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 A172019 #18 Feb 12 2021 04:04:34 %S A172019 5,8,10,12,13,15,16,17,20,21,24,25,26,28,29,30,32,33,34,35,36,37,39, %T A172019 40,41,42,44,45,48,50,51,52,53,55,56,57,58,60,61,63,64,65,66,68,69,70, %U A172019 72,73,74,75,76,77,78,80,82,84,85,87,88,89,90,91,92,93,95,96,97,99,100,101 %N A172019 Numbers k such that 4 divides phi(k) (i.e., A000010(k)). %C A172019 Complement of A097987. %C A172019 The asymptotic density of this sequence is 1 (Dressler, 1975). - _Amiram Eldar_, Feb 12 2021 %H A172019 Rémy Sigrist, <a href="/A172019/b172019.txt">Table of n, a(n) for n = 1..10000</a> %H A172019 Robert E. Dressler, <a href="http://www.numdam.org/item/?id=CM_1975__31_2_115_0">A property of the phi and sigma_j functions</a>, Compositio Mathematica, Vol. 31, No. 2 (1975), pp. 115-118. %t A172019 Select[Range[200], Mod[EulerPhi[#], 4] == 0 &] (* _Geoffrey Critzer_, Nov 30 2014 *) %o A172019 (PARI) is(n)=my(o=valuation(n, 2), p); (o>1 || !isprimepower(n>>o, &p) || p%4<2) && n>4 \\ _Charles R Greathouse IV_, Mar 05 2013 %Y A172019 Cf. A000010, A097987, A066499, A066498, A066500, A066502. %K A172019 easy,nonn %O A172019 1,1 %A A172019 _Giovanni Teofilatto_, Jan 22 2010