# How to prove Taylor's expansion for multivariable function ?

1. Nov 7, 2009

### abcdefg10645

As mentioned in the title~

Could anyone give me a hint or an idea ?

Thanks~

2. Nov 7, 2009

### HallsofIvy

What do you mean by "prove Taylor's expansion"? Taylor's expansion (by which I take it you mean writing a Taylor's series for the function) is defined to be a specific thing. Do you mean "proving the function is equal to its Taylor's series on the interval of convergence"? If so, what restrictions are you putting on the function. Of course, you must have f infinitely differentiable in order that its Taylor's series exist but there exist infinitely differentiable functionsm, whose Taylor's series converge for all x but that are not equal to their Taylor series anywhere except at the base point. One such is $f(x)= e^{-1/x^2}$ is $x\ne 0$, 0 if x= 0. Its Taylor's series, about x= 0, is identically 0 so converges uniformly for all x but is not equal to f(x) anywhere except at x= 0.

3. Nov 7, 2009