where λ λ m a x is the largest eigenvalue of τ τ . The approximation follows from the fact that the eigenvalues of τ τ M are the eigenvalues of τ τ to the power of M, and as M → → ∞ ∞ , the power of the largest eigenvalue becomes much larger than the others. As the trace is the sum of the eigenvalues, the problem of calculating Z reduces to the problem of finding the maximum eigenvalue of τ τ . This in itself is another field of study. However, a standard approach to the problem of finding the largest eigenvalue of τ τ is to find a large family of operators which commute with τ τ . This implies that the eigenspaces are common, and restricts the possible space of solutions. Such a family of commuting operators is usually found by means of the Yang–Baxter equation, which thus relates statistical mechanics to the study of quantum groups.