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 A258715 #14 Feb 19 2024 02:08:47
%S A258715 2,7,1,1,2,8,5,4,0,5,7,0,9,6,3,3,4,4,6,6,6,6,5,2,5,4,2,9,1,8,1,4,7,9,
%T A258715 1,0,4,6,7,9,7,6,5,8,7,7,1,9,8,9,7,5,4,5,6,9,3,7,9,5,7,1,7,0,6,7,9,5,
%U A258715 0,1,8,9,9,9,5,5,4,4,2,8
%N A258715 Decimal expansion of 2*Sum_{k>=0} 1/Product_{i=0..k} (2^(2^i) - 1).
%F A258715 From _Amiram Eldar_, Feb 19 2024: (Start)
%F A258715 Equals 2 * A258714.
%F A258715 Equals 2/A215016 - 3. (End)
%e A258715 2.7112854057096334466665254291814791046797658771989754...
%t A258715 RealDigits[2/NProduct[1 - 1/2^(2^k), {k, 0, Infinity}, WorkingPrecision -> 120] - 3][[1]] (* _Amiram Eldar_, Feb 19 2024 *)
%o A258715 (PARI) 2/prodinf(k = 0, 1 - 1/2^(2^k)) - 3 \\ _Amiram Eldar_, Feb 19 2024
%Y A258715 Cf. A215016, A258714, A258716.
%K A258715 nonn,cons
%O A258715 1,1
%A A258715 _N. J. A. Sloane_, Jun 15 2015