Solution of Second Order Differential Equation with Missing Dependent Variable

Theorem
Let $\map f {x, y', y''} = 0$ be a second order ordinary differential equation in which the dependent variable $y$ is not explicitly present.

Then $f$ can be reduced to a first order ordinary differential equation, whose solution can be determined.

Proof
Consider the second order ordinary differential equation:
 * $(1): \quad \map f {x, y', y''} = 0$

Let a new dependent variable $p$ be introduced:
 * $y' = p$
 * $y'' = \dfrac {\d p} {\d x}$

Then $(1)$ can be transformed into:
 * $(2): \quad \map f {x, p, \dfrac {\d p} {\d x} } = 0$

which is a first order ODE.

If $(2)$ has a solution which can readily be found, it will be expressible in the form:
 * $(3): \quad \map g {x, p}$

which can then be expressed in the form:
 * $\map g {x, \dfrac {\d y} {\d x} } = 0$

which is likewise subject to the techniques of solution of a first order ODE.

Hence such a second order ODE is reduced to the problem of solving two first order ODEs in succession.