# Remove part within square brackets for function

So I've defined a variable as follows:

 q = {{x[t]}, {Φ[t]}}
V=0.5*x[t]^2*m^2+0.5*Φ[t]^2*I


I've had to differentiate with respect to time so:

 dq=D[q,t]


Giving something like:

  q'[t]={{x'[t]}, {Φ'[t]}}


I'd now like to use a Function and have found that the following doesn't work:

fv=Function[{x, Φ}, D[V, q]][1, pi/8]


It seems that x is different to x[t]. Which seems to some regard logical, however it is now impossible to work with the Function.

 Function[{x[t],Φ[t]}, D[V, q]][1, pi/8]


doesn't work either. Is there a way to delete the "[t]" part of each variable? Or is there a better way of going about this?

• What do you want to get in the end? Apr 15, 2016 at 11:37
• I'd like the jacobian to be evaluated and then the values x=1 and phi=pi/8 to be substituted in. So in the end a numerical value. Apr 15, 2016 at 12:07
• Can you write a formula that you want in the end? Apr 15, 2016 at 13:10
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With

q = {x, Φ}
V = (x^2*m^2 + Φ^2*I)/2

fv = Function[{x, Φ}, Evaluate[Total[Flatten[D[V, {q}]]]]][1, pi/8]
(* m^2 + (I pi)/8 *)


gives the desired result, if I understand the question correctly.

If, as suggested in a comment, it is necessary that x and Φ have explicit t dependence, the following can be used instead.

q = {x[t], Φ[t]}
V = (x[t]^2*m^2 + Φ[t]^2*I)/2
fv = Function[{x, Φ}, Evaluate[Total[Flatten[D[V, {q}]]] /. {x[t] -> x, Φ[t] -> Φ}]]
[1, pi/8]


yielding the same result as before.

• I haven't put the entire script in to the question so it may not make sense as to why I started with q = {x[t], [Phi][t]}. But in other parts of my script I need to differentiate with respect to t. For example with D[T,D[q,t]] where T is function dependent on on dq/dt. Apr 15, 2016 at 12:39
• @aenes1519 You can use % /. {x -> x[t], Φ ->Φ[t]} to add the t dependence to the final result. Apr 15, 2016 at 12:46
• Can I also do it the other way around? %/. {x[t]->x, [Phi][t]->[Phi]} I just tried and it seems to stay the same Apr 15, 2016 at 12:52
• Ignore the last comment, I managed to get it to work by understanding what "%" does. Converting from Matlab to Mathematica for symbolic calculations is proving more difficult than hoped for... Apr 15, 2016 at 12:59