  The discrimination of quantum processes, including quantum states, channels,
and superchannels, is a fundamental topic in quantum information theory. It is
often of interest to analyze the optimal performance that can be achieved when
discrimination strategies are restricted to a given subset of all strategies
allowed by quantum mechanics. In this paper, we present a general formulation
of the task of finding the maximum success probability for discriminating
quantum processes as a convex optimization problem whose Lagrange dual problem
exhibits zero duality gap. The proposed formulation can be applied to any
restricted strategy. We also derive necessary and sufficient conditions for an
optimal restricted strategy to be optimal within the set of all strategies. We
provide a simple example in which the dual problem given by our formulation can
be much easier to solve than the original problem. We also show that the
optimal performance of each restricted process discrimination problem can be
written in terms of a certain robustness measure. This finding has the
potential to provide a deeper insight into the discrimination performance of
various restricted strategies.
