Hi,

I still don't see the problem. Here's a smaller example.

A = RandomReal[{-1, 1}, {5, 5}]; B = RandomReal[{-1, 1}, {5, 5}];
sols = Solve[Det[A - w^2*B] == 0, w]

This gives:

{{w -> -0.890446 - 0.570123 I}, {w -> -0.890446 + 0.570123 I}, {w -> -0.604951}, {w -> 0. - 1.01964 I}, 
{w ->  0. + 1.01964 I}, {w -> 0. - 1.82466 I}, {w -> 0. + 1.82466 I}, {w -> 0.604951}, 
{w -> 0.890446 - 0.570123 I}, {w -> 0.890446 + 0.570123 I}}

If you now apply the command I posted earlier:

Select[Chop[w /. sols], (Head[#] == Real && Re[#] > 0) &]

It gives

{0.604951}

which is indeed the only positive real solution that Mathematica found. So what makes you say that this does not work?

Cheers,

Marco

Thanks for your answer, but this tip does not work with my equation Look, i have to solve this equation Solve[Det[kg- w^2*Mg] == 0, w] Knowing that both "kg" and "Mg" are [18x18] matrix Can rewrite your example with my equation. greetings