Re: Re: Can Mathematica Do This?

• To: mathgroup at smc.vnet.net
• Subject: [mg8279] Re: [mg8181] Re: Can Mathematica Do This?
• From: seanross at worldnet.att.net
• Date: Sat, 23 Aug 1997 01:59:22 -0400
• Sender: owner-wri-mathgroup at wolfram.com

```Fahad A Hoymany wrote:
>
> :      Before I go on to investigate this problem, and because I'm not
> : familiar with the capability of Mathematica, could someone tell me if
> : Mathematica can:
>
> : 1) Find the partial derivative of a function of three variables, f(x,y,z),
> :    with respect to each variable?
> : 2) Find the minimum (x,y,z) for f?
>
> :    The function f is not very complicated, involving only first and second
> : order expressions in x,y, and z.
>
> :    I'd very much appreciate a "yes" or "no" answer.
>
>     Thanks to those who e-mailed me with some suggestions.  Although, I am
> now a little more familiar with Mathematica, the problem I need to solve
> seems to be too tough for Mathematica, even though it "looks" like a simple
> problem.  I tried FindMinimum, NDsolve, among other attempts, and I either
> get an error of the form ".... something is not a real number", or
> Mathematica takes the command but never returns an answer!  Here is the
> problem in case some kind soul wants to give it a try:
>
>   Find the minimum of f in c and h
>           f = (c d1+d2) r (2/((c+1) (h-c+1))) + d3 h/2 +
>               (c d4+d5) r (2/((c+2)*(h-c+1)))
>
>   Note that c <= h, and c,h >=0.  If it helps, h < 10,000.  The d's are
> real numbers in the range 0 to 1.0, and r is real in the range 0 to
> 100,000.
>
> Best regards

The "not a real number error" is a generic error message that may mean:
-there are symbols not assigned values in your expression
-there are complex numbers in your expression
-there are text messages in the evaluation of your expression like
ComplexInfinity
-there is a syntax error that doesn't immediately register as such.
-there is something that evaluates to a transformation rule, an
InterpolationFunction object or a list in the evaluation of your
expression.

Before giving up, evaluate your function and see what kind of object it
returns, then try a single derivative and see what it returns, then try
graphing it versus one, then two dimensions.  Try out each command you
use with something you know the answer to to make certain you understand
the syntax as well as the kind of object the Mathematica function
returns.  In some cases, the output of one function must be massaged a
bit before being able to be used as the input to something else.  Good
Luck.

```

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