Let K {\displaystyle {}K} be a field. For polynomials P , Q ∈ K [ X ] {\displaystyle {}P,Q\in K[X]} , Q ≠ 0 {\displaystyle {}Q\neq 0} , the function
where D {\displaystyle {}D} is the complement of the zeroes of Q {\displaystyle {}Q} , is called a rational function.