A178793 - OEIS

%I #19 Mar 05 2015 15:14:02

%S 55,175,551,575,589,609,679,741,741,791,799,805,875,945,987,987,995,

%T 1035,1065,1105,1105,1159,1209,1275,1309,1309,1309,1407,1421,1463,

%U 1463,1463,1495,1495,1551,1581,1581,1611,1625,1639,1651,1665,1665,1665,1695

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

%C Contribution from Gregg Whisler, Jun 21 2010: (Start)

%C 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]

%C 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.

%C (End)

%H Eric W. Weisstein, <a href="http://mathworld.wolfram.com/VisiblePoint.html">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}]] (* _Eric Rowland_ *)

%Y Cf. A157428, A157429, A178794 (corresponding x coordinates), A216467.

%K nonn

%O 1,1

%A _Gregg Whisler_, Jun 15 2010

%E More terms (until the corresponding first x coordinate (21) repeats) from _Gregg Whisler_, Jun 21 2010