If the current can be expressed as an explicit(单值) function of the voltage i=f(D)--A voltage-controlled nonlinear resistor If the voltage can be expressed as an explicit function of the current u=h(i)----A current-controlled nonlinear resistori  i  If the current can be expressed as an explicit(单值) function of the voltage. i = f () ----A voltage-controlled nonlinear resistor. If the voltage can be expressed as an explicit function of the current  = h(i ) ----A current-controlled nonlinear resistor