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 A137243 Number of coprime pairs (a,b) with -n <= a,b <= n. 5
 8, 16, 32, 48, 80, 96, 144, 176, 224, 256, 336, 368, 464, 512, 576, 640, 768, 816, 960, 1024, 1120, 1200, 1376, 1440, 1600, 1696, 1840, 1936, 2160, 2224, 2464, 2592, 2752, 2880, 3072, 3168, 3456, 3600, 3792, 3920, 4240, 4336, 4672, 4832, 5024, 5200, 5568 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Seiichi Manyama, Table of n, a(n) for n = 1..1000 FORMULA a(n) = 4*A018805(n) + 4. - Charles R Greathouse IV, Aug 06 2012. a(n) = a(n-1) + 8*EulerPhi(n), with a(1)=8. - Juan M. Marquez, Apr 24 2015 EXAMPLE a(1)=8 because there are eight coprime pairs (1,1), (1,0), (1,-1), (0,1), (0,-1), (-1,1),(-1,0),(-1,-1) with integral entries of absolute value <= 1. In the same way a(2)=16 as there are sixteen coprime pairs (2,1), (2,-1), (1,2), (1,1), (1,0), (1,-1),(1,-2), (0,1), (0,-1), (-1,2), (-1,1), (-1,0), (-1,-1), (-1,-2), (-2,1), (-2,-1) of integral entries of absolute value <= 2. PROG (PARI) a(n)=4*sum(k=1, n, moebius(k)*(n\k)^2)+4 \\ Charles R Greathouse IV, Aug 06 2012 CROSSREFS Cf. A018805. Sequence in context: A106841 A260711 A139598 * A303026 A068935 A277128 Adjacent sequences:  A137240 A137241 A137242 * A137244 A137245 A137246 KEYWORD nonn AUTHOR Kilian Kilger (kilian(AT)mathi.uni-heidelberg.de), May 11 2008 STATUS approved

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Last modified October 14 07:19 EDT 2019. Contains 327995 sequences. (Running on oeis4.)