Find Solution to Cauchy Problem & Determine Space in $\mathbb{R}^2$

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The heat equation is a simplification of the energy equation for a body.The waves in the glass fiber are a simplification of the electromagnetic field.These are more fundamental and correct than the previous examples.The heat equation is a simplification of the energy equation for a body.The waves in the glass fiber are a simplification of the electromagnetic field.These are more fundamental and correct than the previous examples.So the heat equation is a simplification of the energy equation for a body, and the waves in the glass fiber are a simplification of the electromagnetic field?Yes, exactly.In summary, the conversation discusses a Cauchy problem and determines the space in $\mathbb{R}^2$ where the initial condition
  • #1
evinda
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Hello! (Wave)

I want to find the solution of the following Cauchy problem and determine the space in $\mathbb{R}^2$ where the initial condition defines the solution.

$$u_t+ u_x=4, u|_{t=0}=\sin{x} \text{ for } |x|<1$$

I found that the solution of the above initial value problem is $u(t,x)=4t+\sin{(x-t)}$.

What is meant with the space where the initial condition defines the solution? (Thinking)
 
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  • #2
Hey evinda! (Smile)

It seems a bit like an odd question...

Best I can think of, is that
$$\{(t,x)\in \mathbb R^2\mid |x|<1\} = \mathbb R \times (-1,1)$$
is intended.
A rectangle of infinite length in both directions excluding the boundary. (Thinking)
 
  • #3
I like Serena said:
Hey evinda! (Smile)

It seems a bit like an odd question...

Best I can think of, is that
$$\{(t,x)\in \mathbb R^2\mid |x|<1\} = \mathbb R \times (-1,1)$$
is intended.
A rectangle of infinite length in both directions excluding the boundary. (Thinking)

A ok... (Thinking)
Since $t$ represents the time, will we then have that the space in which the solution is defined is this one:

$$\{(t,x)\in \mathbb{R}^2 \mid |x|<1, t \geq 0\} = [0,+\infty)\times (-1,1)$$

? If so, will it represent a rectangle of infinite length in the upward direction excluding the boundary?
 
  • #4
evinda said:
A ok... (Thinking)
Since $t$ represents the time, will we then have that the space in which the solution is defined is this one:

$$\{(t,x)\in \mathbb{R}^2 \mid |x|<1, t \geq 0\} = [0,+\infty)\times (-1,1)$$

? If so, will it represent a rectangle of infinite length in the upward direction excluding the boundary?

Could be.
It's fairly common that these problems start at time $t=0$, at which time something special happens, such as heat starting to be applied to a body.
But then again, that does not seem to be given, and the solution also holds for negative $t$.
That is, $t=0$ is ultimately only an arbritary reference point in time. (Thinking)
 
  • #5
I like Serena said:
Could be.
It's fairly common that these problems start at time $t=0$, at which time something special happens, such as heat starting to be applied to a body.
But then again, that does not seem to be given, and the solution also holds for negative $t$.
That is, $t=0$ is ultimately only an arbritary reference point in time. (Thinking)

So with $t=0$ we mean the time at which something happens at the solution and we consider that before this time the same happens as after it? (Thinking)
 
  • #6
evinda said:
So with $t=0$ we mean the time at which something happens at the solution and we consider that before this time the same happens as after it? (Thinking)

Let's pick a couple of examples... (Thinking)

Suppose a vertical metal cylinder is originally at room temperature.
And at $t=0$ we start heating it at the bottom, heating it up from the bottom to the top.
Then we are not interested in what happens before $t=0$. We already know.
And it doesn't fit into the differential equation either, since the solution would then not be differentiable at $t=0$.

Another example, suppose rays of light are passing through a glass fiber.
At $t=0$ they pass through the fiber where we are sitting, and it's a sine wave.
Then we can see what happens afterwards ($t > 0$), and we can also trace it back to where it came from ($t < 0$).

Long story short, it should be specified in the problem statement whether $t<0$ is included or not.
 
  • #7
I like Serena said:
Let's pick a couple of examples... (Thinking)

Suppose a vertical metal cylinder is originally at room temperature.
And at $t=0$ we start heating it at the bottom, heating it up from the bottom to the top.
Then we are not interested in what happens before $t=0$. We already know.
And it doesn't fit into the differential equation either, since the solution would then not be differentiable at $t=0$.

Why woulnd't the solution be differentiable at $t=0$ ?
I like Serena said:
Another example, suppose rays of light are passing through a glass fiber.
At $t=0$ they pass through the fiber where we are sitting, and it's a sine wave.
Then we can see what happens afterwards ($t > 0$), and we can also trace it back to where it came from ($t < 0$).

So we have a sine wave when rays of light pass through the glass fiber? And this happens for $t=0$ ? And since rays of light don't change something at the glass fiber, the solution will be the same before and after $t=0$?

I like Serena said:
Long story short, it should be specified in the problem statement whether $t<0$ is included or not.

I see... (Nod)
 
Last edited:
  • #8
evinda said:
Why woulnd't the solution be differentiable at $t=0$ ?

For $x=0$ he solution would be something like:
$$y(t, 0) = \begin{cases}0 &\text{ if }t<0 \\ 1-e^{-t} &\text{ if }t\ge 0\end{cases}$$
And:
$$\lim_{t\to 0^-} y_t(t,0) = 0$$
while
$$\lim_{t\to 0^+} y_t(t,0) = 1$$
Ergo, not differentiable in $t=0$.

evinda said:
So we have a sine wave when rays of light pass through the glass fiber? And this happens for $t=0$ ? And since rays of light don't change something at the glass fiber, the solution will be the same before and after $t=0$?

Since at $t=0$ nothing special happens, the solution doesn't change either around $t=0$. (Thinking)
 
  • #9
I like Serena said:
For $x=0$ he solution would be something like:
$$y(t, 0) = \begin{cases}0 &\text{ if }t<0 \\ 1-e^{-t} &\text{ if }t\ge 0\end{cases}$$
And:
$$\lim_{t\to 0^-} y_t(t,0) = 0$$
while
$$\lim_{t\to 0^+} y_t(t,0) = 1$$
Ergo, not differentiable in $t=0$.

So this problem isn't somehow related with the problem of the #post 1, is it?

How would the differential equation look like? (Thinking)
I like Serena said:
Since at $t=0$ nothing special happens, the solution doesn't change either around $t=0$. (Thinking)

So what would happen at the solution when $t \neq 0$ so that we have a sine wave?
For $t=0$ when the rays of light are passing through the glass fiber the solution will be 0? Or have I understood it wrong? (Worried)
 
  • #10
evinda said:
So this problem isn't somehow related with the problem of the #post 1, is it?

How would the differential equation look like? (Thinking)

No, neither of the examples is for the problem in #1.
They are just examples of IVP's.

It's an example of the heat equation:
$$u_t - \alpha u_{xx} = 0$$

The other example is from the wave equation:
$$u_{tt} = c^2 u_{xx}$$

evinda said:
So what would happen at the solution when $t \neq 0$ so that we have a sine wave?
For $t=0$ when the rays of light are passing through the glass fiber the solution will be 0? Or have I understood it wrong? (Worried)

Not for the problem in post #1, but for the wave equation $\sin(x-ct)$ is a solution everywhere.
 

FAQ: Find Solution to Cauchy Problem & Determine Space in $\mathbb{R}^2$

What is a Cauchy problem?

A Cauchy problem is a type of initial value problem in mathematics that involves finding a solution to a differential equation based on given initial conditions. It is named after the French mathematician Augustin-Louis Cauchy.

How do you find the solution to a Cauchy problem?

To find the solution to a Cauchy problem, one must first determine the differential equation and the initial conditions. The solution can then be found using various methods such as separation of variables, substitution, or using an integrating factor.

What is the importance of solving Cauchy problems?

Solving Cauchy problems is important in many fields of science and engineering, as it allows for the prediction and understanding of how systems will behave over time. It is also a fundamental concept in the study of differential equations and plays a key role in many mathematical models.

What does it mean to determine space in $\mathbb{R}^2$?

Determining space in $\mathbb{R}^2$ refers to finding the set of all possible solutions to a given Cauchy problem. In other words, it is finding the range of values that the dependent variable can take on given the initial conditions and the differential equation.

What are some applications of Cauchy problems in real life?

Cauchy problems have many real-life applications, such as predicting the weather, modeling population growth, and analyzing the behavior of electrical circuits. They are also used in fields such as physics, chemistry, and economics to understand and predict the behavior of systems over time.

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