I read this sentence: Time (t) is not always the independent variable in an ODE since rates are not always expressed in terms of time.
My Question is : what are other independent variables in ordinary differential equation (ODE) except t? if there are, please give some references and some examples
Thank you
A differential equation such as $$ y'=f(x,y) $$ is an equation involving the derivative of one variable with respect to the other. For example from the equation $$ v=\frac {4}{3} \pi r^3$$ we get $$ \frac {dv}{dr} =4\pi r^2$$ which is a differential equation in which the independent variable is $r$ the radius of a sphere.