Why Choose This Function for Solving PDEs?

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In summary, the suggested solution for finding the solution $u(t,x)$ for the given problem involves setting $v$ to be equal to $u$ minus certain terms involving $t$ and $x$. This simplifies the problem by making the boundaries equal to $0$, and allows us to use a Fourier sine series to find a solution for $u$.
  • #1
evinda
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Hello! (Wave)

I am looking at the following exercise:

Find the solution $u(t,x)$ of the problem

$$u_t-u_{xx}=2 \sin{x} \cos{x}+ 3\left( 1-\frac{x}{\pi}\right)t^2, t>0, x \in (0,\pi) \\ u(0,x)=3 \sin{x}, x \in (0,\pi) \\ u(t,0)=t^3, u(t, \pi)=0, t>0$$

At the suggested solution, it is stated that the boundaries are non-zero, so we want to set them equal to $0$.

We set $v=u-\left( 1-\frac{x}{\pi}\right) t^3-\frac{x}{\pi} \cdot 0 \Rightarrow v=u-t^3+t^3 \frac{x}{\pi}$.

Could you explain to me why we take this $v$? Is there a formula that we use that enables us to solve the problem? 🧐
 
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  • #2
evinda said:
At the suggested solution, it is stated that the boundaries are non-zero, so we want to set them equal to $0$.

We set $v=u-\left( 1-\frac{x}{\pi}\right) t^3-\frac{x}{\pi} \cdot 0 \Rightarrow v=u-t^3+t^3 \frac{x}{\pi}$.

Could you explain to me why we take this $v$? Is there a formula that we use that enables us to solve the problem?
Hey evinda!

$v$ is selected so that $v(t,0)=0$ and $v(t,\pi)=0$ to simplify the problem.
Then, if we can find a solution for $v$, we can also find a solution for $u$. 🤔

We can achieve that if we pick $v(t,0)=u(t,0)-t^3$ and $v(t,\pi)=u(t,0)-0$.
Now we still need to introduce $x$ such that both conditions are satisfied.
To do so, we pick $v(t,x)=u(t,x)-A(x)\cdot t^3-B(x)\cdot 0$, where $A(x), B(x)$ are functions of $x$ such that $A(0)=1$ and $A(\pi)=0$, and $B(0)=0$ and $B(\pi)=1$.
Simplest $A$ and $B$ are the linear functions $A(x)=1-\frac x\pi$ respectively $B(x)=\frac x\pi$.
Substitute to find $v(t,x)=u(t,x)-\left(1-\frac x\pi\right)\cdot t^3-\frac x\pi\cdot 0$. 🤔
 
  • #3
In particular, if we have boundary conditions, u(0, t)= 0 and u(L, t)= 0 we can use a "Fourier sine series", $\sum_{n=0}^\infty A_n(t) sin(\frac{n\pi}{L}x)$ rather that the more general (and harder) "Fourier series" with both sine and cosine.
 

FAQ: Why Choose This Function for Solving PDEs?

How do we determine which function to use for our experiment?

Choosing the right function for an experiment depends on the specific goals and variables of the study. Some common factors to consider include the type of data being collected, the relationship between variables, and the expected outcome of the experiment.

Is there a specific method for selecting a function?

There is no one-size-fits-all method for choosing a function. It often involves a combination of knowledge and experience in the field, as well as trial and error. Consulting with other scientists or conducting a literature review can also help in the decision-making process.

How do we know if the chosen function is the most appropriate one?

The appropriateness of a function can be evaluated by analyzing the data and assessing how well the function fits the observed patterns. This can be done through statistical tests, visualizations, and comparing the function to other potential options.

Can we use more than one function in an experiment?

Yes, it is common to use multiple functions in an experiment. This can be especially useful when there are multiple variables or when the relationship between variables is complex. However, it is important to carefully consider the potential impact of using multiple functions and ensure that they are compatible with each other.

What should we do if the chosen function does not fit the data well?

If the chosen function does not fit the data well, it may be necessary to reevaluate the function and consider using a different one. This could also indicate that there are other variables or factors at play that were not initially considered. It is important to carefully analyze the data and make adjustments as needed to ensure accurate and meaningful results.

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