# partial differential equation

• Apr 5th 2010, 05:09 PM
vuze88
partial differential equation
A function $f$ of two variables is said to be homogeneous of degree n if $f(tx,ty)=t^nf(x,y)$ whenever $t>0$.

Show that such a function $f$ satisfies the equation $x\frac{\partial f}{\partial x}+y\frac{\partial f}{\partial y}=nf$
• Apr 5th 2010, 06:04 PM
tonio
Quote:

Originally Posted by vuze88
A function $f$ of two variables is said to be homogeneous of degree n if $f(tx,ty)=t^nf(x,y)$ whenever $t>0$.

Show that such a function $f$ satisfies the equation $x\frac{\partial f}{\partial x}+y\frac{\partial f}{\partial y}=nf$

Derive both sides of $f(tx,ty)=t^nf(x,y)$ wrt $t$ and then evaluate the result at $t=1$ .

Tonio
• Apr 8th 2010, 01:46 AM
vuze88
Quote:

Originally Posted by tonio
Derive both sides of $f(tx,ty)=t^nf(x,y)$ wrt $t$ and then evaluate the result at $t=1$ .

Tonio

Sorry i still dont fully understand. Can you explain this in more detail. Also im not sure whether the f on the RHS means f(x,y) or f(tx,ty)
• Apr 8th 2010, 02:07 AM
tonio
Quote:

Originally Posted by vuze88
Sorry i still dont fully understand. Can you explain this in more detail. Also im not sure whether the f on the RHS means f(x,y) or f(tx,ty)

It is exactly as I wrote it: derivate the left side wrt t, derivate the right side wrt t, they're equal, of course, and now put t=1 .

Tonio
• Apr 8th 2010, 02:37 AM
vuze88
lol yeah but how to i differentiate with respect to t when the function is in terms of tx and ty
• Apr 8th 2010, 03:30 AM
HallsofIvy
Use the chain rule, of course!

$\frac{df(ax)}{dx}= a\frac{df}{dx}$
• Apr 8th 2010, 03:37 AM
Pulock2009
Euler's theorem
Quote:

Originally Posted by HallsofIvy
Use the chain rule, of course!

$\frac{df(ax)}{dx}= a\frac{df}{dx}$

this is what Euler's theorem on partial derivatives says.