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 A216453 Number of points hidden from the central point by a closer point in a hexagonal orchard of order n. 1
 0, 6, 12, 24, 30, 54, 60, 84, 102, 138, 144, 192, 198, 246, 288, 336, 342, 414, 420, 492, 546, 618, 624, 720, 750, 834, 888, 984, 990, 1122, 1128, 1224, 1302, 1410, 1476, 1620, 1626, 1746, 1836, 1980, 1986, 2166, 2172, 2316, 2442, 2586, 2592, 2784, 2826, 3006, 3120, 3288, 3294, 3510, 3600, 3792, 3918 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..5000 Project Euler, Problem 351: Hexagonal orchards. FORMULA a(n) = 6 * (C(n+1,2) - Sum_{i=1..n} phi(i)). - corrected by Piyush Kumar and Robert Israel, Aug 26 2014 a(n) = 6*A063985(n). - Jon Maiga, Jan 12 2019 MATHEMATICA Table[6*Sum[k - EulerPhi[k], {k, n}], {n, 100}] (* Jon Maiga, Jan 12 2019 *) PROG (PARI) for(i=1, 100, print1(6*(binomial(i+1, 2)-sum(X=1, i, eulerphi(X))), ", ")) CROSSREFS Cf. A063985. Sequence in context: A358526 A069171 A071611 * A119500 A260633 A348632 Adjacent sequences: A216450 A216451 A216452 * A216454 A216455 A216456 KEYWORD nonn AUTHOR V. Raman, Sep 07 2012 STATUS approved

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