  Given a set of differential equations whose description involves unknown
parameters, such as reaction constants in chemical kinetics, and supposing that
one may at any time measure the values of some of the variables and possibly
apply external inputs to help excite the system, how many experiments are
sufficient in order to obtain all the information that is potentially available
about the parameters? This paper shows that the best possible answer (assuming
exact measurements) is 2r+1 experiments, where r is the number of parameters.
