FIGURE 1. The four fundamental two-terminal circuit elements: resistor, capacitor, inductor and memristor.

Resistors and memristors are subsets of a more general class of dynamical devices, memristive systems. Note that R, C, L and M can be functions of the independent variable in their defining equations, yielding nonlinear elements. For example, a charge-controlled memristor is defined by a single-valued function M(q).