Introduction - If you have any usage issues, please Google them yourself
In practice, it will often be limited to using the measure defined in (1.2).
In this context, the retro-policy options are of the form u = k (t, y).
This is called output feedback. It is a difficult problem for which
we will show in Chapter 4 that is actually useful to use a system
additional dynamic (referred to as an observer) that will enable effective
more interesting possibilities.