162 Internal stability of $SFL$

The $SFL$ system is internally stable if all the transfer functions in the matrix $\mathrm{\Phi }$ are stable, so

all belong to $𝒮$.

The idea is that we want all the junctions in the diagram to be stable. (Think of the $x,y,z$ as representing the temperature of the components of a tumble drier; we need to ensure that these do not overheat.)