Sascha Rissel on Thu, 02 Jun 2005 12:42:42 +0200

 User functions and their derivatives

```Hello,

I am new to this mailing list and hope, that this is the right place to post my question about Pari.

I got to know Pari at university in a lecture about cryptography. Since then I got to appreciate Pari a lot, not only for its number theoretical capabilities, but also for those in 'classical' mathematics.
So, besides studying computer science, I also work with pupils as a teacher for private lessons.
I often use Pari in analysis concerns, like plotting (e.g. polynomial) functions, solving equations etc...

It all works well, but there's one thing, that I did not figure out how it works yet.

Let's say I define a function (let's choose a simple one)
> f(x) = x^2

I can compute values of this function, like
> f(3)
and get 9 in this case as a result.

Now I would like to derive this function, so I define a function f1 for the derivative of f:
> f1(x) = f(x)' (or f1(x) = deriv(f(x))

The problem that occurs now is, that I am not able to use the derivative f1(x) for computation.
Let's say I want to compute some values of f1:
> f1(3)
The result I get is always 0 and not the expected value 6 (in this example), 'cause the derivative is 2*x of course.
Also plotting of f1(x) does not work. I think this is all the same problem.

So what am I doing wrong?

I have another small question:
If I define variables or functions (like in the case above), how can I undefine these variables and functions, to be able to use the names for other concerns?

Thanks a lot for your help!

Sascha.

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