diffyq

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1.  dirfld( )     This graphs a direction field for a first-order

differential equation in the form dx/dt = f(t,x).  The user is asked to type

in the right-hand side of the equation and has the option to set the graphing

window from within the program.  Once the direction field has been plotted,

the program pauses.  Pressing enter brings a menu that allows the user to

give an initial value, change the window, give a new equation, or end the

program.  If initial value is chosen, a solution curve through that point

will be drawn with either a thick or think pen.  Multiple solution curves can

be graphed by specifying different initial values.



2.  bigdot( )    This is a subroutine that allows solution curves to be

graphed with a thick pen.



3.  twodir( )    This is a similar to dirfld( ), except that it is used with

a system of two first-order equations:  dx/dt = f(x,y) and dy/dt = g(x,y).



4.  phaser( )   Given a system of two first-order equations, the program will

produce two graphs side by side.  First is the time series graph, both

functions x(t) and y(t) are graphed against the independent variable t.  The

second graph is a trajectory in the phase plane with x on the horizontal

axis, y on the vertical.  The user will specify the right-hand sides of the

two equations, window settings for both graphs, and an initial point.  When

graphing the phase plane, the calculator will pause to allow the user to

decide whether or not to continue graphing.  If NO is selected, the

calculator pauses again.  A final enter will restore the screen to full

screen and erase the graphs.