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Can't take a value from an InterpolatingFunction.


Hi all,

I've set up and solved a system of two differential equations, according
to the following prescription;

  solution = 
    NDSolve[{r'[t] == .....equation1......,
             p'[t] == .....equation2......,
             r[0] == 0.,p[0] == p0}, {r, p}, {t, 0, 0.2},
             MaxSteps \[Rule] 300000, Method \[Rule] Automatic]

All has worked very nicely. However, I need now to take the value of r
at t=0.2 and I can't seem to be able to do it. 

I should point out that the Book suggestion r[0.2] /. % doesn't work,
because the above statement is within a For loop and is run many times.
(to collect various r[0.2] values that come out) So, it's within one
big cell.

The error reported sais that solution is not a valid lookup table. Well,
from what I understand, it's a set (?) of two InterpolatingFunctions.
If I knew what kind of entity solution is, I might be able to figure
how to instruct Mathematica to let me access r[0.2].

Any help appreciated, thanks, John

-- 
      Ioannis   I    Ioannou                   phone: (206)-543-1372
      g-2 group, Atomic Physics                fax:   (206)-685-0635
      Department of Physics           
      University of Washington        e-mail: iioannou at u.washington.edu


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