# [Rd] Projects (i.e. wishlist)

**Telford Tendys
**
telford@progsoc.uts.edu.au

*Fri, 15 Sep 2000 15:40:36 +1100*

On Thu, Sep 14, 2000 at 06:46:41PM +0200, Martin Maechler wrote:
>* 2) For Computer Scientists :
*>* "Differentiation" / Symbolic derivatives,..
*>*
*>* Improve the possibilities of D() and deriv(), and document them.
*>* Make these user-extensible.
*>*
*>* Think about the hessian in addition to the gradient.
*
Now that you mention it, I'd like this to work:
>* X <- expression( x )
*>* X + 1
*expression(x + 1)
>* (X + 1) + 2
*expression((x + 1) + 2)
>* simplify(( X + 1 ) + 2 )
*expression( x + 3 )
>* simplify( expand(( X + 2 ) * ( X + 5 ) * ( X + 7 )))
*expression(x^3 + 14 * x^2 + 59 * x + 70)
>* factorise( X^3 + 14 * X^2 + 59 * X + 70 ))
*expression((x + 2) * (x + 5) * (x + 7))
If the roots have small imaginary components due to round off,
that is still OK. If there is a good reason for the operations
to return objects of mode ``call'' rather than mode ``expression''
(like what D() does) that would be OK too, although I prefer
to work with the expression() wrappers on because then the printed
form is identical to what you type in. At any rate, the operations
have to handle objects of mode ``expression'', ``call'' and all the
numericals as their inputs.
And as many others as you get time to do plus a system for
categorising, organising, documenting and extending the available
manipulations...
... I'm sure this is all very useful for statistics, it would
be useful to me at any rate.
- Tel
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