The self-dual point of the two-dimensional random-cluster model is critical for $q\ge 1$

We prove a long-standing conjecture on random-cluster models, namely that the critical point for such models with parameter $q\ge 1$ on the square lattice is equal to the self-dual point $p_{sd}(q)=\sqrt{q}\mathrm{/}(1+\sqrt{q})$. This gives a proof that the critical temperature of the $q$-state Potts model is equal to $\log (1+\sqrt{q})$ for all $q\ge 2$. We further prove that the transition is sharp, meaning that there is exponential decay of correlations in the sub-critical phase. The techniques of this paper are rigorous and valid for all $q\ge 1$, in contrast to earlier methods valid only for certain given $q$. The proof extends to the triangular and the hexagonal lattices as well.