### How to define the ~~derrivative ~~derivative of a function

Hello,

I want to ~~telle ~~tell Sage what expression to use for the ~~derrivative ~~derivative of a funtion *a*. This vould be something like this: *diff(a,t) = b^2*. But this doesn't do the trick...

**Example**
I want to define this set of differential quations:

- aDot = da/dt = b^2,
- bdot = db/da = u.

I want Sage to find the relation between *a* and *u*. For that Sage needs to ~~differenciate ~~differentiate *a* twice:
- diff(a,t,2)
But to be able to calculate d^2/d *t* ^2 *a* = d/d*t *b*^2 = 2 *bu*, Sage needs to to both differential equations.

How can I implemant that?

Thanks alot in advance for your replies!

Johannes