## Instructions for using the Differential Equations Applet |

- Entering/editing differential equations
- Mathematical functions reference
- Mathematical operators reference
- Setting variable/parameter ranges
- Zooming into a region
- Grid controls
- Display controls
- Adding initial conditions
- Plotting solutions
- Choosing the axes of the plot
- Viewing solution tables
- Printing from the Applet
- Detaching the Applet frame from the browser window
- Saving and Loading JOde data
- Advanced Topic: Downloading and running the applet locally
- Advanced Topic: Controlling the applet from HTML
- Advanced Topic: Precedence of operators
- Advanced Topic: User-defined functions and flow-control statements
- Advanced Topic: Documentation auto-generated by Javadoc utility
- Advanced Topic: The file format used by JOde "Save" command
- Recent Changes

- Enter the right hand side of the equation into the area after
eqn #k: dy/dx=

The number*k*is used to identify the equation number in error messages, so that possible problems can be traced to a particular equation. In the default mode, the applet has only one equation. Equation #0, if present, contains initialization code. This code is evaluated only once after its change. It is typically used to define constants and functions. - Any expression in variables
*x*and*y*is allowed, using operators +,-,* and /, and mathematical functions from the following list: sin, cos, tan, arctan, arcsin, exp, ln, sqrt, abs, min, max (these last two are binary). Mathematical constants "E", "PI" and "Infinity" are also available. Examples of valid expressions: - sin(x)/y
- y^2*sin(x)
- min(x,y)
- abs(x^2-y)
- sin(PI*y)
- exp(y)
- E^y
- The equation is entered into the Applet by hitting Return or by pressing the "Submit All" button. Immediately after submission, the slope field and solutions are going to be plotted.

- sin(x)
- Returns sine of x.
- cos(x)
- Returns cosine of x.
- tan(x)
- Returns tangent of x.
- artcan(x)
- Returns inverse tangent of x.
- arcsin(x)
- Returns inverse sine of x.
- abs(x)
- Returns absolute value of x, i.e. x for x>=0 and -x for x< 0.
- exp(x)
- Returns the exponential of x.
- ln(x)
- Returns the natural logarithm of x.
- sqrt(x)
- Returns the square root of x for x>=0. If x< 0
- min(x, y)
- Returns x if x< y and y otherwise.
- max(x, y)
- Returns y if x< y and x otherwise.
- step(x)
- Returns 0 for x < 0 and 1 for x >= 0.
- sgn(x)
- Returns -1 for x < 0, 0 if x=0 and 1 for x > 0.
- iffun(x, y, z)
- Returns y for x is not equal to 0.0, z otherwise. Typically x is written
as a logical expression (see operator reference).
**Note**: This is a true function, i.e. it evaluates all its arguments (x, y, and z) before returning its value.

- E
- The base of natural logarithm.

- PI
- The circumference of a circle of diameter 1.

- Infinity
- Positive infinity.

In addition to the ordinary arithmetical operators +, -, *, /, the following operators are available for use in expressions:

- x^y
- Has a value of x raised to power y.
- x < y
- Has a value of 1.0 if x is less than y and 0.0 otherwise.
- x <= y
- Has a value of 1.0 if x is less or equal to y and 0.0 otherwise.
- x > y
- Has a value of 1.0 if x is greater than y and 0.0 otherwise.
- x >= y
- Has a value of 1.0 if x is greater or equal to y and 0.0 otherwise.
- x == y
- Has a value of 1.0 if x is equal to y and 0.0 otherwise.
- x != y
- Has a value of 1.0 if x is not equal to y and 0.0 otherwise.
- x && y
- Has a value of 1.0 if x and y are different from 0.0. Otherwise it has a value of 0.0.
- x || y
- Has a value of 1.0 if either x or y is different from 0.0. Otherwise it has a value of 0.0.
- x; y
- Has a value of y. The expression x is evaluated for its side effects.
Typically x is an assignment, as in
k=7; k^3-k^2-1

Click your mouse in the canvas area to select an initial condition. A solution passing through the indicated initial condition will be plotted. You can also select the initial condition precisely by typing in values of x and y. This method is described in this section.

**Note:** If there are more variables than 2, only two variables will
be set based on the position of the mouse. The remaining variables will be
set to 0.

The applet tabulates numerical solutions of the differential equations. You can view tabulated values for each initial conditions in a window by clicking on the button labelled "Show table". A window entitled "Solution table" will pop up with all numerically generated solutions, in a form resembling tables in standard textbooks on numerical methods. One can use Cut-And-Paste to copy these tables into a text editor or word processor. The applet generates a solution table every time it recalculates the solution. Old solution tables can be erased by pressing the "Clear" button of the table window.

These are entered into areas labeled as "Min. *x*", "Max. *x*",
"Min. *y*" and "Max*. y*" etc. They are not used in calculations,
until they are entered by either pressing the Return key, or by pressing the
"Submit All" button.

The sizes of the grid in the *x-* and *y-* and other directions
are entered in areas marked as "Num. of segs" . They become effective upon
either hitting the Return key (one item at a time) or by pressing the "Submit
All" button, which updates all input related to entering the formula for dy/dx,
the ranges and the number of grid intervals.

- Slopes
- If checked, slopes are displayed.
- Solutions
- If checked, solutions with selected initial conditions are displayed.
- Init. Conditions
- If checked, every initial condition is marked and labeled.
- Euler/ModifiedEuler/Midpoint/Runge-Kutta/RK4/Runge-Kutta-Fehlberg/RKF
- Selects the algorithm used in calculating solutions.
- Step
- Displays and sets the integration step. Integration step is set automatically
in the following circumstances:
- the number of intervals (fields following "Num. of intervals") is set;
- the "Submit All" button is pressed.

The Applet maintains a list of initial conditions whose solutions will be plotted. You can:

- Add an initial condition by setting the fields following the label "Add init. cond.:". There is a field corresponding to each variable.
- Remove all initial conditions and start anew by pressing the "Clear All" button.
- Initial conditions can be added by clicking them with the mouse.

All solutions displayed by the applet can be tabulated by pressing the "Show table" button in the initial condition entry group of buttons. Tables are displayed in a separate window. The table can be edited or copied into another application (e.g. text editor). This is achieved by first selecting the data to be copied (typically with a mouse) and then with the usual Control-C (Cut) and Control-V (Paste) key combination, or other system-specific Cut/Paste keys. The table data can also be printed directly from the applet by selecting "Pring" from the "File" menu. Further necessary information on printing from the applet is contained in this section.

The best general advice to avoid problems with printing (due to
various bugs in major browsers and Java implementations) is to
*accept the certificate*. The dialog to accept the certificate
should pop up automatically when the page containing the applet is
loaded for the first time. Another method primarily geared towards
older Netscape browsers, is given here.

The page containing the Applet can be printed with all major browsers. However, there are some browser peculiarities. The problem is especially aggravating with Microsoft Internet Explorer, because the applet window is reset to its original state (i.e, your data is erased ):- ) before printing. Thus, in order to print, manipulating the security settings of the browser is required, or downloading a certificate file, which certifies the applet's authenticity. Unfortunately, the number of various browser version combinations and strategies is too numerous to discuss in detail. The included HTML files should take care of making the choice for various Netscape and Internet Explorer versions. The good news is that for versions 6.0+ of the two major browsers the only required action to print is accepting the certificate. Other strategies are explained below.

If you simply use the browsers "Print" button to print the version of the Applet embedded into the Web page, Internet Explorer will initialize the Applet before printing, and thus any changes, initial conditions, etc. will be lost. Therefore you must use the built-in "Print" button of the Applet, or the "Print" from the "File" menu of the Applets's window if running the Applet in a separate frame. You must also accept my security certificate when the Applet loads first by pressing OK when this dialog pops up.

Another approach to printing is by editing the Applet's page with a text editor to enter the Applet parameters directly. Just edit the Applet's Web page with your favorite HTML editor or a text editor (my preferred way).

In order to save the applet data (the formulas you typed in, the initial conditions, algorithms settings etc.) follow this procedure:

- Detach the applet from the browser window if necessary, by pressing the "Frame" button.
- From the "File" menu, select "Save".
- When a file selection dialog pops up, select the name of the file to save the data to.
- Confirm the selection by pressing "OK" in the file dialog pop-up window, or in a similar, system-specific manner.

In order to load the applet from the file you just created, follow these steps:

- Start the applet in the usual manner, in a framed mode. For instance, you can select this link. It does not matter what the content of the applet window is at this point.
- From the "File" menu, select "Load".
- From the resulting file dialog, select the name of the file containing previously saved data.
- Confirm the selection. During this step you will
*overwrite*the current applet data with the data stored in the file, so make sure you want to do this.

These are typically generated by typing in an expression (see section on entering differential equations) which does not conform to the syntax rules of the Applet. The most recent error message appears in the box labeled "Last error". It will hopefully identify the problem by pointing to the column in which the first error occurs. If not, by pressing the button labelled "Show All Errors" all error messages can be examined and used to correct the problem.

- this JOdeApplet.jar jar archive

- index.html
- JOdeApplet.html
- JOdeApplet2D.html
- JOdeApplet3D.html
- ElksCoyotesAndWolves.html
- manual.html
- argdoc.html

java -classpath directory_prefix/(or\)JOdeApplet.jar com.rychlik.jode.JOde "dimension=2" "autonomous=true" ...With the newer versions of Java you will be able to start the applet by executing the

java -jar JOdeApplet.jar "dimension=2" "autonomous=true" ...You can configure the applet by using several "parameter=value" options. All optiona are described in the parameters section. For instance "dimension=2" and "autonomous=2" set the number of differential equations to 2, and the equations are autonomous, i.e. time independent, and thus there is no need to display the independent variable axis.

All settings of the Applet can be set in the containing HTML document by passing parameters to the Applet. Currently, the applet recognizes these parameters, corresponding closely to the quantities which can be set from the graphical user interface (GUI).

- *, /
- +, -
- <, >, <=, >=, ==, !=
- &&, ||
- =
- ;

C=7; function f(x, y) = x + y + C

defines a function which, when called with two arguments, returns their sum plus 7. It is worth noting that every statement of the JOde language is an expression. Function definition expression above returns a value of 0 when evaluated. The function is defined as a side effect.x=1; y=2; x+y

is an expression whose value is 3.x=2; if(x<3) 7 else 1

returns 7.x=1; while(x<10) x=x+2

evaluates to 11. JOde supports " continue" and "break" statements in a way similar to C and Java. However, "continue" and "break" with no argument causes the value of the entire loop to be 0. If given an argument to " break", the entire loop evaluates to that argument. If "continue" is called with an argument, and the next test evaluates to false (i.e. 0.0) then the value of the entire loop will be the value given as an argument to "continue". Thusx=1; while(x<10) x=x+2; if(x==5) break x

returns 5 andx=1; while(x<10) x=x+2; if(x==5) break

return 0. In the following example:x=1; while(x<2) if(x==1) x=3; continue 7; 5

the value of the expression is 7. The semi-colon has very low precedence, and thus in the following examplex=1; while(x<2) if(x==1) x=3; continue (7; 5)

the return value is 5 because of the explicit parenthesis.This software is written in Java, of course. The documentation of a Java program is typically generated as HTML with the help of Javadoc. For those interested in the architecture of our program, here it is.

The files saved by JOde are in the generic format produces by object serialization (Serialization API). It is customary to use the ".ser" extension for these files, and we follow this convention. The saved files will be compatible with future minor revisions of the applet. A major revision may make the saved applets obsolete within a few years. Therefore, if a substantial amount of work was put into a given ODE example, the compatible version of the software (i.e. the file jode.zip) should be kept around.

- A major addition has been the ability to save the applet to a file and load the saved files.
- Switching between framed and unframed versions of the applet can be done by pressing the "Frame" button. Therefore, the links to the framed versions were dropped. The old "*Framed*.html" files still exist in the distribution for backwards compatibility of the Web site.
- The classes in the JAR file now belong to packages. Thus, you need to update your old HTML files which use the applet by replacing "JOdeApplet.class" with "com/rychlik/jode/JOdeApplet.class". Also, the command to run the applet from command line changed (see this section for details).
- Jason Miller's tutorial on using the Applet is part of the distribution now. He also contributed the style sheet used for all pages included with this package.
- The performance improved significantly, using the new JDK 1.4.1 from Sun. The reson for the improvement is not any drastic changes in the code but the fact that the "-O" option actually works now. The byte code produced should be compatible with Java 1.1 specification, which means that the applet should work with older browsers. However, I suspect that the mileage may vary in this respect.

Enjoy,

Marek Rychlik (rychlik@u.arizona.edu)