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 A007371 M4827 #34 Nov 18 2024 05:24:40
%S A007371 12,18,31,32,54,56,80,98,104,108,114,124,126,128,132,140,152,156,182,
%T A007371 186,210,264,272,280,308,320,342,378,390,392,399,403,408,416,440,444,
%U A007371 448,492,522,532,570,572,594,608,630,632,726,762,770,774,780,784,800
%N A007371 Numbers k such that sigma(x) = k has exactly 2 solutions.
%D A007371 M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.
%D A007371 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A007371 Amiram Eldar, <a href="/A007371/b007371.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Donovan Johnson)
%H A007371 M. Abramowitz and I. A. Stegun, eds., <a href="http://www.convertit.com/Go/ConvertIt/Reference/AMS55.ASP">Handbook of Mathematical Functions</a>, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
%H A007371 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%H A007371 Robert G. Wilson v, <a href="/A007015/a007015.pdf">Letter to N. J. A. Sloane, Jul. 1992</a>.
%t A007371 a = Table[ 0, {750} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 751, a[ [ s ] ]++ ], {n, 1, 750} ]; Select[ Range[ 750 ], a[ [ # ] ] == 2 & ]
%o A007371 (PARI) is(n)=sum(k=1,n,sigma(k)==n)==2 \\ _Charles R Greathouse IV_, Mar 09 2014
%o A007371 (PARI) is(k) = invsigmaNum(k) == 2 \\ _Amiram Eldar_, Nov 17 2024, using _Max Alekseyev_'s invphi.gp
%Y A007371 Cf. A000203.
%Y A007371 Number of solutions: A007369 (0), A007370 (1), this sequence (2), A007372 (3), A060660 (4), A060661 (5), A060662 (6), A060663 (7), A060664 (8), A060665 (9), A060666 (10), A060678 (11), A060676 (12).
%K A007371 nonn
%O A007371 1,1
%A A007371 _N. J. A. Sloane_, _Mira Bernstein_, _Robert G. Wilson v_