> I've got a function u depending on a variable x. So I declared:
>
> (%i1) depends(u,x);
>
> I now want to differentiate u with respect to x. So:
>
> (%i2) diff(u,x);
>
> Now I want to substitute the independent variable x with l*X, where l
> is a constant parameter. What I want to obtain is:
>
> d u 1 d u
> ------ = --- -------
> d x l d X
>
My favorite way to do things like this is to use the positional
derivative package:
(%i1) load("pdiff")$
(%i2) diff(u(x/l),x);
(%o2) u[(1)](x/l)/l
On the screen, u[(1)] is u subscript (1). For user documentation,
see the pdf in /share/contrib/pdiff.
Barton