The DFTsinusoids
are all periodic
having periods which divide . That is,
for any
integer . Since a length signal can be expressed as a linear
combination of the DFT sinusoids in the time domain,

it follows that the ``automatic'' definition of beyond the
range is periodic extension, i.e.,
for every integer .

Moreover, the DFT also repeats naturally every samples, since

because
. (The DFT sinusoids behave identically as functions of and
.) Accordingly, for purposes of DFT studies, we may define all
signals in as being single periods from an infinitely long periodic
signal with period samples:

Definition (Periodic Extension): For any signal
, we define

for every integer .

As a result of this convention, all indexing of signals and
spectra^{7.2} can be interpreted modulo , and we may write
to emphasize this. Formally, ``
'' is defined as
with chosen to give in the range .

As an example, when indexing a spectrum , we have that
which can be interpreted physically as saying that the sampling rate
is the same frequency as dc for discrete time signals. Periodic
extension in the time domain implies that the signal input to the DFT
is mathematically treated as being samples of one period of a
periodic signal, with the period being exactly seconds (
samples). The corresponding assumption in the frequency domain is
that the spectrum is exactly zero between frequency samples
. It is also possible to adopt the point of view that the
time-domain signal consists of samples preceded and
followed by zeros. In that case, the spectrum would be
nonzero between spectral samples , and the spectrum
between samples would be reconstructed by means of bandlimited
interpolation [70].