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 A018808 Number of lines through at least 2 points of an n X n grid of points. 17
 0, 0, 6, 20, 62, 140, 306, 536, 938, 1492, 2306, 3296, 4722, 6460, 8830, 11568, 14946, 18900, 23926, 29544, 36510, 44388, 53586, 63648, 75674, 88948, 104374, 121032, 139966, 160636, 184466, 209944, 239050, 270588, 305478, 342480, 383370, 427020 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Seiichi Manyama, Table of n, a(n) for n = 0..1000 (terms 0..100 from T. D. Noe) M. A. Alekseyev, M. Basova, N. Yu. Zolotykh. On the minimal teaching sets of two-dimensional threshold functions. SIAM J. Disc. Math. 29(1), 2015, pp. 157-165. A.-M. Ernvall-Hytonen, K. Matomaki, P. Haukkanen, J. K. Merikoski, Formulas for the number of gridlines, Monatsh. f. Mathem. 164 (2) (2011) 157-170 P. Haukkanen, J. K. Merikoski, Some formulas for numbers of line segments and lines in a rectangular grid, arXiv:1108.1041 [math.CO], 2011. Seppo Mustonen, On lines and their intersection points in a rectangular grid of points [Local copy] FORMULA (1/2) * (f(n, 1) - f(n, 2)) where f(n, k) = Sum ((n - |x|)(n - |y|)); -n < x < n, -n < y < n, (x, y)=k. (1/2) * (f(n, 1) - f(n, 2)) where f(n, k) = Sum ((n - |kx|)(n - |ky|)); -n < kx < n, -n < ky < n, (x, y)=1. - Seppo Mustonen, Apr 18 2009 a(0) = L(0,1) = R1(0) = 0, a(n) = L(n,n) = 2L(n-1,n) - L(n-1,n-1) + R1(n), L(n-1,n) = 2L(n-1,n-1) - L(n-2,n-1) + R2(n), R1(n) = R1(n-1) + 4(phi(n-1) - e(n)), e(n)=0, n even, e(n) = phi((n-1)/2), n odd, R2(n) = (n-1)phi(n-1), n even, R2(n)=(n-1)phi(n-1)/2, n=1 mod 4, R2(n)=0, n=3 mod 4. - Seppo Mustonen, Apr 25 2009 MATHEMATICA L[0]=0; L1[1]=0; R1[1]=0; L[n_]:=L[n]=2*L1[n]-L[n-1]+R1[n] L1[n_]:=L1[n]=2*L[n-1]-L1[n-1]+R2[n] R1[n_]:=R1[n]=R1[n-1]+4*(EulerPhi[n-1]-e[n]) e[n_]:=If[Mod[n, 2]==0, 0, EulerPhi[(n-1)/2]] R2[n_]:= If[Mod[n, 2]==0, (n-1)*EulerPhi[n-1], If[Mod[n, 4]==1, (n-1)*EulerPhi[n-1]/2, 0]] Table[L[n], {n, 0, 37}] (* Seppo Mustonen, Apr 25 2009 *) CROSSREFS Cf. A222267 (lines defined by n X n X n grid of points). A288187 is the main entry for these graphs. Sequence in context: A212689 A027984 A309294 * A027107 A247307 A279215 Adjacent sequences:  A018805 A018806 A018807 * A018809 A018810 A018811 KEYWORD nonn,nice AUTHOR STATUS approved

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Last modified October 21 12:27 EDT 2020. Contains 337914 sequences. (Running on oeis4.)