This type of second-order differential equation is any ODE that does not have a linear form, such as y''(x) + sin(y(x)) = 0 or y''(x) = y'(x)^2.
This type of second-order differential equation is any ODE that does not have a linear form, such as y''(x) + sin(y(x)) = 0 or y''(x) = y'(x)^2.