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 A063507 #19 Jan 12 2018 02:52:48
%S A063507 2,4,9,6,25,10,15,12,21,0,35,18,33,26,39,24,65,34,51,38,45,30,95,36,
%T A063507 69,0,63,52,161,42,87,48,93,0,75,54,217,74,99,76,185,82,123,60,117,66,
%U A063507 215,72,141,0,235,0,329,78,159,98,105,0,371,84,177,122,135,96,305,90,427
%N A063507 Least k such that k - phi(k) = n, or 0 if no such k exists.
%C A063507 Inverse cototient (A051953) sets represented by their minimum, as in A002181 for totient function. Impossible values (A005278) are replaced by zero.
%C A063507 If a(n) > 0, then it appears that a(n) > 1.26n. - _T. D. Noe_, Dec 06 2006
%H A063507 T. D. Noe, <a href="/A063507/b063507.txt">Table of n, a(n) for n=1..10000</a>
%F A063507 a(n)-A051953(a(n)) = n if possible and a(n)=0 if n belongs to A005278.
%e A063507 x = InvCototient[24] = {36, 40, 44, 46}; Phi[x] = Phi[{36, 40, 44, 46}] = {12, 16, 20, 22}; x-Phi[x] = {24, 24, 24, 24}, so a(24) = Min[InvCototient[24]]; a(10) = 0 because 10 is in A005278.
%t A063507 Table[SelectFirst[Range[n^2 + 1], # - EulerPhi[#] == n &] /. k_ /; ! IntegerQ@ k -> 0, {n, 67}] (* _Michael De Vlieger_, Jan 11 2018 *)
%Y A063507 Cf. A051953, A000010, A002181, A005277, A005278.
%Y A063507 Cf. A063748 (greatest solution to x-phi(x)=n).
%Y A063507 Cf. A063740 (number of k such that cototient(k) = n).
%K A063507 nonn
%O A063507 1,1
%A A063507 _Labos Elemer_, Aug 09 2001
%E A063507 Edited by _N. J. A. Sloane_, Oct 25 2008 at the suggestion of _R. J. Mathar_