If all elemental equations are defined for all time, then the system is a continuous-time system.
如果所有的基础方程都定义于整个时间过程，那么，该系统就是连续时间系统。
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If, as in sampling or digital systems, some elemental equations are defined or used only at discrete points in time, a discrete-time system is the result.
When all elemental equations can be described by a set of constant parameter values, as in the familiar RLC circuit, the system is said to be stationary or time-invariant or constant coefficient.