Author 
Comment/Response 
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: , 
