The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

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