### Author Topic: Thanksgiving bonus 5  (Read 3259 times)

#### Victor Ivrii

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##### Thanksgiving bonus 5
« on: October 05, 2018, 05:50:02 PM »
Lagrange equation is of the form

y= x\varphi (y')+\psi (y')
\label{eq1}

with $\varphi(p)-p\ne 0$. To solve it we plug $p=y'$ and differentiate equation:

pdx= \varphi(p)dx + \bigl(x\varphi'(p) +\psi'(p)\bigr)dp.
\label{eq2}

This is a linear ODE with respect to $x$. We find the general solution $x=f(p,C)$ and then $y=f(p,c)\varphi(p)+\psi(p)$:

\left\{\begin{aligned}
&x=f(p,C)\\
&y=f(p,c)\varphi(p)+\psi(p)
\end{aligned}\right.
\label{eq4}

gives us a solution in the parametric form.

(\ref{eq1}) can have a singular solution (or solutions)

y=x\varphi(c)+\psi(c),
\label{eq5}

where $c$ is a root of equation $\varphi(c)-c=0$.

Problem.
Find general and singular solutions to
$$2y - 4xy' - \ln (y') = 0.$$
« Last Edit: October 05, 2018, 06:02:26 PM by Victor Ivrii »

#### Wei Cui

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##### Re: Thanksgiving bonus 5
« Reply #1 on: October 07, 2018, 02:37:38 PM »
Find general and singular solutions to:
$2y-4xy'-\ln(y')=0$

1. We transform the equation into the form:
$y=x\cdot 2y'+\frac{1}{2}\ln(y')$

We plug $p=y'$ and differentiate the equation, we get:

$pdx = 2pdx+2x\cdot dp+\frac{1}{2}\frac{1}{p}\cdot dp$

Then:
$p\frac{dx}{dp}=2p\frac{dx}{dp}+2x+\frac{1}{2p}$

$-px'-2x=\frac{1}{2p}$

$x'+\frac{2}{p}x=-\frac{1}{2p^2}$

Then $p(p) = \frac{2}{p}$, $u(p)=e^{2\int \frac{1}{p}dp} = e^{2lnp}=p^2$,
Multiply both sides with integrating factor $u(p) = p^2$ and we get:

$p^2x'+2px=-\frac{1}{2}$

$(p^2x)'=-\frac{1}{2}$

$p^2x=-\int \frac{1}{2}dp$

$p^2x=-\frac{1}{2}p+C$

$x=-\frac{1}{2p}+\frac{C}{p^2}$

Then the solution in the parametric form is: $\begin{cases}x=-\frac{1}{2p} + \frac{C}{p^2}\\ y=x\cdot 2p + \frac{1}{2}\ln p\end{cases}$

2. I am confused about the singular solution to this question. Since $\varphi(c) = 2c$ and $\varphi(c) -c=0 \implies 2c-c=0\implies c=0$.

However, for $\psi(c)$, since the domain of $\ln x$ is $x>0$. Then $\ln c=\ln 0$ will have no definition. Therefore, how am I supposed to solve the singular solution to the equation $2y-4xy'-\ln(y')=0$?
« Last Edit: October 07, 2018, 04:55:34 PM by Victor Ivrii »

#### Victor Ivrii

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##### Re: Thanksgiving bonus 5
« Reply #2 on: October 07, 2018, 04:57:49 PM »
You do not plug $c=0$ in the general solution since the singular solution cannot be obtained from the general one by a substitution. You use (4)

#### Wei Cui

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• Posts: 16
• Karma: 11
##### Re: Thanksgiving bonus 5
« Reply #3 on: October 07, 2018, 06:35:43 PM »
Find general and singular solutions to:
$2y-4xy'-\ln(y')=0$

1. We transform the equation into the form:
$y=x\cdot 2y'+\frac{1}{2}\ln(y')$

We plug $p=y'$ and differentiate the equation, we get:

$pdx = 2pdx+2x\cdot dp+\frac{1}{2}\frac{1}{p}\cdot dp$

Then:
$p\frac{dx}{dp}=2p\frac{dx}{dp}+2x+\frac{1}{2p}$

$-px'-2x=\frac{1}{2p}$

$x'+\frac{2}{p}x=-\frac{1}{2p^2}$

Then $p(p) = \frac{2}{p}$, $u(p)=e^{2\int \frac{1}{p}dp} = e^{2lnp}=p^2$,
Multiply both sides with integrating factor $u(p) = p^2$ and we get:

$p^2x'+2px=-\frac{1}{2}$

$(p^2x)'=-\frac{1}{2}$

$p^2x=-\int \frac{1}{2}dp$

$p^2x=-\frac{1}{2}p+C$

$x=-\frac{1}{2p}+\frac{C}{p^2}$

Then the solution in the parametric form is: $\begin{cases}x=-\frac{1}{2p} + \frac{C}{p^2}\\ y=x\cdot 2p + \frac{1}{2}\ln p\end{cases}$

2. To find the singular solution, we solve the equation $\varphi(p) -p=0 \implies 2p-p=0 \implies p=0$

It follows from this that $y=C$, we can make direct substitution to make sure that the constant $C$ is equal to 0.

Therefore, the differential equation has the singular solution $y=0$.

#### Victor Ivrii

No, there is no singular solution since you cannot plug $p=0$ into $\ln (p)$.