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 A231986 #16 Feb 07 2025 13:19:50 %S A231986 9,2,7,6,8,9,4,7,5,3,2,2,3,1,3,6,4,0,7,9,5,6,1,3,2,3,8,1,4,5,9,5,4,9, %T A231986 1,7,6,3,0,4,0,4,0,0,6,4,2,4,5,7,4,3,4,0,8,9,9,9,8,6,9,0,4,6,6,9,1,7, %U A231986 4,8,6,1,8,8,5,9,1,4,5,1,8,8,9,3,9,3,7,1,3,1,0,9,9,0,3,1,9,1,2,3,5,3,9,4,4 %N A231986 Decimal expansion of the solid angle (in steradians) subtended by a spherical square of one radian side. %C A231986 In spherical geometry, the solid angle (in steradians) covered by a rectangle with arc-length sides r and s (in radians) equals Omega = 4*arcsin(sin(s/2)*sin(r/2)). For this constant, r = s = 1. %C A231986 Note: It is a common mistake to think that 1 radian squared gives one steradian! See also the discussion in A231984. %D A231986 G. V. Brummelen, Heavenly Mathematics: The Forgotten Art of Spherical Trigonometry, Princeton University Press, 2012, ISBN 978-0691148922. %H A231986 Stanislav Sykora, <a href="/A231986/b231986.txt">Table of n, a(n) for n = 0..2000</a> %H A231986 Wikipedia, <a href="http://en.wikipedia.org/wiki/Solid_angle#Pyramid">Solid angle</a>, Section 3.3 (Pyramid). %H A231986 Wikipedia, <a href="http://en.wikipedia.org/wiki/Steradian">Steradian</a>. %F A231986 Equals 4*arcsin(sin(1/2)^2). %e A231986 0.9276894753223136407956132381459549176304040064245743408999869... %t A231986 RealDigits[4 * ArcSin[Sin[1/2]^2], 10, 120][[1]] (* _Amiram Eldar_, May 16 2023 *) %o A231986 (PARI) 4*asin(sin(1/2)^2) \\ _Charles R Greathouse IV_, Feb 07 2025 %Y A231986 Cf. A072097 (rad/deg), A019685 (deg/rad), A231981 (sr/deg^2), A231982 (deg^2/sr), A231984, A231987 (inverse problem). %K A231986 nonn,cons,easy %O A231986 0,1 %A A231986 _Stanislav Sykora_, Nov 17 2013