`vanderpol.ode`

):x = x_0 y = dx_0 d_x = y d_y = mu * (1 - x**2) * y - x

Note the blank line between the initial conditions and the expression for the derivatives. This is the representation of the form using `y`

as the derivative of `x`

. The integration variable in QSoas is the time `t`

. You can learn more about how to specify differential equations in QSoas from the manual. The game is now simply to use fit-ode to launch the fit interface, but before that, one needs data to serve as a template, generated using generate-buffer:

QSoas> generate-buffer 0 100 1 QSoas> fit-ode vanderpol.ode

Now, the fit interface looks like this:

You can play around with the parameters, and in particular the value of `mu`

... For every modification, hit the Update button, or just Ctrl+U, to see the effect. For data you'd wish to take a closer look at, use the *Push current to stack* action from the *Data...* menu, which creates new buffers on the stack. A nice thing is that QSoas keeps the fit parameters as meta-data, which you can see later on using show:

`mu`

in the terminal above ?This feature of QSoas makes it easy to quickly explore the effect of the parameters of a model on the shape of the resulting curve. I have used that very heavily in my research, I hope others will find it useful too. Enjoy !

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