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 A039766 #16 Mar 15 2019 21:59:38
%S A039766 1,3,21,33,57,69,77,85,93,105,125,129,141,161,175,177,201,205,209,213,
%T A039766 221,237,249,253,309,321,329,345,365,381,393,413,417,437,445,453,473,
%U A039766 475,485,489,493,497,501,517,533,537,565,573,581,597,629,633,649,665
%N A039766 Numbers k such that gcd(phi(k), k-1) = number of divisors of k.
%H A039766 Robert Israel, <a href="/A039766/b039766.txt">Table of n, a(n) for n = 1..10000</a>
%e A039766 phi(21)=12, gcd(12,20)=4, 21 is divisible by {1,3,7,21}.
%p A039766 filter:= t -> igcd(numtheory:-phi(t),t-1) = numtheory:-tau(t):
%p A039766 select(filter,[$1..1000]); # _Robert Israel_, Mar 15 2019
%o A039766 (PARI) isok(n) = gcd(eulerphi(n), n-1) == numdiv(n); \\ _Michel Marcus_, May 30 2014
%Y A039766 Cf. A000005, A000010, A049559.
%K A039766 nonn,easy
%O A039766 1,2
%A A039766 _Olivier Gérard_
%E A039766 Term 1 prepended by _Michel Marcus_, May 30 2014