# Hyperbolic Boundary Valued Problem

1. Dec 18, 2011

### robby991

Hi, I am trying to understand solving boundary valued partial differential equations and it's relation to hyperbolic functions. In one of my problems, there is a PDE and the solution contains the hyperbolic function "cosh". I was just curious if anyone has any information for me to read up on this, I cannot find anything in any book that gives any relation of the cosh function to solution to PDEs. Thanks.

2. Dec 20, 2011

### LCKurtz

Remember that $\cosh(x) = \frac {e^x + e^{-x}} 2$ and $\sinh(x) = \frac {e^x - e^{-x}} 2$, so any DE for which the solutions are exponentials could have the solutions expressed as sinh or cosh functions. The hyperbolic functions come in handy because of their values of 0 or 1 when x = 0 and similarly for their derivatives. For example consider the BVP
$y''-\lambda^2y=0$

$y(0) = 0, y(a) = 0$.
Find the solution to that using the pair {$e^x,e^{-x}$} and compare that to the work using the equivalent pair {$\sinh(x),\sinh(a-x)$}. This extra simplification can come in handy in some fourier series boundary value problems in PDE's.

Last edited: Dec 20, 2011
3. Dec 22, 2011

### robby991

Thank you, I understand now. I am evaluating this problem which I cannot duplicate the solution. It is as follows:

$i(t) =nDF\frac{\partial P}{\partial x} @ x = 0$

Evaluating the following limit

I = Lim i(t) as t→∞

by solving the following equations and boundary conditions:

$\frac{\partial S}{\partial t}=$$D\frac{\partial^{2}S}{\partial x^{2}}$-$\frac{Vm}{Km}S$
$\frac{\partial P}{\partial t}=$$D\frac{\partial^{2} P}{\partial x^{2}}$+$\frac{Vm}{Km}P$

b.c. $\frac{\partial S}{\partial x}$ = 0 @ x = 0, P = 0 @ x = 0, S = $S_{0}$ @ x ≥ d, P = 0 @ x ≥ d

Yields the following:

I=nDF$\frac{S_{0}}{d}$ (1-$\frac{1}{cosh(\alpha d)}$)

where, $\alpha^{2}$ = $\frac{Vm}{KmD}$

I am unable to produce "I" and I was wondering if someone can give me some insight. I don't completely understand the relation between $\frac{\partial P}{\partial x}$ and the 2 diffusion-based equations. When taking the limit, which part is evaluated? Thank you very much.