2.18.2 Integrating factor that depends on \(y\) only

Let

\begin{align} \mu M\left ( x,y\right ) +\mu N\left ( x,y\right ) \frac {dy}{dx} & =d\phi \left ( x,y\right ) \tag {1}\\ & =\frac {\partial \phi }{\partial x}\frac {dx}{dx}+\frac {\partial \phi }{\partial y}\frac {dy}{dx}\nonumber \\ & =\frac {\partial \phi }{\partial x}+\frac {\partial \phi }{\partial y}\frac {dy}{dx} \tag {2}\end{align}

Comparing (1),(2) then

\begin{align*} \frac {\partial \phi }{\partial x} & =\mu M\\ \frac {\partial \phi }{\partial y} & =\mu N \end{align*}

The compatibility condition is \(\frac {\partial ^{2}\phi }{\partial y\partial x}=\frac {\partial ^{2}\phi }{\partial x\partial y}\) then this implies

\begin{align*} \frac {\partial }{\partial y}\left ( \frac {\partial \phi }{\partial x}\right ) & =\frac {\partial }{\partial x}\left ( \frac {\partial \phi }{\partial y}\right ) \\ \frac {\partial \mu M}{\partial y} & =\frac {\partial \mu N}{\partial x}\\ \mu _{y}M+\mu M_{y} & =\mu _{x}N+\mu N_{x}\\ \mu _{y}M & =\mu _{x}N+\mu N_{x}-\mu M_{y}\\ \mu _{y}M & =\mu _{x}N+\mu \left ( N_{x}-M_{y}\right ) \\ \mu _{y} & =\frac {\mu _{x}N}{M}+\frac {1}{M}\mu \left ( N_{x}-M_{y}\right ) \end{align*}

Assuming \(\mu \equiv \mu \left ( y\right ) \) then \(\mu _{x}=0\) and the above simplifies to

\begin{align*} \mu _{y} & =\frac {1}{M}\mu \left ( N_{x}-M_{y}\right ) \\ \frac {d\mu }{dy}\frac {1}{\mu } & =\frac {1}{M}\left ( N_{x}-M_{y}\right ) \end{align*}

Let \(\frac {1}{M}\left ( N_{x}-M_{y}\right ) =B\). If \(B\equiv B\left ( y\right ) \) which depends only on \(y\) then we can solve the above.

\begin{align*} \frac {d\mu }{dy}\frac {1}{\mu } & =B\left ( y\right ) \\ \mu & =e^{\int Bdy}\end{align*}

Let \(\overline {M}=\mu M,\overline {N}=\mu N\) then the ode

\[ \overline {M}\left ( x,y\right ) +\overline {N}\left ( x,y\right ) y^{\prime }=0 \]

is now exact.