# Thread: vector calculus and partial diff

1. ## vector calculus and partial diff

I was hoping to get help on the following problems. Hope to c a rpl soon from somebody

2. Originally Posted by prasoonmishra
I was hoping to get help on the following problems. Hope to c a rpl soon from somebody
Why don’t you simply post your questions on the forum?
Many of us object to opening attachements.

3. i could not write the equation and laplacian operator so i had to attach the file...

4. Originally Posted by prasoonmishra
i could not write the equation and laplacian operator so i had to attach the file...
It is clearly in Latex.
$$\nabla$$ gives $\nabla$.

5. Hint: Some of the possible solutions to these problems involve using Lanczos iteration, so make sure you are familiar with this algorithm.

Hint: I highly recommend that you avoid trying to solve for the eigenvalues of a non-hermetian matrix in any of the following problems. If your approach requires this, consider doing something else.

1. Consider a box with length, width and height given by L. The box encompasses the region described by 0<=x<=L, 0<=y<=L, and 0<=z<=L. A scalar field inside the box satisfies the differential equation:
$\nabla^2\Psi=-a\Psi$ (1)

Note that the differential operator here is known as the Laplacian. An equivalent of writing the equation is:

$\Delta\Psi=-a\Psi$ (2)

Here a is a positive constant that is equal to $30/L^2$.

The field is 0 on the surfaces y=0, y=L, x=0, x=L and finally the surface z=L. On the surface z=0, the field has the functional form:

$\Psi(x,y)=(1-|x-\frac{L}{2}|\frac{2}{L})(1-|y-\frac{L}{2}|\frac{2}{L})$ (3)

Solve for y as a function of x, y, and z inside the box. Your final solution must be an analytic expression, though it can involve an infinite sum.

2. Consider the system of nonlinear partial differential equations:

$\partial f/ \partial x=\partial g/ \partial y +af^2$

$\partial f/ \partial y = \partial g/ \partial x$ (1)

Here a is a small positive constant, such that a <<1. The boundary conditions are:

$f(x,0)= \sin(\frac{\pi}{L} 100x)$ (2)

$f(0,y)=0$ (3)
$f(L,y)=0$ (4)
$g(x,0)=0$ (5)

Solve for f(x,y) and g(x,y) for 0<=x<=L and y>=0 to first order in a. Your solution should be a finite sum of elementary functions. Numerical approximations are not to be used.

6. Now i have posted the question directly. Look into the questions please...