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Student Support Forum: 'Problem in solving a differential equation' topicStudent Support Forum > General > Archives > "Problem in solving a differential equation"

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Daniele Lupo
09/27/07 06:14am

Hi to all.

I've a problem with a differential equation, that I'm trying to solve with mathematica 6.0.1


When I try to solve it in a symbolic way, I obtain an error

\[Infinity]::indet: "Indeterminate expression \
ComplexInfinity+ComplexInfinity encountered. "

Commands that I write are followings:

eq = Derivative[2][f][t] + (2*l + m)*Derivative[1][f][t] +
l^2*f[t] == ((m + Sqrt[m*(4*l + m)])/(2*Sqrt[m*(4*l + m)]))*
Exp[(-(t/2))*((2*l + m) - Sqrt[m*(4*l + m)])] +
((-m + Sqrt[m*(4*l + m)])/(2*Sqrt[m*(4*l + m)]))*
Exp[(-(t/2))*((2*l + m) + Sqrt[m*(4*l + m)])]


DSolve[{eq, f[0] == 0, Derivative[1][f][0] == 0}, f, t]


I know that there's a solution, so I've tried to solve it numerically, and I must to notice that, instead, NDSolve works fine:

fun = f /.
NDSolve[{Evaluate[eq /. {m -> .5, l -> .8}], f[0] == 0, f'[0] == 0},
f, {t, 0, 10}][[1]]

Plot[fun[x], {x, 0, 10}]

I obtain the right graph.

Now, if Mathematica can't solve it in symbolic way, it shoud return unevaluated expression, instead of the error, right?

What's wrong? There's something that I can do to solve it simbolically?

Attachment: problem.nb, URL: ,

Subject (listing for 'Problem in solving a differential equation')
Author Date Posted
Problem in solving a differential equation Daniele Lupo 09/27/07 06:14am
Re: Problem in solving a differential equation yehuda ben-s... 09/27/07 7:24pm
Re: Problem in solving a differential equation Daniele Lup 10/01/07 2:40pm
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