Is a nonlinear a function

Answer 1

Any function in which the dependent variable is not exactly proportional to the zeroth or first power of the independent variable. E.g.: y(x) = a*(x+x^3); y(x) = a*exp(x); y(x) = a*sin(x); etc.

This may be extended to differential equations by stating a nonlinear differential equation is one in which some function depends on a derivative which is not to the zeroth or first power. An example is: dy/dx - (y(x))^2 = a.

Note, all of the values for "a" in these examples are meant to be constants.