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# Classroom Tips and Techniques: A Note on Parametric Plotting

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Classroom Tips and Techniques: A Note on Parametric Plotting

Robert J. Lopez

Emeritus Professor of Mathematics and Maple Fellow

Maplesoft

Introduction

Recently, I was asked how to draw a curve defined parametrically by the two equations , in the case where explicit solutions cannot be obtained. Of course, when such explicit functions can be written, Maple's plot command will suffice. This article describes two approaches for the case when remain defined implicitly.

Method 1

One approach to obtaining a point on the curve is to set , constant, in , and solve numerically for and . Of course, the devil is always in the details - for some functions manual intervention might be necessary to insure that Maple's fsolve command can find such a solution.  And drawing a graph requires multiple points to be found, so this numeric solving has to be iterated.

Method 2

Assuming that the equations , define a function for a suitable -interval, the identities

can be differentiated with respect to to generate the ordinary differential equations

Initial conditions at can be determined by solving , for . Then, Maple's dsolve command can be used to generate a numeric solution of the resulting initial value problem, and its odeplot command can be used to graph the resulting solution. The options provided in these two commands are robust enough to simplify the work of obtaining a graph based on numeric calculations.

Initializations

Example 1

We begin with a simple example that has an explicit resolution. Let the curve be defined implicitly by the equations

Of course, the first equation defines a circle with center at the origin and radius , while the second defines a line with slope that passes through the origin.  Figure 1 provides an animation of these two curves whose intersection defines the points on the curve given parametrically by

Figure 1   As varies, the intersection of the line and the circle defines the curve

For an implicit representation of the curve Maple provides

For an explicit parametric representation of this curve, Maple provides

In Figure 2, a graph of one branch of this curve (in green) is added to the animation in Figure 1.

Figure 2   One branch of drawn in green

In Figure 2, the curve, defined parametrically (but explicitly), is graphed with the plot command, using the appropriate syntax for a parametric plot, namely, a list of the form

Example 2

The equations

can be solved for explicitly, but they are the equations that accompanied the user's question. These explicit functions are given by

from which Figure 3, a graph of the parametrically defined curve, can be drawn.

Figure 3   Parametric curve drawn from the explicit

A strictly numeric solution of this graphing problem requires iterating the numeric solution of the equations , for specific values of . Let's look at the requisite calculations for

Since we want to draw a graph, we need to put the solution into the form

We can tediously repeat these calculations for additional values of , or we can execute them in the following loop.

Once all the points have been computed, they are stored in the list . Figure 4 provides a graph of these points, along with a linear interpolant. To improve this graph, we would have to compute at more values of , but as approaches 1, the spacing between the computed points increases. The most efficient way to improve Figure 4 is to use a nonuniform grid, but that adds to the computational complexity.

Figure 4   Numerically computed points for uniformly spaced values of

Example 3

Using the equations from Example 2, we next obtain the desired graph by solving a system of differential equations. To this end, write the equations with the parameter-dependence of and explicitly stated.

Then, differentiate both equations with respect to the parameter

Determine initial conditions, typically at the starting value of the parameter. We use after experimenting with where the first equation reduces to . A numeric solution cannot be generated from this point, so we stepped ever so slightly away from this singularity.

Figure 5 shows the result of plotting the numerically calculated solution of the differential equations representing the given system of algebraic equations. Refining the graph is much easier with the combination of the dsolve and odeplot commands. The range option in dsolve allows us to use the refine option in odeplot, thereby automatically increasing the number of points used in the plot. Setting causes odeplot to use all stored points for drawing the graph.

Figure 5   Parametric plot obtained as a solution of a system of ODEs

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