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 A211680 #17 Oct 14 2023 19:05:34
%S A211680 135,819,3375,6975,1485,9009,2295,13923,2565,15561,3105,18837,3915,
%T A211680 23751,4185,25389,4995,30303,5535,33579,5805,35217,6345,38493,25137,
%U A211680 40131,7155,43407,7965,48321,8235,49959,9045,54873,9585,58149,9855,59787,10665,64701
%N A211680 Pairs of deficient numbers having the same value of sigma(k)/k in the order that they are found.
%C A211680 These are pairs of friendly deficient numbers. The terms a(2k) are in order, but terms a(2k-1) may be out of order.
%C A211680 Many of these pairs (a,b) have the property that (k*a,k*b) is another pair for some integer k. See A212610 for primitive pairs.
%C A211680 From _Suyash Pandit_, Oct 13 2023: (Start)
%C A211680 The first (but not smallest) even term of this sequence is n=1278316. It is friendly to m=1680705, with sigma(n)/n = sigma(m)/m = 336/169.
%C A211680 The first pair of even terms in this sequence is (n,m) = (366776,1581644246) with sigma(n)/n = sigma(m)/m = 720/361.
%C A211680 It is possible to have more than two deficient numbers with the same value of sigma(n)/n. For example, the numbers 119129783409, 217416788955, and 1318995186327 all satisfy sigma(n)/n = 3584/1891. (End)
%H A211680 T. D. Noe, <a href="/A211680/b211680.txt">Table of n, a(n) for n = 1..2860</a>
%t A211680 nn = 10^5; t = DivisorSigma[1, Range[nn]]/Range[nn]; t2 = Transpose[Select[Tally[t], #[[1]] < 2 && #[[2]] > 1 &]][[1]]; Sort[Table[Flatten[Position[t, t2[[n]]]], {n, Length[t2]}], #1[[2]] < #2[[2]] &]
%Y A211680 Cf. A005100 (deficient numbers), A212608, A212609 (the pairs separated).
%Y A211680 Cf. A212610 (primitive pairs).
%K A211680 nonn
%O A211680 1,1
%A A211680 _T. D. Noe_, May 23 2012