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 A249521 #23 Jun 19 2023 06:41:24
%S A249521 2,2,5,6,7,5,8,3,3,4,1,9,1,0,2,5,1,4,7,7,9,2,3,1,7,8,0,6,2,4,3,0,9,0,
%T A249521 3,4,3,3,7,6,2,0,2,5,1,7,3,1,5,9,9,5,4,2,7,3,7,6,3,4,2,8,8,6,8,4,2,5,
%U A249521 6,9,8,7,3,7,6,5,9,7,3,6,5,7,9,4,6,9,7,4,6,4,0,8,0,8,4,2,9,4,5,3,7,7,2
%N A249521 Decimal expansion of 4/sqrt(Pi), the average distance between two random Gaussian points in three dimensions.
%C A249521 Coordinates are independent normally distributed random variables with mean 0 and variance 1.
%H A249521 G. C. Greubel, <a href="/A249521/b249521.txt">Table of n, a(n) for n = 1..5000</a>
%H A249521 Steven R. Finch, <a href="/A102519/a102519.pdf">Random Triangles</a>, Jan 21 2010. [Cached copy, with permission of the author]
%H A249521 <a href="/index/Tra#transcendental">Index entries for transcendental numbers</a>.
%F A249521 Equals Sum_{k>=0} k!/(k+3/2)!. - _Amiram Eldar_, Jun 19 2023
%e A249521 2.25675833419102514779231780624309034337620251731...
%t A249521 RealDigits[4/Sqrt[Pi], 10, 103] // First
%o A249521 (PARI) 4/sqrt(Pi) \\ _G. C. Greubel_, Jan 09 2017
%Y A249521 Cf. A002161 (the analog constant in two dimensions), A087197, A190732 (the analog constant in one dimension).
%K A249521 nonn,cons,easy
%O A249521 1,1
%A A249521 _Jean-François Alcover_, Oct 31 2014