# Math Help - Directional Derivative implying Continuity

1. ## Directional Derivative implying Continuity

I'm having problems with how to prove this statement:

Suppose that a function g:R^n-->R has the property that Dug(0)=0, where Dug(0) is the directional derivative of g in the direction of unit vector u at the point (0,...,0).
Does it follow that g is continuous at 0? Prove or give an example showing that it is false.

Thanks a lot!

2. Hi AKTilted,

I have assumed in your question that $\mathbf{u}$ can be any unit vector. If this isn't true then I don't think it can be shown.

If this is true for any unit vector $\mathbf{u}$, then it is true for each $\mathbf{u}_{i} = (0,\ldots,1,\ldots,0),$ where 1 is in the $i^{\text{th}}$ position. Then we have that
$D_{u}g(0)=\nabla g(0) \cdot \mathbf{u}_{i} = \frac{\partial g}{\partial x_{i}}(0) = 0.$
Now since $g$ does not change in any direction from 0, then it must be a constant function near 0, which is continuous.