The state at any future time, $mathbf{x}(t_1)$, may be determined exactly given knowledge of the initial state, $mathbf{x}(t_0)$, and the time history of the inputs, $mathbf{u}(t)$, between $t_0$ and $t_1$ by integrating Eq.(1). Though the state variables themselves are not unique, there is a minimum number of state variables, $n$, required in a given system for the above to hold true. $n$ is referred to as the system order and determines the dimensionality of the state-space. The system order usually corresponds to the number of independent energy storage elements in the system.