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