A178793 These are the y coordinates of isolated visible lattice points in the plane.

55, 175, 551, 575, 589, 609, 679, 741, 741, 791, 799, 805, 875, 945, 987, 987, 995, 1035, 1065, 1105, 1105, 1159, 1209, 1275, 1309, 1309, 1309, 1407, 1421, 1463, 1463, 1463, 1495, 1495, 1551, 1581, 1581, 1611, 1625, 1639, 1651, 1665, 1665, 1665, 1695

Offset: 1

Author

Gregg Whisler, Jun 15 2010

Keywords

• nonn

Comments

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

Crossrefs

Cf. A157428, A157429, A178794 (corresponding x coordinates), 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, 2]] (* Eric Rowland *)

Extensions

More terms (until the corresponding first x coordinate (21) repeats) from Gregg Whisler, Jun 21 2010