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.

The main problems which invariant theory research are the set of generators of invariants, the defining relations of the generators of invariants, the structure of invariant ring, and so on.

The property of the invariant set and measure's dimension of the IFS are main objects in the studying of fractal geometry.