I wonder if somebody can help me with this problem. I am triying to use dsolve to evaluate the stress trayectories for a problem in elasticity. These means to solve numerically a nonlinear ode but with a rather complicated user-defined functions at the right hand side of the equation. I have used this function as a known function but in this way dsolve just does not work, it does not even start and it seems to me it does not recognize the independente as well as the dependent variables. Would be grateful if somebody can give me a hint here.. Thank in advance Raul Manasevich