We determine optimal L-p-properties for the solutions of the general nonlin
ear elliptic system in the plane of the form
f (z) over bar = H(z, f(z)), h is an element of L-p(C),
where H is a measurable function satisfying \H(z, omega (1)) - H(z, omega (
2))\ less than or equal to k\omega (1) - omega (2)\ and k is a constant k <
1.
We also establish the precise invertibility and spectral properties in L-p(
C) for the operators
I - T<mu>, I - muT, and T - mu,
where T is the Beurling transform. These operators are basic in the theory
of quasi-conformal mappings and in linear and nonlinear elliptic partial di
fferential equations (PDEs) in two dimensions. In particular we prove inver
tibility in L-p(C) whenever 1 + \\mu\\(infinity) < p < 1+1/\\mu\\(infinity)
.
We also prove related results with applications to the regularity of weakly
quasiconformal mappings.