Auxiliary random variables are a fundamental tool in information theory. They were initially introduced by Gallager to characterize the capacity region of a degraded broadcast channel. Today, auxiliary random variables are widely utilized in establishing "single-letter" outer bounds for various network information theory problems. Auxiliary random variables (in single-letter form) can serve as proxies to represent the underlying "messages" (information available at the encoders) in the n-letter problem formulation. Such auxiliary random variables can be called the "transmitter side" auxiliary random variables. Interestingly, the concept of auxiliary random variables can also be applied on the receiver side, leading to the notion of" auxiliary receivers:' This talk discusses some applications of auxiliary receivers in deriving new outer bounds for the capacity regions of the broadcast channel, interference channel, and relay channel.
