A178794 These are the x coordinates of the isolated visible lattice points in the plane.

21, 99, 115, 369, 495, 475, 195, 259, 265, 225, 375, 741, 741, 649, 323, 377, 399, 1001, 1001, 441, 987, 609, 755, 1001, 545, 645, 1035, 407, 1275, 153, 645, 1275, 51, 1221, 485, 35, 805, 715, 441, 595, 1015, 221, 1001, 1183, 371, 391, 575, 519, 645, 1065

Offset: 1

Author

Gregg Whisler, Jun 15 2010

Keywords

• nonn

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.

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

Crossrefs

Cf. A178793 (corresponding y coordinates), A157428, A157429, A216467.

Programs

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,2000}][[All, 1]] (* Eric Rowland *)

Extensions

More terms (until the initial 21 repeats) from Gregg Whisler, Jun 21 2010