This is an example of a continuous autonomous dynamical system, commonly written in the form x ˙ = f ( x ) {\displaystyle {\dot {x}}=f(x)} . The number of molecules of each reactant used up each time a reaction occurs is constant, as is the number of molecules produced of each product. These numbers are referred to as the stoichiometry of the reaction, and the difference between the two (i.e. the overall number of molecules used up or produced) is the net stoichiometry.