An equation describing how some quantity varies at a given point in time in terms of other quantities (or their derivatives) at that point in time. Ubiquitous in science.
Employed by programmers with math backgrounds or by programmers assisted by a mathematics specialist. It is often necessary for a programmer to employ their use along with other such areas as probability and statistics, fuzzy logic, and numerical analysis.
DifferentialEquations are used to solve physical relationships between mass, energy, charge, space, position, time, and temperature. This makes them useful in:
Mathlab and Maple are two software packages designed to manipulate DifferentialEquations.
Sources of additional information:
Electronic Journal of Differential Equations - http://ejde.math.txstate.edu/
Mathematics And Programming - A Revolution in the art of Reasoning. http://www.cs.nott.ac.uk/~rcb/inaugural.pdf
The ExponentialFunction can be coded thusly:
function expFun($x0,$k,$t) { $x = $x0*exp($k*$t); return $x; }