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I have just started studying directional derivative from the book "Mathematical Analysis" written by T.M. Apostol.I find that the directional derivative is the generalization of partial derivative.Partial derivative represents the rate of change of a function due to small change of one of the independent variables involved where as directional derivative represents rate of change of the function due to the small change of a point in it's domain along any arbitrary direction.

So, the concept of directional derivative is as follows :

Let $\mathbf{f} : S \longrightarrow \mathbb R^{m}$ be a vector valued function defined over $S \subset \mathbb R^{n}$.Suppose we are to find out the rate of change of $\mathbf{f}$ when we move from a point $\mathbf{c}$ of $S$ to a nearby point $\mathbf{c} + \mathbf{u}$ along a line segment. Since each point of the line can be taken as $\mathbf{c} + h\mathbf{u}$ for some $h \in \mathbb R$.So we can take $h$ sufficiently small so that $\mathbf{c} + h\mathbf{u}$ is in $S$.Then the quantity

$$\lim_{h \rightarrow 0} \frac {\mathbf{f}(\mathbf{c} + h\mathbf{u}) - \mathbf{f}(\mathbf{c})} {h}$$

if it exists is called the directional derivative of $\mathbf{f}$ at $\mathbf{c} \in S$ in the direction of $\mathbf{u}$.

But I find some difficulty here.According to the lecture note of my teacher it is considered that $||\mathbf{u}|| = 1$.For this reason the directional derivative of a given function $\mathbf{f}$ at a point along some certain direction may differ.Now I have a question.

"Is there any significance of considering $||\mathbf{u}|| = 1$?"

If the answer to my question is affirmative one then why?This creates lots of trouble to me.Please help me in understanding this concept.

Thank you in advance.

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    $\begingroup$ Some authors require normalization; others don't. The advantage to not normalizing is that it makes the directional derivative a linear operator. I have answered this question here: math.stackexchange.com/questions/809376/… $\endgroup$ – symplectomorphic Dec 27 '16 at 6:20
  • $\begingroup$ Yes, there is a significance of considering $||u|| = 1.$ Because, $\frac{u}{||u||}$ is the unit vector, $\frac{hu}{||u||}$ represents the vector displacement while moving a distance of $h*||u|| = h$. $\endgroup$ – bat_of_doom Dec 27 '16 at 6:26
  • $\begingroup$ I find difficulty in understanding the concept in the link provided by you @symplectomorphic.In fact I fail to compare my defination of directional derivative to your's. Please be more explicit reminding the matter that I am a novice to this concept. $\endgroup$ – Arnab Chatterjee. Dec 27 '16 at 9:01
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The directional derivative $\mathrm D_u f(x)$ can be seen as the derivative of $$ \gamma(t) = f(x + tu) $$ at $0$. If we allow $\lVert u \rVert \neq 1$, we can define $\phi(t) = f(x + tu/\lVert u\rVert)$. Then $$ \gamma'(t) = \frac{\mathrm d}{\mathrm dt}f(x + tu) = \frac{\mathrm d}{\mathrm dt}f\left(x + (t\lVert u \rVert) \frac{u}{\| u\|} \right) = \|u\| \phi'(t) $$ which results in different directional derivatives along the same direction. This might be undesirable.

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  • $\begingroup$ Do you mean @Henry W. $f'(x + tu) = D_u f(x + tu)$? $\endgroup$ – Arnab Chatterjee. Dec 27 '16 at 9:18
  • $\begingroup$ Sorry,I also feel trouble to understand the fact that $\gamma' (t) = ||u|| \phi' (t)$.Please be more explicit. $\endgroup$ – Arnab Chatterjee. Dec 27 '16 at 9:42
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It's a very nice idea to view $D_u f(x)$ as a function of $u$, with $x$ fixed. In convex analysis, it turns out that if $f$ is convex then the function $u \mapsto D_u f(x)$ is a convex function of $u$, and in fact is the support function of the set $\partial f(x)$ (under mild assumptions). This is a beautiful connection between two different notions of "derivative" that are useful in convex analysis. But we could not make this statement if $u$ were required to be a unit vector.

So I think the elegant / beautiful thing to do is to allow $u$ to be any vector, not necessarily a unit vector.

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