Question to first order PDE with Characteristics Method

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We have the given problem

\begin{align} -2yu_{x} + u_{y} + 2yu &= 2y \tag 1 \\ u(1,y) &= 1 + e^{-1-2y^2} \tag 2 \end{align}

where $x > 0, y \in \mathbb{R^*}$. We can modify the formula $(1)$ to get

\begin{align} -2yu_{x} + u_{y} &= 2y - 2yu \\ \implies -2yu_{x} + u_{y} &= 2y(1-u) \\ \implies -u_{x} + \frac{u_{y}}{2y} &= 1-u \\ \implies u_{x} - \frac{u_{y}}{2y} &= u-1 \end{align}

so our problem would be

\begin{align} u_{x} - \frac{u_{y}}{2y} &= u-1 \tag 3 \\ u(1,y) &= 1 + e^{-1-2y^2} \tag 4 \end{align}

The method of characteristics gives

$$\dfrac{dx}{1} = -2y dy = \dfrac{du}{u-1}$$


Edited after @MatthewCassell suggestion

Now, we have

\begin{align} \dfrac{dx}{1} = -2y dy \implies C_{1} &= y^2+x \end{align}

and

\begin{align} \dfrac{dx}{1} = \dfrac{du}{u-1} \implies u &= C_{2} e^x + 1 \\ \implies C_{2} &= \frac{u-1}{e^x} \end{align}

The general solution of the PDE expressed in the form of implicit equation

$$C_2=G(C_1)$$

So $$u=e^x G(x+y^2)+1$$

Now, using the initial condition $u(1,y)=1+e^{-1-2y^2}$ gives

$$1+e^{-1-2y^2} = e G(1+y^2)+1 \implies G(1+y^2) = \frac{e^{-1-2y^2}}{e}$$

I get something strange (from what I've seen at least). Is this possible?

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Solution

\begin{align} -2yu_{x} + u_{y} &= 2y - 2yu \\ \implies -2yu_{x} + u_{y} &= 2y(1-u) \\ \implies -u_{x} + \frac{u_{y}}{2y} &= 1-u \\ \implies u_{x} - \frac{u_{y}}{2y} &= u-1 \end{align}

so our problem would be

\begin{align} u_{x} - \frac{u_{y}}{2y} &= u-1 \tag 3 \\ u(1,y) &= 1 + e^{-1-2y^2} \tag 4 \end{align}

The method of characteristics gives

$$\dfrac{dx}{1} = -2y dy = \dfrac{du}{u-1}$$

Now, we have

\begin{align} \dfrac{dx}{1} = -2y dy \implies C_{1} &= y^2+x \end{align}

and

\begin{align} \dfrac{dx}{1} = \dfrac{du}{u-1} \implies u &= C_{2} e^x + 1 \\ \end{align}

The general solution of the PDE expressed in the form of implicit equation

$$C_2=G(C_1)$$

So $$u=e^x G(x+y^2)+1$$

Now, using the initial condition $u(1,y)=1+e^{-1-2y^2}$ gives

$$1+e^{-1-2y^2} = e G(1+y^2)+1 \implies G(1+y^2) = \frac{e^{-1-2y^2}}{e}$$

Then, \begin{align} G(1+y^2) &= e^{-2-2y^2}\\ &=e^{-2(1+y^2)} \end{align}

So we can see that

\begin{align} G(x+y^2) &= e^{-2(x+y^2)}\\ \implies u&=e^x \cdot e^{-2(x+y^2)} +1\\ &=e^{-x-2y^2} +1 \end{align}

which satisfies the PDE and boundary condition