  The sum capacity of a class of discrete memoryless interference channels is
determined. This class of channels is defined analogous to the Gaussian
Z-interference channel with weak interference; as a result, the sum capacity is
achieved by letting the transceiver pair subject to the interference
communicates at a rate such that its message can be decoded at the unintended
receiver using single user detection. Moreover, this class of discrete
memoryless interference channels is equivalent in capacity region to certain
discrete degraded interference channels. This allows the construction of a
capacity outer-bound using the capacity region of associated degraded broadcast
channels. The same technique is then used to determine the sum capacity of the
discrete memoryless interference channel with mixed interference. The above
results allow one to determine sum capacities or capacity regions of several
new discrete memoryless interference channels.
