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A018808 Number of lines through at least 2 points of an n X n grid of points. 11
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)

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.

S. Mustonen, On lines and their intersection points in a rectangular grid of points

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).

Sequence in context: A109164 A212689 A027984 * A027107 A247307 A279215

Adjacent sequences:  A018805 A018806 A018807 * A018809 A018810 A018811

KEYWORD

nonn,nice

AUTHOR

David W. Wilson

STATUS

approved

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Last modified November 13 13:15 EST 2018. Contains 317149 sequences. (Running on oeis4.)