       Module[ ... ] anomoly??

• To: mathgroup at smc.vnet.net
• Subject: [mg3666] Module[ ... ] anomoly??
• From: "Stephen L. McConnell" <scawa at intersurf.com>
• Date: Tue, 2 Apr 1996 21:56:44 -0500
• Organization: Louisiana State University Dept of Chem
• Sender: owner-wri-mathgroup at wolfram.com

```I have written a series of instructions which plot the vector field of a
differential equation and then plot solution curves over that vector
field.  I am attempting to put it in a Module with the hope of creating
a Differential Equations package for use with C.H. Edwards and David
Penney's book on Differential Equations and Boundary Value Problems.

I appear to have an anomoly when I try to modularize the solution.

The first few lines which create the vector field are

f[x_,y_]=-y-Sin[x]
p1=PlotVectorField[{1,f[x,y]},
{x,-3,3},
{y,-3,3},
Frame->True,
Axes->True]

These lines use the PlotVectorField package from Wickham-Jones
"Mathematica Graphics" book.  The vector field works just as advertised
and my solution curves when I solve the Diffeq are right on.

However when I modularize it and use the following lines:

Needs["Graphics`PlotField`"]
test[fun_,xmin_,xmax_,ymax_,ymin_]:=
Module[{p1,x,y,i,a},
p1=PlotVectorField[{1,fun[x,y]},
{x,xmin,xmax},
{y,ymin,ymax},
Frame->True,
Axes->True];
]

g[x_,y_]:=-y-Sin[x]

test[g,-3,3,-3,3]

The arrows on the vector field are exactly the opposite direction, and
within the module, NDSolve cannot solve the diffeq... Interpolating
fuction says the value is larger than can be represented.

Why is the vector field oposite in direction?
How can I write a general module or fuction which will allow me to use
as a calling parameter my function ei:

test[-y-Sin[x],-3,3,-3,3]

Thanks for the help in advance.
--
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```

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