A differential equation or system of Ordinary Differential Equations is said to
be autonomous if it does not explicitly contain the independent variable (usually denoted ). A second-order
autonomous differential equation is of the form , where
. By the Chain
Rule, can be expressed as

For an autonomous ODE, the solution is independent of the time at which the initial conditions are applied. This means that all particles pass through a given point in phase space. A nonautonomous system of first-order ODEs can be written as an autonomous system of ODEs by letting and increasing the dimension of the system by 1 by adding the equation

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1999-05-25