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 A386530 #9 Aug 25 2025 04:35:46 %S A386530 2,9,5,2,8,8,2,1,2,2,8,0,6,2,3,1,1,6,8,6,8,1,5,0,8,9,8,3,0,9,6,8,9,4, %T A386530 7,1,1,8,6,0,3,9,8,5,3,3,6,9,8,2,4,6,3,4,2,9,9,1,1,4,9,7,3,4,3,2,1,8, %U A386530 7,0,6,8,6,6,3,0,9,1,1,1,7,1,0,1,9,0,6,7,9,6 %N A386530 Decimal expansion of the largest dihedral angle, in radians, in an elongated pentagonal rotunda (Johnson solid J_21). %C A386530 This is the dihedral angle between a triangular face and a square face (at the edge where the prism and rotunda parts of the solid meet). %C A386530 Also the analogous dihedral angle in Johnson solids J_40-J_43. %H A386530 Paolo Xausa, <a href="/A386530/b386530.txt">Table of n, a(n) for n = 1..10000</a> %H A386530 Wikipedia, <a href="https://en.wikipedia.org/wiki/Elongated_pentagonal_rotunda">Elongated pentagonal rotunda</a>. %F A386530 Equals arccos(-sqrt(2*(5 + sqrt(5))/15)) = arccos(-sqrt(2*(5 + A002163)/15)). %e A386530 2.9528821228062311686815089830968947118603985336982... %t A386530 First[RealDigits[ArcCos[-Sqrt[2*(5 + Sqrt[5])/15]], 10, 100]] (* or *) %t A386530 First[RealDigits[Max[PolyhedronData["J21", "DihedralAngles"]], 10, 100]] %Y A386530 Cf. other J_21 dihedral angles: A019669, A228824, A344075, A387191. %Y A386530 Cf. A384213 (J_21 volume), A179637 (J_21 surface area - 10). %Y A386530 Cf. A002163. %K A386530 nonn,cons,easy,new %O A386530 1,1 %A A386530 _Paolo Xausa_, Aug 22 2025