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 A384213 #11 May 23 2025 10:14:01 %S A384213 1,4,6,1,1,9,7,1,8,1,1,0,6,2,8,3,5,5,7,6,3,3,8,7,2,2,4,7,0,7,9,4,9,1, %T A384213 5,8,9,3,5,5,7,6,3,1,3,6,8,2,9,4,1,4,2,5,1,0,3,1,4,9,9,5,0,5,6,9,3,5, %U A384213 3,9,6,1,9,9,2,2,4,6,1,7,5,7,0,3,0,6,9,0,4,7 %N A384213 Decimal expansion of the volume of an elongated pentagonal rotunda with unit edge. %C A384213 The elongated pentagonal rotunda is Johnson solid J_21. %H A384213 Paolo Xausa, <a href="/A384213/b384213.txt">Table of n, a(n) for n = 2..10000</a> %H A384213 Wikipedia, <a href="https://en.wikipedia.org/wiki/Elongated_pentagonal_rotunda">Elongated pentagonal rotunda</a>. %F A384213 Equals (45 + 17*sqrt(5) + 30*sqrt(5 + 2*sqrt(5)))/12 = (45 + 17*A002163 + 30*sqrt(5 + A010476))/12. %F A384213 Equals the largest real root of 1296*x^4 - 19440*x^3 + 2340*x^2 + 70200*x + 43525. %e A384213 14.611971811062835576338722470794915893557631368294... %t A384213 First[RealDigits[(45 + 17*Sqrt[5] + 30*Sqrt[5 + Sqrt[20]])/12, 10, 100]] (* or *) %t A384213 First[RealDigits[PolyhedronData["J21", "Volume"], 10, 100]] %Y A384213 Cf. A179637 (surface area - 10). %Y A384213 Cf. A002163, A010476, A384138, A384140, A384144. %K A384213 nonn,cons,easy %O A384213 2,2 %A A384213 _Paolo Xausa_, May 23 2025