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 A331373 #19 Sep 03 2025 21:22:48
%S A331373 1,2,5,3,4,9,8,7,5,5,6,9,9,9,5,3,4,7,1,6,4,3,3,6,0,9,3,7,9,0,5,7,9,8,
%T A331373 9,4,0,3,6,9,2,3,2,2,0,8,3,3,2,0,1,3,4,1,7,0,6,3,8,3,4,7,1,6,6,4,0,9,
%U A331373 5,2,4,8,2,0,4,8,9,8,7,1,7,0,8,9,0,2,4
%N A331373 Decimal expansion of Sum_{k>=2} 1/(k! - 1).
%C A331373 Erdős was interested in the question whether this constant is irrational.
%D A331373 Paul Erdős, Some of my favourite unsolved problems, in A. Baker, B. Bollobás and A. Hajnal (eds.), A tribute to Paul Erdős, Cambridge University Press, 1990, p. 470.
%H A331373 Thomas Bloom, <a href="https://www.erdosproblems.com/68">Is Sum_{n>=2} 1/(n!-1) irrational?</a>, Erdős Problems.
%H A331373 Paul Erdős, <a href="https://www.renyi.hu/~p_erdos/1988-22.pdf">On the irrationality of certain series: problems and results</a>, in Alan Baker (ed.), New Advances in Transcendence Theory, Cambridge University Press, 1988, p. 102.
%H A331373 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, p. 62.
%H A331373 Terence Tao, <a href="https://github.com/teorth/erdosproblems/blob/main/README.md#table">Erdős problem database</a>, see no. 68.
%e A331373 1.25349875569995347164336093790579894036923220833201...
%t A331373 RealDigits[Sum[1/(k! - 1), {k, 2, 300}], 10, 100][[1]]
%o A331373 (PARI) suminf(k=2, 1/(k!-1)) \\ _Michel Marcus_, May 03 2020
%Y A331373 Cf. A033312, A331372, A327826.
%K A331373 nonn,cons,changed
%O A331373 1,2
%A A331373 _Amiram Eldar_, May 03 2020