The convergence theorem essentially delineates sufficient conditions for the existence of the
Fourier transform and its inverse and is analogous to the convergence theorem of the Fourier series,
i.e., Theorem 2.1. Note that periodic signals are not absolutely integrable as the area under the graph
of | ˜x(t)| over the infinite range −∞ ≤ t ≤∞is infinite, and a similar problem arises in connection
with impulse signals which comprise infinitely tall and infinitesimally thin pulses. The application
of the Fourier transform to signals that do not satisfy the convergence theorem will be examined in
Sec. 6.2.