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 A334071 #9 Apr 14 2020 03:27:41
%S A334071 2,0,8,5,5,6,1,8,8,2,5,9,7,4,1,4,6,5,2,6,3,0,3,8,7,0,8,3,2,8,4,7,4,7,
%T A334071 5,6,1,6,1,2,1,0,7,7,6,0,3,2,3,8,7,3,7,1,9,4,7,5,7,9,3,8,8,3,1,3,6,1,
%U A334071 8,0,1,3,3,7,1,0,5,8,8,2,1,7,1,0,3,1,9
%N A334071 Decimal expansion of Sum_{k >= 1} 2^(-phi(k)), where phi is the Euler totient function (A000010).
%C A334071 This constant is irrational (Erdős and Graham, 1980).
%H A334071 Paul Erdős and Ronald L. Graham, <a href="http://www.math.ucsd.edu/~fan/ron/papers/80_11_number_theory.pdf">Old and new problems and results in combinatorial number theory</a>, L'enseignement Mathématique, Université de Genève, 1980.
%e A334071 2.08556188259741465263038708328474756161210776032387...
%t A334071 RealDigits[Sum[2^(-EulerPhi[n]), {n, 1, 2000}], 10, 100][[1]]
%o A334071 (PARI) suminf(k=1, 1/2^eulerphi(k)) \\ _Michel Marcus_, Apr 14 2020
%Y A334071 Cf. A000010, A334072.
%K A334071 nonn,cons
%O A334071 1,1
%A A334071 _Amiram Eldar_, Apr 13 2020