# Math Help - u_{xxyy}+u_{xx}+u_{yy}=0

1. ## u_{xxyy}+u_{xx}+u_{yy}=0

$u_{xxyy}+u_{xx}+u_{yy}=0$

Separation of variables

With mixed partial derivatives, is it true that separation of variables wont work?

2. Originally Posted by dwsmith
$u_{xxyy}+u_{xx}+u_{yy}=0$

Separation of variables

With mixed partial derivatives, is it true that separation of variables wont work?
Dear dwsmith,

Let $u(x,y)=f(x).g(x)$

$u_{xxyy}+u_{xx}+u_{yy}=0$

$f''(x).g''(y)+f''(x).g(y)+g''(y).f(x)=0$

$f''(x)\{g''(y)+g(y)\}=-g''(y).f(x)$

$\dfrac{f''(x)}{f(x)}=-\dfrac{g''(y)}{g''(y)+g(y)}$

So this partial differential equation is seperable.