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 A002181 M2421 N0957 #44 Oct 10 2023 16:26:10
%S A002181 1,3,5,7,15,11,13,17,19,25,23,35,29,31,51,37,41,43,69,47,65,53,81,87,
%T A002181 59,61,85,67,71,73,79,123,83,129,89,141,97,101,103,159,107,109,121,
%U A002181 113,177,143,127,255,131,161,137,139,213,185,149,151,157,187,163,249,167,203,173
%N A002181 Least number k such that phi(k) = m, where m runs through the values (A002202) taken by phi.
%C A002181 Inverse of Euler totient function.
%C A002181 A051445 without the zeros. The values of m are in A002180.
%C A002181 According to Guy, the first even term is for 2m = 16842752 = 257*2^16. If there are only five Fermat primes, then terms will be even for 2m = 2^r for all r > 31. This was discussed in problem E3361. - _T. D. Noe_, Aug 14 2008
%D A002181 J. W. L. Glaisher, Number-Divisor Tables. British Assoc. Math. Tables, Vol. 8, Camb. Univ. Press, 1940, p. 64.
%D A002181 R. K. Guy, Unsolved problems in number theory, B39.
%D A002181 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A002181 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A002181 T. D. Noe, <a href="/A002181/b002181.txt">Table of n, a(n) for n = 1..10000</a>
%H A002181 R. D. Carmichael, <a href="/A002180/a002180.pdf">A table of the values of m corresponding to given values of phi(m)</a>, Amer. J. Math., 30 (1908), 394-400. [Annotated scanned copy]
%H A002181 T. D. Noe, <a href="http://www.sspectra.com/math/16842752.txt">Numbers Like 16842752</a>.
%H A002181 William P. Wardlaw, L. L. Foster and R. J. Simpson, <a href="http://www.jstor.org/stable/2323869">Problem E3361</a>, Amer. Math. Monthly, Vol. 98, No. 5 (May, 1991), 443-444.
%H A002181 K. W. Wegner, <a href="/A002180/a002180_1.pdf">Values of phi(x) = n for n from 2 through 1978</a>, mimeographed manuscript, no date [Annotated scanned copy]
%F A002181 a(n) = A061026(A002202(n)). - _Flávio V. Fernandes_, Oct 08 2023
%t A002181 With[{ep=EulerPhi[Range[1000]]},Flatten[Table[Position[ep,n,{1},1],{n,200}]]] (* _Harvey P. Dale_, Apr 10 2015 *)
%Y A002181 Cf. A058277, A006511, A002202, A061026.
%K A002181 nonn
%O A002181 1,2
%A A002181 _N. J. A. Sloane_
%E A002181 Offset and initial term corrected Oct 07 2007
%E A002181 Revised definition from _T. D. Noe_, Aug 14 2008