This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A261991 #23 Oct 13 2015 03:21:07 %S A261991 18,36,48,54,60,72,84,90,96,108,120,126,132,144,162,168,180,192,198, %T A261991 210,216,234,240,252,264,270,288,300,306,324,330,336,342,360,378,384, %U A261991 390,396,414,420,432,450,462,468,480,486,504,522,528,540,546,558,576,588 %N A261991 Indices of abundant repunits: numbers n such that sigma((10^n-1)/9) > 2*(10^n-1)/9. %C A261991 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. %H A261991 Hiroaki Yamanouchi, <a href="/A261991/b261991.txt">Table of n, a(n) for n = 1..92</a> %e A261991 a(1)=18 since (10^18-1)/9 is abundant (abundancy ~ 2.09). %t A261991 Select[Range@ 60, DivisorSigma[1, (10^# - 1)/9] > 2 (10^# - 1)/9 &] (* _Michael De Vlieger_, Sep 08 2015 *) %o A261991 (PARI) is(n)=sigma(10^n\9,-1)>2 \\ _Charles R Greathouse IV_, Sep 07 2015 %Y A261991 Cf. A002275, A005101, A103759. %K A261991 nonn %O A261991 1,1 %A A261991 _Sergio Pimentel_, Sep 07 2015 %E A261991 a(27)-a(54) from _Hiroaki Yamanouchi_, Oct 13 2015