ode45 - Does matlab keep track of derivatives which can be recalled?

I've run into an issue with a system of coupled 2nd order nonlinear ODEs. I've converted the system to a system of 1st order ODEs like normal, but its not in the format that ode45 requires. ode45 requires

x_dot = f(t,x), where x is the state vector

My form is

x_dot = f(t,x,x_dot)

where the function also depends on the derivative. I searched all around to see if anyone else has had this issue, and only found one post on a forum. It just said to proceed normally with ode45 and use the old values (previous step) of x_dot to calculate the rhs.

This makes sense, but ode45 doesn't work that way (not that I know of). I only have access to the old state values and not their derivatives.

Any ideas?

Answer 1

[Babak]

Re-formulate your equations from

 x_dot = f(t,x,x_dot)

to

 g(t,x,x_dot) = f(t,x,x_dot) - x_dot = 0

and then use

 ode15i

to integrate the set of equations in g().

[JSL]

Excellent! Thanks!