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plotting - How to find and count the points that are elements of ${(x,y)inmathbb{Z}^2:x^2+y^2le16 wedge |y|gt x}$?


I am given the following set:


$\qquad \{(x,y)\in\mathbb{R}^2:x^2+y^2\le16 \wedge |y|\gt x\}$


I want find and count the number of points in this set that have integer coordinates.


I drew this to help me visualize the problem.



Plot


ClearAll["Global`*"];
Plot[{y /. Solve[x^2 + y^2 == 16, y], x, -x}, {x, -5, 5},
PlotStyle -> {Solid, Dashed, Dashed},
AspectRatio -> 1,
PlotRange -> {{-5, 5}, {-5, 5}},
PlotTheme -> "Detailed",
GridLines -> {{-4, -3, -2, -1, 0, 1, 2, 3, 4}, {-4, -3, -2, -1, 0, 1, 2, 3, 4}}]

I want to mark all of those points which are inside the set red as well as count them.



What should I do?



Answer



Just solve it:


sol = Solve[x^2 + y^2 <= 16 && Abs[y] > x, {x, y}, Integers];

Get how many:


Length@sol

Make the points (set point size or use Disk if you want bigger):


gr = Graphics[{Red, Point[{x, y}] /. sol}];


Then show them together:


Show[{Plot[{y /. Solve[x^2 + y^2 == 16, y], x, -x}, {x, -5, 5}, 
PlotStyle -> {Solid, Dashed, Dashed}, AspectRatio -> 1,
PlotRange -> {{-5, 5}, {-5, 5}}, PlotTheme -> "Detailed",
GridLines -> {{-4, -3, -2, -1, 0, 1, 2, 3, 4}, {-4, -3, -2, -1, 0,
1, 2, 3, 4}}], gr}]

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