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Universality in the two-dimensional dilute Baxter-Wu model

We study the question of universality in the two-dimensional spin-$1$ Baxter-Wu model in the presence of a crystal field $Δ$. We employ extensive numerical simulations of two types, providing us with complementary results: Wang-Landau sampling at fixed values of $Δ$ and a parallelized variant of the multicanonical approach performed at constant temperature $T$. A detailed finite-size scaling analysis in the regime of second-order phase transitions in the $(Δ, T)$ phase diagram indicates that the transition belongs to the universality class of the $4$-state Potts model. Previous controversies with respect to the nature of the transition are discussed and possibly attributed to the presence of strong finite-size effects, especially as one approaches the pentacritical point of the model.