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 A216453 #32 Jan 12 2019 08:06:54
%S A216453 0,6,12,24,30,54,60,84,102,138,144,192,198,246,288,336,342,414,420,
%T A216453 492,546,618,624,720,750,834,888,984,990,1122,1128,1224,1302,1410,
%U A216453 1476,1620,1626,1746,1836,1980,1986,2166,2172,2316,2442,2586,2592,2784,2826,3006,3120,3288,3294,3510,3600,3792,3918
%N A216453 Number of points hidden from the central point by a closer point in a hexagonal orchard of order n.
%H A216453 Vincenzo Librandi, <a href="/A216453/b216453.txt">Table of n, a(n) for n = 1..5000</a>
%H A216453 Project Euler, <a href="http://projecteuler.net/problem=351">Problem 351: Hexagonal orchards</a>.
%F A216453 a(n) = 6 * (C(n+1,2) - Sum_{i=1..n} phi(i)). - corrected by _Piyush Kumar_ and _Robert Israel_, Aug 26 2014
%F A216453 a(n) = 6*A063985(n). - _Jon Maiga_, Jan 12 2019
%t A216453 Table[6*Sum[k - EulerPhi[k], {k, n}], {n, 100}] (* _Jon Maiga_, Jan 12 2019 *)
%o A216453 (PARI) for(i=1,100,print1(6*(binomial(i+1,2)-sum(X=1,i,eulerphi(X))), ", "))
%Y A216453 Cf. A063985.
%K A216453 nonn
%O A216453 1,2
%A A216453 _V. Raman_, Sep 07 2012