A differential equation characterizing the functions

, which arise when e
Atis expressed as

, is derived. It is shown that the set of functions

is linearly independent over any nonzero interval. Using this fact, a constructive proof is given for the well-known criterion for a linear time-invariant system to be controllable, namely, rank
![[B|AB| ... |A^{n-1}B] = n](/images/tex/4165.gif)
.