We give an elementary proof of the global well-posedness for the critical 2D dissipative quasi-geostrophic equation. The argument is based on a non-local maximum principle involving appropriate moduli of continuity.

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We are going to give necessary and sufficient conditions for two weight norm inequalities for Haar multipliers operators and for square functions. We also give sufficient conditions for two weight norm inequalities for the Hilbert transform.1991 Mathematics Subject Classification. 42B20, 42A50, 47B35.

We establish borderline regularity for solutions of the Beltrami equation f z −µfz = 0 on the plane, where µ is a bounded measurable function, µ ∞ = k < 1. What is the minimal requirement of the type f ∈ W 1,q loc which guarantees that any solution of the Beltrami equation with any µ ∞ = k < 1 is a continuous function? A deep result of K. Astala says that f ∈ W 1,1+k+ε loc suffices if ε > 0. On the other hand, O. Lehto and T. Iwaniec showed that q < 1 + k is not sufficient. In [2], the following question was asked: What happens for the borderline case q = 1 + k? We show that the solution is still always continuous and thus is a quasiregular map. Our method of proof is based on a sharp weighted estimate of the Ahlfors-Beurling operator. This estimate is based on a sharp weighted estimate of a certain dyadic singular integral operator and on using the heat extension of the Bellman function for the problem. The sharp weighted estimate of the dyadic operator is obtained by combining J. Garcia-Cuerva and J. Rubio de Francia's extrapolation technique and two-weight estimates for the martingale transform from [26].

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