$L^1$ solutions to one-dimensional BSDEs with sublinear growth generators in $z$

This paper aims at solving a one-dimensional backward stochastic differential equation (BSDE for short) with only integrable parameters. We first establish the existence of a minimal $L^1$ solution for the BSDE when the generator $g$ is stronger continuous in $(y,z)$ and monotonic in $y$ as well as it has a general growth in $y$ and a sublinear growth in $z$. Particularly, the $g$ may be not uniformly continuous in $z$. Then, we put forward and prove a comparison theorem and a Levi type theorem on the minimal $L^1$ solutions. A Lebesgue type theorem on $L^1$ solutions is also obtained. Furthermore, we investigate the same problem in the case that $g$ may be discontinuous in $y$. Finally, we prove a general comparison theorem on $L^1$ solutions when $g$ is weakly monotonic in $y$ and uniformly continuous in $z$ as well as it has a stronger sublinear growth in $z$. As a byproduct, we also obtain a general existence and unique theorem on $L^1$ solutions. Our results extend some known works.