# Harmonic Functions (Partial Derivatives)

• Oct 26th 2010, 11:16 PM
ZeroVector
Harmonic Functions (Partial Derivatives)
The Laplace operator $\Delta$ is defined by $\Delta f = f_{xx} + f_{yy}$. A function $u(x,y)$ satisfying the Laplace equation $\Delta u = 0$ is called harmonic.
Show that $u(x,y) = x$ is harmonic.

First thing, though: What does it mean for a function to be harmonic? The explanation they give is confusing to me. Can you show me how to do this so I can understand how to do my other problems related to this?
• Oct 27th 2010, 03:02 AM
drumist
Suppose $f(x,y) = 2x^2 - 2y^2$. (This is just an example.)

Then $f_{xx} = 4$ and $f_{yy} = -4$, which means $\Delta f = f_{xx} + f_{yy} = 4 - 4 = 0$. Therefore, $f(x,y)$ is harmonic. If this value had been nonzero, then $f(x,y)$ would not be harmonic.

Does this make sense?
• Oct 27th 2010, 05:45 AM
HallsofIvy
You are told that $\nabla f= f_{xx}+ f_{yy}$ and you were told that u being "harmonic" means that u satisfies $\nabla u= 0$. Putting those together, u(x,y) is "harmonic" if and only if $u_{xx}+ u_{yy}= 0$. Is that true for u(x,y)= x?