You had to create dqdt manually (or else it would have evaluated to 0
already) so instead of using `q` use `f(q)` where `f=Function('f')`. When
you do this something like `(q*Derivative(f(q),t)).subs(q,2)` it will now
become `2*Derivative(f(2),t)` instead of 0.
Else, as you say, the masking is easy: reps = {a:Dummy() for a in
eq.atoms(Derivative)};eq.xreplace(reps).subs(q,2).xreplace({v:k for k,v in
reps.items()})`
/c
On Wednesday, September 17, 2025 at 3:15:04 PM UTC-5 frank....@gmail.com
wrote:
> Hi all,
>
> I constantly run into something like cos(q)*dqdt and I want to substitute
> a specific value for q without it affecting dqdt say q = 0 but dqdt is
> non-zero. Is there a way to do this without having to make a change of
> variables (ie dqdt = u1 then subs for q =0)?
>
> Thank,
> Frank
>
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