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 A206448 #13 Mar 13 2023 07:35:43
%S A206448 24,31,42,48,56,60,72,84,90,96,120,124,126,144,156,168,180,186,192,
%T A206448 210,216,224,228,234,240,248,252,270,280,288,294,312,320,324,336,342,
%U A206448 360,372,378,384,392,399,403,408,416,420,432,434,448,450,456,468,480,504
%N A206448 Values of sigma(n) of composite numbers n such that sigma(n) = sigma(d) has solution for any other composite number d in increasing order.
%e A206448 Number 24 is in sequence because 24 = sigma(x) for composite numbers x = 14 and 15.
%t A206448 nn = 600; t = Table[If[PrimeQ[n], -1, DivisorSigma[1, n]], {n, nn}]; t2 = Rest[Select[Sort[Tally[t]], #[[2]] > 1 &]]; Select[Transpose[t2][[1]], # <= nn &] (* _T. D. Noe_, Feb 13 2012 *)
%Y A206448 Cf. A206447 (composite numbers n such that sigma(n) = sigma(d) has solution for any other composite number d), A000203.
%K A206448 nonn
%O A206448 1,1
%A A206448 _Jaroslav Krizek_, Feb 07 2012
%E A206448 Corrected by _T. D. Noe_, Feb 13 2012