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 A141255 Total number of line segments between points visible to each other in a square n X n lattice. 3
 0, 6, 28, 86, 200, 418, 748, 1282, 2040, 3106, 4492, 6394, 8744, 11822, 15556, 20074, 25456, 32086, 39724, 48934, 59456, 71554, 85252, 101250, 119040, 139350, 161932, 187254, 215136, 246690, 280916, 319346, 361328, 407302, 457180, 511714, 570232 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A line segment joins points (a,b) and (c,d) if the points are distinct and gcd(c-a,d-b)=1. LINKS S. Mustonen, On lines going through a given number of points in a rectangular grid of points [From Seppo Mustonen, May 13 2010] FORMULA a(n) = A114043(n) - 1. EXAMPLE The 2 x 2 square lattice has a total of 6 line segments: 2 vertical, 2 horizonal and 2 diagonal. MATHEMATICA Table[cnt=0; Do[If[GCD[c-a, d-b]<2, cnt++ ], {a, n}, {b, n}, {c, n}, {d, n}]; (cnt-n^2)/2, {n, 20}] Contribution from Seppo Mustonen, May 13 2010: (Start) (* This recursive code is much more efficient. *) a[n_]:=a[n]=If[n<=1, 0, 2*a1[n]-a[n-1]+R1[n]] a1[n_]:=a1[n]=If[n<=1, 0, 2*a[n-1]-a1[n-1]+R2[n]] R1[n_]:=R1[n]=If[n<=1, 0, R1[n-1]+4*EulerPhi[n-1]] R2[n_]:=(n-1)*EulerPhi[n-1] Table[a[n], {n, 1, 37}] (End) CROSSREFS Cf. A141224. Sequence in context: A222198 A302650 A055711 * A091321 A125310 A138874 Adjacent sequences:  A141252 A141253 A141254 * A141256 A141257 A141258 KEYWORD nonn AUTHOR T. D. Noe, Jun 17 2008 STATUS approved

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Last modified October 21 20:44 EDT 2019. Contains 328315 sequences. (Running on oeis4.)