Another classification of differential equations depends on the number of unknown functions that are involved. If there is a single function to be determined, then one equation is sufficient. However, if there are two or more unknown functions, then a system of equations is required. For example, the Lotka-Volterra, or predator-prey, equations are important in ecological modeling. They have the form
where 
and 
are the respective populations of the prey
and predator species. The constants 
and 
are
based on empirical observations and depend on the particular species
that are being studied.
