Inside your function the following variables are not defined: C, K, M, u
Global variables are only accessible inside functions that declare them as global (or inside nested functions of a function that declared them global.)
Avoid using global.
Note: there is no point in taking inv(M) inside the function, as you do not change M inside the function. Either use M\(F-K*y(u:end)-C*y(1:u-1)) or else calculate inv(M) outside and pass in the inverse.
Note: using interp1() with no interpolation method specified results in linear interpolation. However, linear interpolation of a variable results in the derivative of F being discontinuous, which is something that will cause problems with the ode*() routines, as the ode*() routines rely upon the second derivative of all statements to be continuous. If you must interpolate, then use 'spline' method.
