, 48, 60, 84, 132, 210, etc. It would be interesting to find the first index not a multiple of 6. LINKS Hiroaki Yamanouchi, Table of n, a(n) for n = 1..92 EXAMPLE a(1)=18 since (10^18-1)/9 is abundant (abundancy ~ 2.09). MATHEMATICA Select[Range@ 60, DivisorSigma[1, (10^# - 1)/9] > 2 (10^# - 1)/9 &] (* Michael De Vlieger, Sep 08 2015 *) PROG (PARI) is(n)=sigma(10^n\9, -1)>2 \\ Charles R Greathouse IV, Sep 07 2015 CROSSREFS Cf. A002275, A005101, A103759. Sequence in context: A346137 A303282 A073332 * A109428 A329611 A166403 Adjacent sequences:  A261988 A261989 A261990 * A261992 A261993 A261994 KEYWORD nonn AUTHOR Sergio Pimentel, Sep 07 2015 EXTENSIONS a(27)-a(54) from Hiroaki Yamanouchi, Oct 13 2015 STATUS approved

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Last modified December 2 05:01 EST 2021. Contains 349437 sequences. (Running on oeis4.)