Barton Willis wrote:
>>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
>
>
Hi Barton,
thanks for the hint. I wasn't aware of that package. It's true, it
improves the clarity. But unfortunately, it doesn't solve my problem.
What I have done so far, is
(%i1) expr1:diff(u(x),x)
(%i2) subst(l*X,x,expr1)
(%o2) d u(lX)
------------
d(lX)
How can I get
1 d u(X)
-- ------------
l d(X)
from (%i2)? Do you have any idea?
Many thanks.
Marc