%I #8 Dec 31 2012 08:59:49
%S 21,99,115,369,495,475,195,259,265,225,375,741,741,649,323,377,399,
%T 1001,1001,441,987,609,755,1001,545,645,1035,407,1275,153,645,1275,51,
%U 1221,485,35,805,715,441,595,1015,221,1001,1183,371,391,575,519,645,1065
%N These are the x coordinates of the isolated visible lattice points in the plane.
%C 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.
%C Contribution from Gregg Whisler, Jun 21 2010: (Start)
%C a(n) is also A157429 + 1.
%C These are also the x coordinates of the isolated visible lattice points in Z^2. (End)
%H Eric W. Weisstein, <a href="http://mathworld.wolfram.com/VisiblePoint.html">MathWorld: Visible Point</a>
%t 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
%Y Cf. A178793 (corresponding y coordinates), A157428, A157429, A216467.
%K nonn
%O 1,1
%A _Gregg Whisler_, Jun 15 2010, Jun 22 2010
%E More terms (until the initial 21 repeats) from Gregg Whisler, Jun 21 2010