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A177725 Number of line segments connecting exactly 9 points in an n x n grid of points 0
0, 0, 0, 0, 0, 0, 0, 0, 20, 48, 84, 128, 180, 240, 308, 384, 504, 640, 792, 960, 1144, 1344, 1560, 1792, 2144, 2528, 2944, 3392, 3872, 4384, 4928, 5504, 6248, 7040, 7880, 8768, 9704, 10688, 11720, 12800, 14264, 15808, 17432, 19136, 20920, 22784, 24728, 26752 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,9

COMMENTS

a(n) is also the number of pairs of points visible to each other exactly through 7 points in an n x n grid of points.

LINKS

Table of n, a(n) for n=1..48.

S. Mustonen, On lines going through a given number of points in a rectangular grid of points

FORMULA

See Mathematica code.

MATHEMATICA

j=8;

a[n_]:=a[n]=If[n<=j, 0, 2*a1[n]-a[n-1]+R1[n]]

a1[n_]:=a1[n]=If[n<=j, 0, 2*a[n-1]-a1[n-1]+R2[n]]

R1[n_]:=R1[n]=If[n<=j, 0, R1[n-1]+4*S[n]]

R2[n_]:=(n-1)*S[n]

S[n_]:=If[Mod[n-1, j]==0, EulerPhi[(n-1)/j], 0]

Table[a[n], {n, 1, 50}]

CROSSREFS

Sequence in context: A234259 A135286 A338235 * A264444 A053245 A115882

Adjacent sequences:  A177722 A177723 A177724 * A177726 A177727 A177728

KEYWORD

nonn

AUTHOR

Seppo Mustonen, May 13 2010

STATUS

approved

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Last modified September 18 12:13 EDT 2021. Contains 347527 sequences. (Running on oeis4.)