3
$\begingroup$

I want to get a function as output form DSolve. For Example :

sol = DSolve[{Q''[t] + 40 Q'[t] + 625 Q[t] == 100*Cos[10*t], Q[0] == 0, Q'[0] == 0}, Q[t], t]

I want to use Q[t] as a function to use in an another equation.

$\endgroup$
10
$\begingroup$

It's not too hard:

dsaad[t_] = Q[t] /. First @ DSolve[{Q''[t] + 40 Q'[t] + 625 Q[t] == 100*Cos[10*t],
                                   Q[0] == 0, Q'[0] == 0}, Q, t]

Notes:

  1. I instructed DSolve[] to return the pure function Q (through the second argument) instead of the function itself to ease the replacement.

  2. I used Set[] (=) instead of SetDelayed[] (:=), so that the replacement is done at once before the definition takes place.


Another possibility is to use DifferentialRoot[] instead:

dsaad[x_] := DifferentialRoot[Function[{Q, t},
               {Q''[t] + 40 Q'[t] + 625 Q[t] == 100*Cos[10*t], Q[0] == 0, Q'[0] == 0}]][x]
| improve this answer | |
$\endgroup$
  • $\begingroup$ I understand all of it but I didn't get the "First". What does it? $\endgroup$ – DSaad Jul 24 '12 at 14:28
  • $\begingroup$ @DSaad The First is meant to extract the solution as it is returned by Mathematica as a List of List. $\endgroup$ – PlatoManiac Jul 24 '12 at 15:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.