is thus periodic with period 
.
is often written as 
e.g. in S&K because the DTFT is the z-transform with
values of z restricted to the unit circle 
.
The inverse transform is given by :
Notice that while x(n) is a discrete sequence is defined for a continuous
range of 
i.e. is a continuous function of .
Example :
Find the DTFT of x(n)=u(n+4)-u(n-5) as illustrated in the diagram.
Substitute r=m+4 :
Calculate the sum from the finite geometric sum formula :
