Thanks a lot for your answer.
I was solving my system in a symilar way that you propos. From the matrix form, I get the equations and then I use the dsolve command with numeric option. You are setting the problem in a more efficent way. Nevertheless, I still can't solve my problem and I don't understand way!
I try to explain you:
I have matrix formed by Nm*Nc components (I'm solving the problem of a Nc charges moving upon a bridge, considering Nm modes of vibration). I have no problems to solve for example Nc = 20 charges with Nm = 1 modes (so then 21 ode equations of 2nd degree). This problem takes in my processor 1.3 seconds. The problem comes when I want to take into consideration more than one modes of vibration. If I try with Nm = 2 and Nc = 2 (only two charges and two modes), it takes 10.8 seconds to solve (less equations but more complicated). With Nc = 2 and Nm = 3 it goes to 120 seconds. With Nc = 2 and Nm = 4 it doesn't work and after 500 sec I get this error message:
Error, (in expand/bigprod) object too large
The problem is well set (I have programmed a Newmark method that works with matrix and compared results with a commercial finite element code). I don't understand what hapens, because if I work with equations, it seems not to be a problem with the number, but with the complexity. I know that some of the complexity comes because I use the Heaviside function in order to consider the charge only when it is under the bridge.
Since it seems to be a numeric problem, I wanted to set it in a more efficent way. I was reading in the forums that it's better to transform a 2nd order system into a 1st order system with the procedure option (as indicated in the maple>help>dsolve/numeric/IVP. I have been trying also with the numeric options and procedures, but no results.
Have you any idea of what kind of problem do I have? I lift the file here if you are intereste in have a look. Anyway thanks a lot for your answer (and sorry about my poor english!)
View 11156_MIS.mw on MapleNet or Download 11156_MIS.mw
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