Subs for variable without affecting it's derivative

[Frank Senkel]

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

[Chris Smith]

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

[peter.st...@gmail.com]

I believe sympy.physics.mechanics.msubs(….)  also does not go ‘inside’ the time derivatives (?)

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