Describing an equation or expression that is not linear or a dynamical system governed by non-linear equations. Many areas of mathematics focus on linear aspects, in part because there are many important linear differential equations and a rich theory, as in linear algebra, can be developed. Also, even non-linear systems can be approximated to some extent by linear ones (see linearization, linear theory of equilibria). However, there are many important non-linear equations and systems, and their general properties and behaviour are completely different, often exhibiting chaos (see Lorenz attractor) with little hope of analytical solutions, though it is possible to qualitatively analyse them.