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 A078888 #12 Nov 12 2020 05:17:27
%S A078888 1,6,3,6,8,1,9,7,2,7,1,6,8,6,8,0,1,7,9,1,1,7,2,9,7,2,5,8,9,3,9,0,9,2,
%T A078888 0,0,6,0,5,2,4,4,8,5,4,1,5,9,3,3,6,8,2,5,3,2,7,8,6,2,2,1,0,3,5,9,7,2,
%U A078888 5,1,1,8,5,9,2,9,2,3,5,7,5,0,2,5,1,1,7,3,9,7,8,4,0,1,2,7,2,9,4,3,8,1,8,4,7
%N A078888 Decimal expansion of Sum {n>=0} 1/7^(2^n).
%H A078888 Aubrey J. Kempner, <a href="https://doi.org/10.1090/S0002-9947-1916-1501054-4">On Transcendental Numbers</a>, Transactions of the American Mathematical Society, volume 17, number 4, October 1916, pages 476-482.
%H A078888 <a href="/index/Tra#transcendental">Index entries for transcendental numbers</a>
%F A078888 Equals -Sum_{k>=1} mu(2*k)/(7^k - 1), where mu is the Möbius function (A008683). - _Amiram Eldar_, Jul 12 2020
%e A078888 0.163681972716868017911...
%t A078888 RealDigits[ N[ Sum[1/7^(2^n), {n, 0, Infinity}], 110]][[1]]
%o A078888 (PARI) suminf(n=0, 1/7^(2^n)) \\ _Michel Marcus_, Nov 11 2020
%Y A078888 Cf. A165425.
%Y A078888 Similar sums: A007404, A078885, A078585, A078886, A078887, A078889, A078890, A036987.
%K A078888 cons,nonn
%O A078888 0,2
%A A078888 _Robert G. Wilson v_, Dec 11 2002