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 A097987 #21 Jul 23 2020 03:27:03 %S A097987 1,2,3,4,6,7,9,11,14,18,19,22,23,27,31,38,43,46,47,49,54,59,62,67,71, %T A097987 79,81,83,86,94,98,103,107,118,121,127,131,134,139,142,151,158,162, %U A097987 163,166,167,179,191,199,206,211,214,223,227,239,242,243,251,254,262,263,271 %N A097987 Numbers k such that 4 does not divide phi(k), where phi is Euler's totient function (A000010). %C A097987 The asymptotic density of this sequence is 0 (Dressler, 1975). - _Amiram Eldar_, Jul 23 2020 %H A097987 Ivan Neretin, <a href="/A097987/b097987.txt">Table of n, a(n) for n = 1..10000</a> %H A097987 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. %F A097987 a(n)=1, 2, 4, p^k, 2*p^k, with prime p == 3 (mod 4). %t A097987 Select[Range@275, ! Divisible[EulerPhi[#], 4] &] (* _Ivan Neretin_, Aug 24 2016 *) %o A097987 (PARI) is(n)=my(o=valuation(n,2),p); (o<2 && isprimepower(n>>o,&p) && p%4>1) || n<5 \\ _Charles R Greathouse IV_, Feb 21 2013 %Y A097987 Essentially the same as A066499. %Y A097987 Cf. A000010. %Y A097987 Complement of A172019. %K A097987 nonn %O A097987 1,2 %A A097987 _Lekraj Beedassy_, Sep 07 2004 %E A097987 Corrected and extended by _Vladeta Jovovic_, Sep 08 2004