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 A255605 #18 May 07 2021 08:09:15
%S A255605 1,6,15,27,43,61,84,110,139,172,208,247,290,337,387,440,497,557,621,
%T A255605 688,758,832,910,990,1075,1163,1254,1348,1446,1548,1653,1761,1873,
%U A255605 1988,2107,2229,2355,2484,2616,2752,2892,3034,3181,3330,3483,3640,3800,3963,4130,4301,4474
%N A255605 Integer part of the area of a pentagon with side length n.
%C A255605 Column 3 of A255604.
%H A255605 Harvey P. Dale, <a href="/A255605/b255605.txt">Table of n, a(n) for n = 1..1000</a>
%F A255605 a(n) = floor(5*n^2/(4*tan(Pi/5))), n >= 1.
%t A255605 Table[IntegerPart[5*n^2/(4*Tan[Pi/5])], {n, 51}] (* _Michael De Vlieger_, Mar 18 2015 *)
%t A255605 With[{c=Sqrt[25+10*Sqrt[5]]/4},Table[IntegerPart[c*n^2],{n,60}]] (* _Harvey P. Dale_, Aug 17 2020 *)
%o A255605 (PARI){for(n=1,100,a=floor(5*n^2/(4*tan(Pi/5)));print1(a,", "))}
%o A255605 (PARI) my(c=25+10*quadgen(20)); a(n) = sqrtint(floor(c*n^4))>>2; \\ _Kevin Ryde_, May 07 2021
%Y A255605 Cf. A102771, A255604.
%K A255605 nonn
%O A255605 1,2
%A A255605 _Kival Ngaokrajang_, Feb 27 2015