To determine the frequency response of a continuous-time filter, the transfer function formula_1 is evaluated at formula_43 which is on the formula_3 axis. Likewise, to determine the frequency response of a discrete-time filter, the transfer function formula_45 is evaluated at formula_46 which is on the unit circle, formula_5. The bilinear transform maps the formula_3 axis of the "s"-plane (of which is the domain of formula_1) to the unit circle of the "z"-plane, formula_5 (which is the domain of formula_45), but it is not the same mapping formula_52 which also maps the formula_3 axis to the unit circle. When the actual frequency of formula_54 is input to the discrete-time filter designed by use of the bilinear transform, then it is desired to know at what frequency, formula_55, for the continuous-time filter that this formula_54 is mapped to.