A Szemer\'edi-Trotter type theorem, sum-product estimates in finite quasifields, and related results

We prove a Szemer'edi-Trotter type theorem and a sum-product estimate in the setting of finite quasifields. These estimates generalize results of the fourth author, of Garaev, and of Vu. We generalize results of Gyarmati and S'ark"ozy on the solvability of the equations $a+b=cd$ and $ab+1=cd$ over a finite field. Other analogous results that are known to hold in finite fields are generalized to finite quasifields.