  While capacities of discrete memoryless channels are well studied, it is
still not possible to obtain a closed-form expression for the capacity of an
arbitrary discrete memoryless channel. This paper describes an elementary
technique based on Karush Kuhn Tucker (KKT) conditions to obtain (1) a good
upper bound of a discrete memoryless channel having an invertible positive
channel matrix and (2) a closed-form expression for the capacity if the channel
matrix satisfies certain conditions related to its singular value and its
Gershgorin disk.
