OFFSET
1,1
COMMENTS
Contribution from Gregg Whisler, Jun 21 2010: (Start)
a(n) is also A157428 + 1. [Charles R Greathouse IV points out that this is false, since (1308, 1274) is in (A157428, A157429) but not in (A178793, A178794). Oct 17 2012]
An isolated lattice point is surrounded (in a Moore neighborhood of r=1) in the Z^2 lattice of points by 8 points that are not visible from the origin.
(End)
LINKS
Eric W. Weisstein, 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}]] (* Eric Rowland *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Gregg Whisler, Jun 15 2010
EXTENSIONS
More terms (until the corresponding first x coordinate (21) repeats) from Gregg Whisler, Jun 21 2010
STATUS
approved