Partial Derivatives

Let $f : D \subseteq \mathbb{R}^n \to \mathbb{R}$. The partial derivative of $f$ with respect to $x_i$ at $\mathbf{a} = (a_1, \ldots, a_n)$ is

$\frac{\partial f}{\partial x_i}(\mathbf{a}) = \lim_{h \to 0} \frac{f(a_1, \ldots, a_i + h, \ldots, a_n) - f(a_1, \ldots, a_n)}{h}$

Provided the limit exists. This is the rate of change of $f$ in the direction of the $x_i$-axis, Holding all other variables fixed.

Notation. Common notations for the partial derivative with respect to $x_i$ include $f_{x_i}$, $\partial_i f$And $\frac{\partial f}{\partial x_i}$. We use these interchangeably.

Theorem 1.1 (Clairaut’s Theorem / Schwarz’s Theorem). If $f_{xy}$ and $f_{yx}$ are continuous on an Open set containing $(a, b)$Then

$\frac{\partial^2 f}{\partial x \partial y}(a,b) = \frac{\partial^2 f}{\partial y \partial x}(a,b)$

Proof. Define the second-order difference function

$\Delta(h, k) = f(a+h,\, b+k) - f(a+h,\, b) - f(a,\, b+k) + f(a, b)$

For $h, k \neq 0$. Define $\phi(s) = f(s, b+k) - f(s, b)$. Then $\Delta(h,k) = \phi(a+h) - \phi(a)$. By the Mean Value Theorem, there exists $\theta_1 \in (0, 1)$ such that

$\Delta(h, k) = h \cdot \phi'(a + \theta_1 h) = h \left[f_x(a + \theta_1 h,\, b+k) - f_x(a + \theta_1 h,\, b)\right]$

Apply the Mean Value Theorem again to the function $g(t) = f_x(a + \theta_1 h,\, t)$ on $[b, b+k]$. There exists $\theta_2 \in (0, 1)$ such that

$\Delta(h, k) = hk \cdot f_{xy}(a + \theta_1 h,\, b + \theta_2 k)$

Similarly, by reversing the order of application, there exist $\theta_3, \theta_4 \in (0,1)$ such That

$\Delta(h, k) = hk \cdot f_{yx}(a + \theta_3 h,\, b + \theta_4 k)$

For $h, k \neq 0$ we have

$f_{xy}(a + \theta_1 h,\, b + \theta_2 k) = f_{yx}(a + \theta_3 h,\, b + \theta_4 k)$

Taking the limit as $(h, k) \to (0, 0)$ and using continuity of $f_{xy}$ and $f_{yx}$We obtain $f_{xy}(a, b) = f_{yx}(a, b)$. $\blacksquare$

Intuition. Clairaut’s theorem tells us that, under a mild regularity condition (continuity of the Mixed second partials), the order in which we differentiate does not matter. Without this Condition, the mixed partials may differ.

Definition. $f : D \subseteq \mathbb{R}^n \to \mathbb{R}$ is differentiable at $\mathbf{a}$ if There exists a linear map $L : \mathbb{R}^n \to \mathbb{R}$ such that

$\lim_{\mathbf{h} \to \mathbf{0}} \frac{f(\mathbf{a} + \mathbf{h}) - f(\mathbf{a}) - L(\mathbf{h})}{\lVert \mathbf{h} \rVert} = 0$

When $f$ is differentiable at $\mathbf{a}$The linear map $L$ is given by the gradient.

Remark. Existence of all partial derivatives at a point does not imply differentiability at That point. The canonical counterexample is

$f(x,y) = \begin{cases} \dfrac{xy}{x^2 + y^2} & \mathrm{if\ }(x,y) \neq (0,0), \\ 0 & \mathrm{if\ }(x,y) = (0,0). \end{cases}$

Both $f_x(0,0)$ and $f_y(0,0)$ exist (and equal $0$), yet $f$ is not even continuous at the origin, Hence not differentiable.

The gradient of $f$ at $\mathbf{a}$ is

$\nabla f(\mathbf{a}) = \left(\frac{\partial f}{\partial x_1}(\mathbf{a}), \ldots, \frac{\partial f}{\partial x_n}(\mathbf{a})\right)$

The linear approximation of $f$ near $\mathbf{a}$ is

$f(\mathbf{a} + \mathbf{h}) \approx f(\mathbf{a}) + \nabla f(\mathbf{a}) \cdot \mathbf{h}$

Theorem 1.2. If all partial derivatives of $f$ exist and are continuous in a neighbourhood of $\mathbf{a}$Then $f$ is differentiable at $\mathbf{a}$.

Remark. Functions whose partial derivatives exist and are continuous on an open set $U$ are called $C^1(U)$. Theorem 1.2 says $C^1 \implies$ differentiable. The converse is false: there exist Differentiable functions whose partial derivatives are not continuous.

Proposition. If $f$ is differentiable at $\mathbf{a}$Then $f$ is continuous at $\mathbf{a}$.

Proof. From the definition of differentiability:

$f(\mathbf{a} + \mathbf{h}) - f(\mathbf{a}) = L(\mathbf{h}) + \varepsilon(\mathbf{h})\lVert \mathbf{h} \rVert$

Where $L$ is linear and $\varepsilon(\mathbf{h}) \to 0$ as $\mathbf{h} \to \mathbf{0}$. As $\mathbf{h} \to \mathbf{0}$ Both terms on the right vanish, so $f(\mathbf{a} + \mathbf{h}) \to f(\mathbf{a})$. $\blacksquare$

The directional derivative of $f$ at $\mathbf{a}$ in the direction of a unit vector $\mathbf{u}$ is

$D_{\mathbf{u}} f(\mathbf{a}) = \lim_{h \to 0} \frac{f(\mathbf{a} + h\mathbf{u}) - f(\mathbf{a})}{h}$

Theorem 1.3. If $f$ is differentiable at $\mathbf{a}$Then

$D_{\mathbf{u}} f(\mathbf{a}) = \nabla f(\mathbf{a}) \cdot \mathbf{u}$

Proof. Since $f$ is differentiable at $\mathbf{a}$

$\frac{f(\mathbf{a} + h\mathbf{u}) - f(\mathbf{a})}{h} = \frac{\nabla f(\mathbf{a}) \cdot (h\mathbf{u}) + \varepsilon(h\mathbf{u}) \lVert h\mathbf{u} \rVert}{h}$

$= \nabla f(\mathbf{a}) \cdot \mathbf{u} + \varepsilon(h\mathbf{u}) \lVert \mathbf{u} \rVert$

Where $\varepsilon(\mathbf{h}) \to 0$ as $\mathbf{h} \to \mathbf{0}$. Taking $h \to 0$ gives the result. $\blacksquare$

Corollary 1.4. The gradient points in the direction of steepest ascent, and $\lVert \nabla f \rVert$ Is the rate of steepest ascent.

Proof. By the Cauchy—Schwarz inequality, $\lvert \nabla f \cdot \mathbf{u} \rvert \leq \lVert \nabla f \rVert \cdot \lVert \mathbf{u} \rVert = \lVert \nabla f \rVert$ With equality when $\mathbf{u}$ is parallel to $\nabla f$. $\blacksquare$

Theorem 1.5 (Multivariable Chain Rule). If $\mathbf{g} : \mathbb{R}^m \to \mathbb{R}^n$ is Differentiable at $\mathbf{a}$ and $f : \mathbb{R}^n \to \mathbb{R}$ is differentiable at $\mathbf{g}(\mathbf{a})$Then

$\nabla (f \circ \mathbf{g})(\mathbf{a}) = J\mathbf{g}(\mathbf{a})^T \nabla f(\mathbf{g}(\mathbf{a}))$

Where $J\mathbf{g}$ is the Jacobian matrix of $\mathbf{g}$.

Proof. Write $h(t) = f(\mathbf{g}(\mathbf{a} + t\mathbf{v}))$ for a fixed direction $\mathbf{v}$. Then

$\frac{h(t) - h(0)}{t} = \frac{f(\mathbf{g}(\mathbf{a} + t\mathbf{v})) - f(\mathbf{g}(\mathbf{a}))}{t}$

Let $\mathbf{k} = \mathbf{g}(\mathbf{a} + t\mathbf{v}) - \mathbf{g}(\mathbf{a})$. By differentiability of $\mathbf{g}$ $\mathbf{k} = J\mathbf{g}(\mathbf{a})(t\mathbf{v}) + o(t)$And $\mathbf{k} \to \mathbf{0}$ as $t \to 0$. By Differentiability of $f$:

$f(\mathbf{g}(\mathbf{a}) + \mathbf{k}) - f(\mathbf{g}(\mathbf{a})) = \nabla f(\mathbf{g}(\mathbf{a})) \cdot \mathbf{k} + o(\lVert \mathbf{k} \rVert)$

$= \nabla f(\mathbf{g}(\mathbf{a})) \cdot [J\mathbf{g}(\mathbf{a})(t\mathbf{v}) + o(t)] + o(t)$

Dividing by $t$ and taking $t \to 0$:

$h'(0) = \nabla f(\mathbf{g}(\mathbf{a})) \cdot J\mathbf{g}(\mathbf{a})\mathbf{v} = [J\mathbf{g}(\mathbf{a})^T \nabla f(\mathbf{g}(\mathbf{a}))] \cdot \mathbf{v}$

Since $\mathbf{v}$ was arbitrary, $\nabla h(0) = J\mathbf{g}(\mathbf{a})^T \nabla f(\mathbf{g}(\mathbf{a}))$. $\blacksquare$

Problem. Let $f(x, y) = x^2 y$ and let $x = \cos t$, $y = \sin t$. Find $\frac{d}{dt} f(\cos t, \sin t)$ Using the chain rule, and verify by direct substitution.

Problem. Let $f(x, y) = x^2 y + \sin(xy)$. Compute $\nabla f$ and find the directional derivative At $(1, \pi)$ in the direction $\mathbf{u} = (1/\sqrt{2}, 1/\sqrt{2})$.

Solution.

$\frac{\partial f}{\partial x} = 2xy + y\cos(xy)$

$\frac{\partial f}{\partial y} = x^2 + x\cos(xy)$

$\nabla f(1, \pi) = (2\pi + \pi\cos(\pi), 1 + \cos(\pi)) = (2\pi - \pi, 1 - 1) = (\pi, 0)$

$D_{\mathbf{u}} f(1, \pi) = \nabla f(1, \pi) \cdot \mathbf{u} = \pi \cdot \frac{1}{\sqrt{2}} + 0 = \frac{\pi}{\sqrt{2}}$ $\blacksquare$

Problem. Let $f(x, y, z) = x^2 y\, e^z + \sin(xz)$. Compute $\nabla f$ and evaluate it at $(1, 0, \pi)$.

Problem. Find the directional derivative of $f(x,y) = x^2 y^3$ at $(1, -1)$ in the direction of $\mathbf{v} = (3, -4)$.

Suppose $F(x, y, z) = 0$ defines $z$ implicitly as a function of $x$ and $y$ near a point $(a, b, c)$ with $F_z(a, b, c) \neq 0$. By the Implicit Function Theorem, there exists a $C^1$ function $\varphi$ defined on a neighbourhood of $(a, b)$ such that $\varphi(a, b) = c$ and $F(x, y, \varphi(x, y)) = 0$.

Differentiating $F(x, y, \varphi(x, y)) = 0$ with respect to $x$:

$F_x + F_z \cdot \frac{\partial z}{\partial x} = 0 \implies \frac{\partial z}{\partial x} = -\frac{F_x}{F_z}$

Similarly, $\frac{\partial z}{\partial y} = -\frac{F_y}{F_z}$.

Proposition 1.6 (Implicit Function Theorem, special case). If $F : \mathbb{R}^3 \to \mathbb{R}$ is $C^1$ and $F(a,b,c) = 0$ with $F_z(a,b,c) \neq 0$Then there exist neighbourhoods $U$ of $(a,b)$ and $V$ of $c$ and a unique $C^1$ function $\varphi : U \to V$ with $\varphi(a,b) = c$ and $F(x, y, \varphi(x,y)) = 0$ for all $(x,y) \in U$.

Problem. If $x^2 y + y^2 z + z^2 x = 3$Find $\frac{\partial z}{\partial x}$ and $\frac{\partial z}{\partial y}$ at the point $(1, 1, 1)$.

Theorem 1.7 (Taylor’s Theorem). Let $f : U \subseteq \mathbb{R}^n \to \mathbb{R}$ be of class $C^{k+1}$ On an open convex set $U$And let $\mathbf{a} \in U$. Then for all $\mathbf{x} \in U$:

$f(\mathbf{x}) = f(\mathbf{a}) + \nabla f(\mathbf{a}) \cdot (\mathbf{x} - \mathbf{a}) + \frac{1}{2!}(\mathbf{x} - \mathbf{a})^T H_f(\mathbf{a})(\mathbf{x} - \mathbf{a}) + \cdots + R_k$

Where $H_f$ is the Hessian matrix and the remainder $R_k$ can be written in Lagrange form:

$R_k = \frac{1}{(k+1)!} \sum_{\lvert \alpha \rvert = k+1} \frac{(k+1)!}{\alpha!} D^{\alpha} f(\mathbf{c})\, (\mathbf{x} - \mathbf{a})^{\alpha}$

For some $\mathbf{c}$ on the line segment joining $\mathbf{a}$ and $\mathbf{x}$.

For $n = 2$ and $k = 2$The second-order Taylor expansion is:

$f(a+h, b+k) = f(a,b) + f_x h + f_y k + \frac{1}{2}\left(f_{xx} h^2 + 2f_{xy} hk + f_{yy} k^2\right) + R_2$

Where all partial derivatives are evaluated at $(a, b)$ and the remainder is

$R_2 = \frac{1}{6}\left(f_{xxx} h^3 + 3f_{xxy} h^2 k + 3f_{xyy} hk^2 + f_{yyy} k^3\right)\Big|_{\mathbf{c}}$

Proof (sketch). Define $\phi(t) = f(\mathbf{a} + t(\mathbf{x} - \mathbf{a}))$ for $t \in [0, 1]$. Apply the single-variable Taylor theorem to $\phi$ at $t = 0$:

$\phi(1) = \phi(0) + \phi'(0) + \frac{1}{2!}\phi''(0) + \cdots + \frac{1}{k!}\phi^{(k)}(0) + \frac{1}{(k+1)!}\phi^{(k+1)}(\tau)$

For some $\tau \in (0, 1)$. By the multivariable chain rule, $\phi'(t) = \nabla f(\mathbf{a} + t(\mathbf{x}-\mathbf{a})) \cdot (\mathbf{x}-\mathbf{a})$And higher Derivatives involve higher-order partial derivatives of $f$. Substituting $\mathbf{c} = \mathbf{a} + \tau(\mathbf{x}-\mathbf{a})$ yields the result. $\blacksquare$

:::caution Common Pitfalls

• Existence $\neq$ continuity of partials. A function can have all partial derivatives at a point yet fail to be continuous (hence not differentiable) there.
• Existence $\neq$ differentiability. Even if all partials exist at a point, the function need not be differentiable. Continuity of the partials in a neighbourhood (i.e., $C^1$) is sufficient but not necessary.
• Clairaut’s theorem requires continuity. Without continuity of the mixed partials, the equality $f_{xy} = f_{yx}$ can fail.
• Normalise the direction vector. The formula $D_{\mathbf{u}} f = \nabla f \cdot \mathbf{u}$ assumes $\lVert \mathbf{u} \rVert = 1$. If the direction is given by a non-unit vector $\mathbf{v}$Divide by $\lVert \mathbf{v} \rVert$ first.

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