A geometric condition for the invertibility of Toeplitz operators on the Bergman space

Invertibility of Toeplitz operators on the Bergman space and the related Douglas problem are long standing open problems. In this paper we study the invertibility problem under a novel geometric condition on the images of the symbols, which relaxes the standard positivity condition. We show that under our geometric assumption, the Toeplitz operator $T_\varphi $ is invertible if and only if the Berezin transform of $|\varphi |$ is invertible in $L^{\infty }$. It is well known that the Douglas problem is still open for harmonic functions. We study a class of rather general harmonic polynomials and characterize the invertibility of the corresponding Toeplitz operators. We also give a number of related results and examples.