  A new concept, called quasi-linear transfer functions (QLTF), which can be
used to characterize the output frequency behaviour of nonlinear systems, is
introduced based on the well-known Volterra series representation. By using the
new concept of QLTF, it can be proved that the input and output frequency
behaviour of a given system can be expressed using a number of one-dimensional
functions with a form similar to that of the traditional frequency response
function for linear systems. Two algorithms, which can be used to determine the
valid range of the associated output frequencies of arbitrary order nonlinear
subsystems with both a multitone and general inputs, are provided. The results
obtained provide a new important insight into the output frequency
characteristics of nonlinear systems and have many potential applications in
nonlinear systems analysis and nonlinear structure detection
