The claim that average cost equals marginal cost at the minimum of the average cost curve is a standard textbook microeconomic identity that applies regardless of market structure, including monopolies, and does not require a citation.
The claim is a fundamental microeconomic identity true by definition of cost curves and calculus, making it common knowledge. None of the retrieved papers discuss the microeconomic cost curve identity regarding average and marginal cost minima; they focus on unrelated topics like energy economics, health, and policy.