Essentially semi-commuting singular integral operators with Cauchy kernel on $L^2$

In this paper, we completely characterize when the semi-commutator of two Cauchy singular integral operators on $L^2$ is compact. Our main idea is to study Cauchy singular integral operators via Hankel operators, Toeplitz operators and function algebras. Moreover, we obtain a necessary and sufficient condition for Cauchy singular integral operators to be essentially normal.