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Devil's curve
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<iframe 
    src="https://jsxgraph.uni-bayreuth.de/share/iframe/devils-curve" 
    style="border: 1px solid black; overflow: hidden; width: 550px; aspect-ratio: 55 / 65;" 
    name="JSXGraph example: Devil's curve" 
    allowfullscreen
></iframe>
This code has to 
<div id="board-0-wrapper" class="jxgbox-wrapper " style="width: 100%; ">
   <div id="board-0" class="jxgbox" style="aspect-ratio: 12/ 10; width: 100%;" data-ar="12/ 10"></div>
</div>

<script type = "text/javascript"> 
    /*
    This example is licensed under a 
    Creative Commons Attribution 4.0 International License.
    https://creativecommons.org/licenses/by/4.0/
    
    Please note you have to mention 
    The Center of Mobile Learning with Digital Technology
    in the credits.
    */
    
    const BOARDID = 'board-0';

    const board = JXG.JSXGraph.initBoard(BOARDID, {
        boundingbox: [-5, 5, 7, -5],
        axis: true
    });
     
    var a = board.create('slider', [
        [1, 4], [4, 4], [0, 1.5, 3]
    ], {
        name: 'a'
    });
    var b = board.create('slider', [
        [1, 3.5], [4, 3.5], [0, 1, 3]
    ], {
        name: 'b'
    });
     
    var c = board.create('curve', [
      (theta) => Math.sqrt( (b.Value()**2 * Math.sin(theta)**2 - a.Value()**2 * Math.cos(theta)**2 ) /
                     (Math.sin(theta)**2 - Math.cos(theta)**2 ) ),
        [0, 0],
        0, 2 * Math.PI
    ], {
        strokeWidth: 3,
        curveType: 'polar'
    });
 </script> 
/*
This example is licensed under a 
Creative Commons Attribution 4.0 International License.
https://creativecommons.org/licenses/by/4.0/

Please note you have to mention 
The Center of Mobile Learning with Digital Technology
in the credits.
*/

const BOARDID = 'your_div_id'; // Insert your id here!

const board = JXG.JSXGraph.initBoard(BOARDID, {
    boundingbox: [-5, 5, 7, -5],
    axis: true
});
 
var a = board.create('slider', [
    [1, 4], [4, 4], [0, 1.5, 3]
], {
    name: 'a'
});
var b = board.create('slider', [
    [1, 3.5], [4, 3.5], [0, 1, 3]
], {
    name: 'b'
});
 
var c = board.create('curve', [
  (theta) => Math.sqrt( (b.Value()**2 * Math.sin(theta)**2 - a.Value()**2 * Math.cos(theta)**2 ) /
                 (Math.sin(theta)**2 - Math.cos(theta)**2 ) ),
    [0, 0],
    0, 2 * Math.PI
], {
    strokeWidth: 3,
    curveType: 'polar'
});
<jsxgraph width="100%" aspect-ratio="12/ 10" title="Devil's curve" description="This construction was copied from JSXGraph examples database: BTW HERE SHOULD BE A GENERATED LINKuseGlobalJS="false">
   /*
   This example is licensed under a 
   Creative Commons Attribution 4.0 International License.
   https://creativecommons.org/licenses/by/4.0/
   
   Please note you have to mention 
   The Center of Mobile Learning with Digital Technology
   in the credits.
   */
   
   const board = JXG.JSXGraph.initBoard(BOARDID, {
       boundingbox: [-5, 5, 7, -5],
       axis: true
   });
    
   var a = board.create('slider', [
       [1, 4], [4, 4], [0, 1.5, 3]
   ], {
       name: 'a'
   });
   var b = board.create('slider', [
       [1, 3.5], [4, 3.5], [0, 1, 3]
   ], {
       name: 'b'
   });
    
   var c = board.create('curve', [
     (theta) => Math.sqrt( (b.Value()**2 * Math.sin(theta)**2 - a.Value()**2 * Math.cos(theta)**2 ) /
                    (Math.sin(theta)**2 - Math.cos(theta)**2 ) ),
       [0, 0],
       0, 2 * Math.PI
   ], {
       strokeWidth: 3,
       curveType: 'polar'
   });
</jsxgraph>

Devil's curve

Curves
Equation: \[ y^{2}(y^{2}-b^{2})=x^{2}(x^{2}-a^{2}). \] Polar form: \[ r= \sqrt {\frac {b^{2}\sin ^{2}\theta -a^{2}\cos ^{2}\theta }{\sin ^{2}\theta -\cos ^{2}\theta }}. \]
// Define the id of your board in BOARDID

const board = JXG.JSXGraph.initBoard(BOARDID, {
    boundingbox: [-5, 5, 7, -5],
    axis: true
});
 
var a = board.create('slider', [
    [1, 4], [4, 4], [0, 1.5, 3]
], {
    name: 'a'
});
var b = board.create('slider', [
    [1, 3.5], [4, 3.5], [0, 1, 3]
], {
    name: 'b'
});
 
var c = board.create('curve', [
  (theta) => Math.sqrt( (b.Value()**2 * Math.sin(theta)**2 - a.Value()**2 * Math.cos(theta)**2 ) /
                 (Math.sin(theta)**2 - Math.cos(theta)**2 ) ),
    [0, 0],
    0, 2 * Math.PI
], {
    strokeWidth: 3,
    curveType: 'polar'
});

license

This example is licensed under a Creative Commons Attribution 4.0 International License.
Please note you have to mention The Center of Mobile Learning with Digital Technology in the credits.