OFFSET
1,6
COMMENTS
a(n) is also the number of pairs of points visible to each other exactly through 4 points in an n x n grid of points.
LINKS
FORMULA
See Mathematica code.
MATHEMATICA
j=5;
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
KEYWORD
nonn
AUTHOR
Seppo Mustonen, May 13 2010
STATUS
approved