What is the general form of the notation u_{x_1 , ..,x_n}?

[duckywitbigfeet]

ok i am pretty good with the topic but i need help with this notation i actually have never seen it before i was wondering if anyone could put in a way that i might have seen it..

the problem is u_xx+u_yy = 0

 

[d_leet]

ok i am pretty good with the topic but i need help with this notation i actually have never seen it before i was wondering if anyone could put in a way that i might have seen it..

the problem is u_xx+u_yy = 0

$$u_{xx}=\frac{\partial^{2}u}{\partial x^{2}}$$

And similarly for u_yy, just replace x with y.

 

[Nesk]

If I remember correctly, the general form of that notation is:
$$u_{x_1 , ..,x_n} \equiv \frac{\partial ^n u}{\partial x_n... \partial x_1}$$