A178794 - OEIS

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

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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.

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

Table of n, a(n) for n=1..50.

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

CROSSREFS

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

Sequence in context: A200255 A212406 A288750 * A140370 A124949 A267681

Adjacent sequences: A178791 A178792 A178793 * A178795 A178796 A178797

KEYWORD

nonn

AUTHOR

Gregg Whisler, Jun 15 2010

EXTENSIONS

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

STATUS

approved