OFFSET
1,1
COMMENTS
An isolated lattice point is surrounded (in a Moore neighborhood) by 8 points that are not visible from the origin. I have also submitted the corresponding sequence of denominators.
Contribution from Gregg Whisler, Jun 21 2010: (Start)
a(n) is also A157429 + 1.
These are also the x coordinates of the isolated visible lattice points in Z^2. (End)
LINKS
Eric W. Weisstein, MathWorld: Visible Point
MATHEMATICA
Table[Replace[Select[First/@Position[Partition[CoPrimeQ[n, Range[n]], 3, 1], {False, True, False}, {1}]+1, Outer[CoPrimeQ, n+ {-1, 1}, #1+{-1, 0, 1}]=={{False, False, False}, {False, False, False}}&], {{}-> Sequence[], list_:->Sequence@@ ({#1, n}&)/@list}], {n, 5000}]] program by Eric Rowland
CROSSREFS
KEYWORD
nonn
AUTHOR
Gregg Whisler, Jun 15 2010, Jun 22 2010
EXTENSIONS
More terms (until the initial 21 repeats) from Gregg Whisler, Jun 21 2010
STATUS
approved