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 A006872 M2984 #57 Feb 19 2024 01:49:08
%S A006872 1,3,15,26,39,45,74,104,111,117,122,146,175,183,195,219,296,314,333,
%T A006872 357,386,471,488,549,554,555,579,584,585,608,626,646,657,794,831,842,
%U A006872 914,915,939,962,1071,1082,1095,1191,1226,1256,1263,1292,1322,1346
%N A006872 Numbers k such that phi(k) = phi(sigma(k)).
%D A006872 S. W. Golomb, Equality among number-theoretic functions, Abstract 882-11-16, Abstracts Amer. Math. Soc., 14 (1993), 415-416.
%D A006872 R. K. Guy, Unsolved Problems in Number Theory, B42.
%D A006872 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A006872 Antti Karttunen, <a href="/A006872/b006872.txt">Table of n, a(n) for n = 1..20000</a> (terms 1..1000 from T. D. Noe, terms 1001..12394 from Marius A. Burtea)
%H A006872 S. W. Golomb, <a href="/A006872/a006872.pdf">Letter to N. J. A. Sloane, Oct. 1992</a>
%H A006872 S. W. Golomb, <a href="/A006872/a006872_1.pdf">Equality among number-theoretic functions</a>, Unpublished manuscript. (Annotated scanned copy)
%t A006872 Select[Range@ 1350, EulerPhi@ # == EulerPhi@ DivisorSigma[1, #] &] (* _Michael De Vlieger_, Jan 01 2019 *)
%o A006872 (PARI) lista(nn) = {for (i=1, nn, if (eulerphi(i)==eulerphi(sigma(i)), print1(i, ", ")););} \\ _Michel Marcus_, May 25 2013
%o A006872 (Haskell)
%o A006872 a006872 n = a006872_list !! (n-1)
%o A006872 a006872_list = filter (\x -> a000010' x == a000010' (a000203' x)) [1..]
%o A006872 -- _Reinhard Zumkeller_, Jul 14 2015
%o A006872 (Magma) [n:n in [1..2000]| EulerPhi(SumOfDivisors(n)) eq EulerPhi(n)]; // _Marius A. Burtea_, Jan 01 2019
%Y A006872 Cf. A000010, A000203, A062401, A353637 (characteristic function).
%Y A006872 Positions of zeros in A353636.
%Y A006872 Setwise difference of A353684 and A353683, and also of A353685 and A353686.
%Y A006872 Intersection of A353684 and A353685.
%Y A006872 Subsequences: A260021, A353634, A353635, A353679 (odd terms).
%Y A006872 Cf. also A033631, A033632, A067880, A137815, A260020.
%K A006872 nonn
%O A006872 1,2
%A A006872 _Jeffrey Shallit_, _N. J. A. Sloane_
%E A006872 More terms from _Jud McCranie_