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A178794
These are the x coordinates of the isolated visible lattice points in the plane.
2
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
1,1
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.
Contribution 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)
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}]] program by Eric Rowland
CROSSREFS
Cf. A178793 (corresponding y coordinates), A157428, A157429, A216467.
Sequence in context: A200255 A212406 A288750 * A140370 A124949 A267681
KEYWORD
nonn
AUTHOR
Gregg Whisler, Jun 15 2010, Jun 22 2010
EXTENSIONS
More terms (until the initial 21 repeats) from Gregg Whisler, Jun 21 2010
STATUS
approved