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 A336686 #24 Jun 23 2023 23:27:05
%S A336686 2,5,0,1,3,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,9,0,5,0,0,6,2,6,4,5,9,
%T A336686 9,8,5,0,0,7,2,3,7,9,3,7,6,0,2,1,9,3,6,9,0,0,6,0,6,9,0,2,1,3,6,1,4,9,
%U A336686 9,1,4,9,6,1,1,6,8,6,2,4,1,8,1,7,8,9,1,9,8,9,9,3,3,9,4,3,7,4,1,5,7,4,4,1
%N A336686 Decimal expansion of Sum_{k>=0} 1/(k!)!.
%C A336686 The sum has 18 8's in a row within the first 23 decimal places.
%C A336686 1801/720 approximates this constant to 23 significant digits.
%e A336686 2.5013888888888888888888905006...
%p A336686 evalf(sum(1/(k!)!, k=0..infinity), 112); # _Alois P. Heinz_, Nov 20 2020
%t A336686 RealDigits[Sum[1/(k!)!, {k, 0, 4}], 10, 100][[1]] (* _Amiram Eldar_, Nov 21 2020 *)
%o A336686 (PARI) suminf(k=0, 1/(k!)!)
%Y A336686 Cf. A000197, A336810.
%K A336686 nonn,cons
%O A336686 1,1
%A A336686 _Daniel Hoyt_, Nov 20 2020