\[ \newcommand{\R}{\mathbb{R}} \newcommand{\Z}{\mathbb{Z}} \newcommand{\lbrac}{\left(} \newcommand{\rbrac}{\right)} \newcommand\pd[2]{\displaystyle\frac{\partial #1}{\partial #2}} \]

Topics: Mathematics, Physics, Standing Waves, Python

Maxima and Minima of an n-th Dimensional Function - The Hessian Matrix

Updated May 25, 2024

“As you will find in multivariable calculus, there is often a number of solutions for any given problem.”

- John Nash

Local Maximum, Local Minimum, and Extremum

Consider an $n$-dimensional function $z=f(\mathbf{x})$ defined by $f: D\subseteq \R^n \to \R$, where $\mathbf{x}=(x_1, x_2, \ldots, x_n)$ is an $n$-tuple of real numbers. Also consider a point $\mathbf{a} = (a_1, a_2, \ldots, a_n)$ defined in the domain $D\subseteq\R^n$.

Local Maximum. The function $z=f(\mathbf{x})$ attains its local maximum at $\mathbf{a}$ if there exists a neighborhood $N_\varepsilon(\mathbf{a})\in D$ of $\mathbf{a}$ such that $f(\mathbf{x})\leq f(\mathbf{a})$, $\forall \mathbf{x}\in N_\varepsilon(\mathbf{a}) \backslash \{\mathbf{a}\}$.
Local Minimum. The function $z=f(\mathbf{x})$ attains its local minimum at $\mathbf{a}$ if there exists a neighborhood $N_\varepsilon(\mathbf{a})\in D$ of $\mathbf{a}$ such that $f(\mathbf{x})\geq f(\mathbf{a})$, $\forall \mathbf{x}\in N_\varepsilon(\mathbf{a}) \backslash \{\mathbf{a}\}$.
Extremum. The function $z=f(\mathbf{x})$ attains its maximum, or minimum at point $\mathbf{a}$ is generally called an extremum at point $\mathbf{a}$. Point $\mathbf{a}$ might sometimes be called the extreme point of the function and $f(\mathbf{a})$ is called the extreme value of the function $z=f(\mathbf{x})$.

We call the plural of maximum maxima, the plural of minimum minima, and the plural of extremum extrema.

If the function $z=f(\mathbf{x})$ has continuous first partial derivatives in some neighborhood of point $\mathbf{a}$ and obtain its extrema at $\mathbf{a}$, then

$$\pd{f}{x_i}(a_1,a_2,...,a_n)=\pd{f(\mathbf{a})}{x_i}=0\quad\text{for}\; i=1,2,...,n$$

That is, for each first partial derivative of $f$,

\[ \left\{ \begin{array}{cc} \pd{f(\mathbf{a})}{x_1}=f_{x_1}(a_1,a_2,...,a_n)=0 \\[0.1em] \pd{f(\mathbf{a})}{x_2}=f_{x_2}(a_1,a_2,...,a_n)=0 \\[0.1em] \vdots \\[-0.1em] \pd{f(\mathbf{a})}{x_n}=f_{x_n}(a_1,a_2,...,a_n)=0 \end{array} \right., \]

Stationary Points, Critical Points, and Saddle Points

Consider an $n$-dimensional function $z=f(\mathbf{x})$, $f: D\subseteq \R^n \to \R$, where $\mathbf{x}=(x_1, x_2, \ldots, x_n)$ is an $n$-tuple of real numbers. Assume that the function is defined at a point $\mathbf{a} = (a_1, a_2, \ldots, a_n)\in D\subseteq\R^n$.

Stationary Point. The points where all of the partial derivatives of $f$ equals 0 is called the stationary points of $f$. That is,
$$\pd{f}{x_i}(a_1,a_2,...,a_n)=\pd{f(\mathbf{a})}{x_i}=0\quad\forall\; i=1,2,...,n$$
Critical Point. The point $\mathbf{a}=(a_1, a_2, \ldots ,a_n)\in D$ is called a critical point if at least one of the partial derivatives of $f$ at that point equals 0 or undefined.
Saddle Point. The point $\mathbf{a}=(a_1,a_2,\ldots,a_n)\in D$ is called a saddle point if it is a stationary point and for all $V_\varepsilon(\mathbf{a})$, and there exists $\mathbf{y}=(y_1, y_2,\ldots,y_n)\in V_\varepsilon$ and $\mathbf{z}=(z_1,z_2,\ldots,z_n)\in V_\varepsilon$ such that $$f(\mathbf{y})< f(\mathbf{a})< f(\mathbf{z}).$$

The Hessian Matrix and the Sylvester Criterion

The Hessian Matrix. Consider a function $z=f(\mathbf{x})$ and $\mathbf{x}$ defined as before, and let $f$ be a function whose second partial derivatives are continuous on its domain $D$. Consider also a point $\mathbf{a}=(a_1,a_2,...,a_n)\in D\subseteq \R^n$. By then, the matrix

$$H_f(\mathbf{a})= \begin{bmatrix} f_{x_1x_1}(\mathbf{a}) & f_{x_1x_2}(\mathbf{a}) & \cdots & f_{x_1x_n}(\mathbf{a})\\ f_{x_2x_1}(\mathbf{a}) & f_{x_2x_2}(\mathbf{a}) & \cdots & f_{x_2x_n}(\mathbf{a})\\ \vdots & \vdots & \ddots & \vdots\\ f_{x_nx_1}(\mathbf{a}) & f_{x_nx_2}(\mathbf{a}) & \cdots & f_{x_nx_n}(\mathbf{a}) \end{bmatrix}$$

is called the Hessian matrix of the function $f$ at point $\mathbf{a}$. Since $f$ has continuous second partial derivatives, $H_f(M)$ must be a symmetric square matrix; that is, $H_f(\mathbf{a})=H_f(\mathbf{a})^{\mathsf{T}}$.

Quadratic Form. A quadratic form, to put simply, is just a second-degree multi-variable polynomial such as

$$\phi(x,y,z) = ax^2 + bxy + cy^2 + dyz + ez^2 + fxz.$$

One interesting about quadratic forms is that every symmetric matrix defines a quadratic form; that is, given that $\mathbf{A} = [a]_{ij}$, the polynomial

$$\phi(\mathbf{x}) = \sum_{i=1}^{n}\sum_{j=1}^{n}a_{ij}x_ix_j = \mathbf{x}^{\mathsf{T}}\mathbf{A}\mathbf{x}$$

will results in a quadratic form without requiring us to combining terms. Therefore, $H_f(\mathbf{a})$ can most definitely be treated as a matrix associated with a quadratic form.

Definite Matrix. A symmetric real matrix $\mathbf{A}$ is called positive definite if the associated quadratic form $$\phi( \mathbf{x})=\mathbf{x}^{\mathsf{T}}\mathbf{A}\mathbf{x}$$ is positive for every column ($n\times 1$), non-zero vector $\mathbf{x}$ in $\R^n$. Similarly, if $\phi(\mathbf{x})$ only yields negative values, then $\mathbf{A}$ is called negative definite. Finally, if $\phi$ produces both negative and positive values, then $\mathbf{A}$ is said to be indefinite. There are also other cases such as positive semi-definite or negative semi-definite matrices. In general,

$\mathbf{A}$ is positive-definite if and only if $\mathbf{x}^{\mathsf{T}}\mathbf{A}\mathbf{x}>0,\;\forall \mathbf{x}\in\R^n \backslash\{\mathbf{0}\}$.
$\mathbf{A}$ is negative-definite if and only if $\mathbf{x}^{\mathsf{T}}\mathbf{A}\mathbf{x}< 0,\;\forall \mathbf{x}\in\R^n \backslash\{\mathbf{0}\}$.
$\mathbf{A}$ is semi-positive-definite if and only if $\mathbf{x}^{\mathsf{T}}\mathbf{A}\mathbf{x}\geq 0,\;\forall \mathbf{x}\in\R^n$.
$\mathbf{A}$ is semi-negative-definite if and only if $\mathbf{x}^{\mathsf{T}}\mathbf{A}\mathbf{x}\leq 0,\;\forall \mathbf{x}\in\R^n$.

Sub-matrices. Given an $n\times n$ matrix $\mathbf{A}$ with $(i,j)$-th entry, where $a_{ij}=a_{ji}$. Let $A^{(k)}$ denote the $k\times k$ submatrix taken from the top left corner of $\mathbf{A}$. These matrices are called major sub-matrices of $A$. That is,

$$\mathbf{A}^{(k)}= \begin{bmatrix} a_{11} & a_{12} & \cdots & a_{1k} \\ a_{21} & a_{22} & \cdots & a_{2k} \\ \vdots & \vdots & \ddots & \vdots \\ a_{k1} & a_{k2} & \cdots & a_{kk} \end{bmatrix},\quad 1\leq k\leq n$$

In particular,

$$\mathbf{A}^{(1)}= \begin{bmatrix} a_{11} \end{bmatrix},\quad \mathbf{A}^{(2)}= \begin{bmatrix} a_{11} & a_{12}\\ a_{21} & a_{22}\\ \end{bmatrix}, \quad \mathbf{A}^{(3)}= \begin{bmatrix} a_{11} & a_{12} & a_{13}\\ a_{21} & a_{22} & a_{23}\\ a_{31} & a_{32} & a_{33} \end{bmatrix},\cdots, \mathbf{A}^{(n)}=\mathbf{A} $$

Let $\Delta_k=\det(\mathbf{A}^{k})\;,1\leq k\leq n$. For instance, $\Delta_1=a_{11}$ and $\Delta_n =\det(\mathbf{A})$.

Let $\mathbf{A}$ be an $n\times n$ symmetric matrix. Then,

$\mathbf{A}$ is positive definite, denote $\mathbf{A}\succ 0$ if and only if $\Delta_1>0,\Delta_2>0,...,\Delta_n>0$.
$\mathbf{A}$ is negative definite, denote $\mathbf{A}\prec 0$ if and only if the signs of $\Delta_i$ alternates, with $\Delta_1 < 0$; that is,
$$(-1)^1\Delta_1>0, \;(-1)^2\Delta_2>0,\ldots,\;(-1)^n\Delta_n>0.$$
$\mathbf{A}$ is indefinite if there exists a $\Delta_k$ that breaks both patterns above such that $\Delta_k$ is in the wrong sign.
The Sylvester criterion is inconclusive ($\mathbf{A}$ can be either positive or negative definite, or indefinite) if there exists a $\Delta_k$ that breaks the first two patterns such that $\Delta_k=0$.

The Maximum/Minimum Algorithm for an n-Variable Function

The General Case

The General Second Derivative Test. To identify all the stationary (and critical points) of $f(\mathbf{x})$ and test if they are the extrema of $f$, we follow the following procedure:

Find $D(f)$, the domain of $f(\mathbf{x})$. This is for us to determine whether to keep a critical point or not.
Solve the equation $\nabla f(\mathbf{x})=\mathbf{0}$; that is, solve the system of $n$-equations
\[ \left\{ \begin{array}{cc} \pd{f(\mathbf{x})}{x_1}=f_{x_1}(x_1,x_2,...,x_n)=0 \\[0.1em] \pd{f(\mathbf{x})}{x_2}=f_{x_2}(x_1,x_2,...,x_n)=0 \\[0.1em] \vdots \\[-0.1em] \pd{f(\mathbf{x})}{x_n}=f_{x_n}(x_1,x_2,...,x_n)=0 \end{array} \right., \]
to find all stationary points of $f(\mathbf{x})$ (if they exist). Label these points, for instance, $\mathbf{a}_0=(a_1^0, \;a_2^0,...,\;a_n^0)$, $\mathbf{a}_1=(a_1^1,\;a_2^1,\ldots,\;a_n^1),\ldots$. Next, find all points where the partial derivatives $f_{x_1}(\mathbf{x}), f_{x_2}(\mathbf{x}),...,f_{x_n}(\mathbf{x})$ are undefined, we now have the set of all critical points of $f(\mathbf{x})$.

Remark. One reminder to note is that all critical points that need to be considered must lie inside the domain $D$. If not, just discard them.
If $\mathbf{a_0}$ is a stationary point, calculate its Hessian matrix at $\mathbf{a_0}$; that is, find $H_f(\mathbf{a_0})$. After that, apply Sylvester's Criterion on $H_f(\mathbf{a_0})$ by calculating the values of $\Delta_1,\Delta_2,...,\Delta_n$ at $\mathbf{a_0}$.
1. If $\mathrm{H}_f(\mathbf{a_0})\succ 0$, then $\mathbf{a_0}$ is a minimum of the function $f(\mathbf{x})$.
2. If $\mathrm{H}_f(\mathbf{a_0})\prec 0$, then $\mathbf{a_0}$ is a maximum of the function $f(\mathbf{x})$.
3. If $\det (\Delta_k)\neq 0,\;1\leq k\leq n,$ but $\mathrm{H}_f(\mathbf{a_0})$ itself is neither positive nor negative definite, then $\mathbf{a_0}$ is not an extremum. A similar statement is that if there exist two points $\mathbf{y}=(y_1,y_2,...,y_n)$ and $\mathbf{z}=(z_1,z_2,...,z_n)$ such that $\mathbf{y}H_f(\mathbf{a_0})\mathbf{y}^{\mathsf{T}}>0$ and $\mathbf{z}H_f(\mathbf{a_0})\mathbf{z}^{\mathsf{T}}< 0$, then $\mathbf{a_0}$ is not an extremum of $f(\mathbf{x})$.
4. If there exists a point $\mathbf{y}=(y_1,y_2,...,y_n)$ such that $\mathbf{y}^{\mathrm{T}}\mathrm{H}_f(\mathbf{a_0})\mathbf{y}=0$; that is, if there is at least one major sub-matrix of $\mathrm{H}_f(\mathbf{a_0})$ equals to 0, then the test is inconclusive.
Repeat the same process again for the stationary point $\mathbf{a_1}$.

The Special Case: $n=2$

For a 2-variable function $z=f(x,y)$, the maximum/minimum algorithm can be simplified a bit. After finding all stationary and critical points of $f(x,y)$, we can follow these steps to identify the extrema of $f$:

For each stationary point $(x_0,y_0)$, calculate the second partial derivatives of $f$ at that point: $f_{xx}(x_0,y_0)$, $f_{yy}(x_0,y_0)$, and $f_{xy}(x_0,y_0)$.
Calculate $\Delta_2(x_0,y_0)=f_{xx}(x_0,y_0)f_{yy}(x_0,y_0)-[f_{xy}(x_0,y_0)]^2$.
1. If $\Delta_2(x_0,y_0)>0$ and $f_{xx}(x_0,y_0)>0$, then $(x_0,y_0)$ is a minimum of $f(x,y)$.
2. If $\Delta_2(x_0,y_0)>0$ and $f_{xx}(x_0,y_0)< 0 $, then $(x_0,y_0)$ is a maximum of $f(x,y)$.
3. If $\Delta_2(x_0,y_0)< 0$, then $(x_0,y_0)$ is not an extremum of $f(x,y)$.
4. If $\Delta_2(x_0,y_0)=0$, then the test is inconclusive.

Example Problems with Solutions

In 3D Space

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=x^2-xy+y^2-2x+y$$

Solution

We first need to find the domain of $f$, which is just simply $D=\R^2$ since there is no point in $\R^2$ where the function is undefined. We then solve for $\nabla f(x,y)=(0,0)$, that is, solve for the system of equations

\begin{equation*} \begin{cases} f_x(x,y)=2x-y-2=0\\[0.5em] f_y(x,y)=2y-x+1=0 \end{cases} \quad\Rightarrow\quad \begin{cases} x=1\\[0.5em] y=0 \end{cases} \end{equation*}

The function $f$ has one critical point $(x,y)=(1,0)$. The next step is to find second partial derivatives of $f$: $$f_{xx}(x,y)=2, \;f_{xy}(x,y)=1,\;\text{and}\;f_{yy}(x,y)=2$$ At $(1,0)$, we have $f_{xx}(1,0)=2$, $f_{xy}(1,0)=1$ and $f_{yy}(1,0)=2$. Since, $\Delta_2(1,0)=f_{xx}f_{yy}-f_{xy}^2=3>0$ and $f_{xx}(1,0)=2>0$, therefore, $(1,0)$ is a minimum of $f(x,y)$ and $f_{\min} = f(1, 0) = -1$.

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=\sqrt{x^2+y^2}$$

Solution

The first step, again, is to find the domain of $f: D=\R^2$. We then solve for $\nabla f(x,y)=(0,0)$

\begin{equation*} \begin{cases} f_x(x,y)=\dfrac{x}{\sqrt{x^2+y^2}}=0\\[0.5em] f_y(x,y)=\dfrac{y}{\sqrt{x^2+y^2}}=0\\ \end{cases} \end{equation*}

If we simply solve for $x$ and $y$, we will immediately get $(x, y) = (0, 0)$. This solution is invalid because the condition of the system requires $(x, y)\neq (0,0)$.

Based on the condition of the equations system, the function $f$ has no stationary point, but it has one critical point $(0,0)$ since that point is undefined at both $f_x$ and $f_y$. Notice that $f(x,y)=\sqrt{x^2+y^2}\geq f(0,0)=0,$, $\forall (x,y)\in\R^2$. Therefore, by definition, $(0,0)$ is the minimum of the function and $f_{\min}=0$.

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=xy+\dfrac{50}{x}+\dfrac{20}{y}$$

Solution

Domain of $f$: $D=\R^2\backslash\{(0,0)\}$. We then solve for $\nabla f(x,y)=(0,0)$

\[ \begin{cases} f_x(x,y)=y-\dfrac{50}{x^2}=0\\[1em] f_y(x,y)=x-\dfrac{20}{y^2}=0 \end{cases} \]

For this system to be well-defined, $x$ and $y$ must not be equal to zero. We have already defined that in the domain of $f$, so now, we can simply multiple both sides of the first equations by $x$, both sides of the second equation by $y$, we will obtain

\[ \begin{cases} x^2y=50\\ xy^2=20 \end{cases} \quad\Rightarrow{y = 50/x^2 \to x^2y = 50}\quad x\left(\frac{50}{x^2}\right)^2=\frac{50^2}{x^3}=20\;\Rightarrow \begin{cases} x=50\\ y=2 \end{cases}. \]

The next step is to find the partial derivatives of $f$:

$$f_{xx}=\frac{100}{x^3},\;f_{xy}=1,\;f_{yy}=\frac{40}{y^3}$$

At $(5,2)$, $f_{xx}=4/5$, $f_{xy}=1$, $f_{yy}=5$, and thus $\Delta_2=3>0$. Therefore, $(5,2)$ is a local minimum of $f$ and $f_{\min}=30$.

For this particular function, we can verify this quite easily using the AM-GM inequality. The inequality requires $x$ and $y$ to be non-negative, which they are, so we don't have to worry about them. Simply apply the inequality and we may obtain

$$ xy + \frac{50}{x} + \frac{20}{y}\geq 3\cdot\sqrt[3]{(xy)\cdot \frac{50}{x} \cdot \frac{20}{y}} = 3\cdot 10 = 30, $$

with equality if and only if

$$xy = \frac{50}{x} = \frac{20}{y}.$$

Solve for these equations one by one, we will obtain $(x, y) = (5, 2)$, verifying our solution that uses the Hessian matrix.

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=xy\ln(x^2+y^2)$$

Solution

Domain of $f$: $D=\R^2\backslash\{(0,0)\}$. We then solve for $\nabla f(x,y)=(0,0)$

\begin{equation*} \begin{cases} f_x(x,y)=y\left(\ln(x^2+y^2)+\dfrac{2x^2}{x^2+y^2}\right)=0\quad (1)\\[1em] %%% f_y(x,y)=x\left(\ln(x^2+y^2)+\dfrac{2y^2}{x^2+y^2}\right)=0\quad (2) \end{cases} \end{equation*}

This system branches out to several solutions:

If $y=0$, substitute into equation (2), we obtain $x\ln(x^2)=0$, and thus $x=0$ (We reject this solution since $(0,0)\notin D$) or $\ln(x^2)=0$, which gives two stationary points $(1,0)$ and $(-1,0)$.
If $x=0$, substitute into equation (1), we obtain $y\ln(y^2)=0$, and thus $y=0$ (We reject this solution since $(0,0)\notin D$) or $\ln(y^2)=0,$ which gives two more stationary points $(0,1)$ and $(0,-1)$.
If $x\neq 0$ and $y\neq 0$, the system of equations above is equivalent to
\begin{equation*} \begin{cases} \ln(x^2+y^2)+\dfrac{2x^2}{x^2+y^2}=0\quad (3)\\[1em] %%% \ln(x^2+y^2)+\dfrac{2y^2}{x^2+y^2}=0\quad (4) \end{cases} \end{equation*}
Adding $(3)$ and $(4)$, we obtain $2\ln(x^2+y^2)+2=0\;\Rightarrow\; x^2+y^2=\dfrac{1}{e}$. Substitute this into (3) (or (4)), we obtain the quadratic equation
$$-1+2ex^2=0\quad\Rightarrow\quad x = \pm \frac{1}{\sqrt{2e}}$$
If we substitute these solutions to the equation $x^2+y^2=\dfrac{1}{e}$, we can obtain the $y$'s: If $x = 1/\sqrt{2e}$, then
$$y = \pm \dfrac{1}{\sqrt{2e}},$$
similarly, if $x = -1/\sqrt{2e}$, then we have two more of the same $y$
$$y = \pm \dfrac{1}{\sqrt{2e}},$$
This gives four more stationary points:
$$\left(\dfrac{1}{\sqrt{2e}},\dfrac{1}{\sqrt{2e}}\right),\;\left(\dfrac{1}{\sqrt{2e}},-\dfrac{1}{\sqrt{2e}}\right),\;\left(-\dfrac{1}{\sqrt{2e}},\dfrac{1}{\sqrt{2e}}\right),\; \text{and} \;\left(-\dfrac{1}{\sqrt{2e}},-\dfrac{1}{\sqrt{2e}}\right).$$

We now have a total of eight stationary points. Now let's find the second partial derivatives of $f$ and $\Delta_2(x,y)$.

$$f_{xx}=\frac{2x^3+6xy^3}{(x^2+y^2)^2},\;f_{xy}=\ln(x^2+y^2)+\frac{2y^2}{x^2+y^2}+\frac{2x^4-2xy^2}{(x^2+y^2)^2},\;\text{and}\;f_{yy}=\frac{6yx^3+2xy^3}{(x^2+y^2)^2}$$

For this problem, it is easier to calculate each second partial derivative at a point independently (just use a good calculator that allows variable-passing). We now set up a table:

Points	$f_{xx}(x_0,y_0)$	$f_{xy}(x_0,y_0)$	$f_{yy}(x_0,y_0)$	$\Delta_2=f_{xx}f_{yy}-f_{xy}^2$	Conclusion
$(0,1)$	$0$	$2$	$0$	$-4<0$	Saddle point at $(0,1,0)$
$(0,-1)$	$0$	$2$	$0$	$-4<0$	Saddle point at $(0,-1,0)$
$(1,0)$	$0$	$2$	$0$	$-4<0$	Saddle point at $(1,0,0)$
$(-1,0)$	$0$	$2$	$0$	$-4<0$	Saddle point at $(-1,0,0)$
$\left(\dfrac{1}{\sqrt{2e}},\dfrac{1}{\sqrt{2e}}\right)$	$2>0$	$0$	$2$	$4>0$	Local minimum, $f_{\min}=-\frac{1}{2e}$
$\left(\dfrac{1}{\sqrt{2e}},-\dfrac{1}{\sqrt{2e}}\right)$	$-2<0$	$0$	$-2$	$4>0$	Local maximum, $f_{\max}=\dfrac{1}{2e}$
$\left(-\dfrac{1}{\sqrt{2e}},\dfrac{1}{\sqrt{2e}}\right)$	$-2<0$	$0$	$-2$	$4>0$	Local maximum, $f_{\max}=\dfrac{1}{2e}$
$\left(-\dfrac{1}{\sqrt{2e}},-\dfrac{1}{\sqrt{2e}}\right)$	$2>0$	$0$	$2$	$4>0$	Local minimum, $f_{\min}=-\frac{1}{2e}$

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=y^2-2y\cos x,\quad -1\leq x\leq 7$$

Solution

Domain of $f$: $D=\{(x,y)\in\R^2 | -1\leq x\leq 7\}$. We then solve for $\nabla f(x,y)=(0,0)$

$$ \begin{cases} f_x(x,y)=2y\sin x=0\\[0.5em] f_y(x,y)=2y-2\cos x=0 \end{cases}\quad \Rightarrow\quad \begin{cases} x=k\pi,\; y=0\\[0.5em] y=\cos x \end{cases} $$

For $x=k\pi$, since $-1\leq x\leq 7$, we have $x\in \{0,\;\pi,\;2\pi\}$. This gives us three stationary points.

If $x=0$, then $y=\cos (0)=1\;\Rightarrow\;(0,1)$.
If $x=\pi$, then $y=\cos (\pi)=-1\;\Rightarrow\;(\pi,-1)$.
If $x=2\pi$, then $y=\cos (2\pi)=1\;\Rightarrow\;(2\pi,1)$.

For $y=0$, we then have $y=\cos x=0$, and thus $x=\pi/2$ and $x=3\pi/2$ (since $-1\leq x\leq 7$), therefore we have two more stationary point $(\pi/2,0)$ and $(3\pi/2,0)$. We then find the second partial derivatives of $f$ and $\Delta_2(x,y)$

$$f_{xx}=2y\cos x,\;f_{xy}=2\sin x,\;\text{and}\;f_{yy}=2$$

$$\Delta_2(x,y)=f_{xx}f_{yy}-f_{xy}^2=4y\cos x-4\sin^2 x=4(y\cos x-\sin^2 x)$$

For this kind of problem, we should draw a table to govern all the stationary points.

Points	$f_{xx}(x_0,y_0)$	$\Delta_2(x_0,y_0)$	Conclusion
$(0,1)$	$2>0$	$4>0$	Relative minimum, $f_{\min}=-1$
$(\pi,-1)$	$2>0$	$4>0$	Relative minimum, $f_{\min}=-1$
$(2\pi,1)$	$2>0$	$4>0$	Relative minimum, $f_{\min}=-1$
$(\frac{\pi}{2},0)$	$0$	$-4<0$	Saddle point at $(\pi/2,0,0)$
$(\frac{3\pi}{2},0)$	$0$	$-4<0$	Saddle point at $(3\pi/2,0,0)$

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=2x^4+y^4-x^2-2y^2$$

Solution

Domain of $f$: $D=\R^2$. We now solve for $\nabla f(x,y)=0$

\begin{equation*} \begin{cases} f_x(x,y)=8x^3-2x=0\\ f_y(x,y)=4y^3-4y=0 \end{cases} \Rightarrow\; \begin{cases} x=0,\;x=\dfrac{1}{2},\;x=-\dfrac{1}{2}\\ y=0,\;y=1,\;y=-1 \end{cases} \end{equation*}

This system of equations gives $3\cdot 3=9$ stationary points, namely

$$(0,0),\;(0,1),\;(0,-1),\;\left(\frac{1}{2},0\right),\;\left(\frac{1}{2},1\right),\;\left(\frac{1}{2},-1\right),\left(-\frac{1}{2},0\right),\;\left(-\frac{1}{2},1\right),\;\text{and}\;\left(-\frac{1}{2},-1\right)$$

We then follow the procedure and find the partial derivatives of $f$ and $\Delta_2$:

$$f_{xx}=24x^2-2,\;f_{xy}=0,\;f_{yy}=12y^2-4,\;\text{and}\;\Delta_2(x,y)=(24x^2-2)(12y^2-4)$$

Now, we draw a table, of course.

Points	$f_{xx}(x_0,y_0)$	$\Delta_2(x_0,y_0)$	Conclusion
$(0,0)$	$-2<0$	$8>0$	Local maximum, $f_{\max}=0$
$(0,1)$	$-2<0$	$-16<0$	Saddle Point at $(0,1,-1)$
$(0,-1)$	$-2<0$	$-16<0$	Saddle Point at $(0,-1,-1)$
$\left(\frac{1}{2},0\right)$	$4>0$	$-16<0$	Saddle Point at $(\frac{1}{2},0,-\frac{1}{8})$
$\left(\frac{1}{2},1\right)$	$4>0$	$32<0$	Local minimum, $f_{\min}=-\frac{9}{8}$
$\left(\frac{1}{2},-1\right)$	$4>0$	$32>0$	Local minimum, $f_{\min}=-\frac{9}{8}$
$\left(-\frac{1}{2},0\right)$	$4>0$	$-16<0$	Saddle Point at $(-\frac{1}{2},0,-\frac{1}{8})$
$\left(-\frac{1}{2},1\right)$	$4>0$	$32>0$	Local minimum, $f_{\min}=-\frac{9}{8}$
$\left(-\frac{1}{2},-1\right)$	$4>0$	$32>0$	Local minimum, $f_{\min}=-\frac{9}{8}$

Find all local maximum, minimum, and saddle points of the following function $$f(x,y)=x+y+4\sin x\sin y$$

Solution

Domain of $f$: $D=\R^2$. We then solve for $\nabla f(x,y)=(0,0)$:

\begin{equation*} \begin{cases} f_{x}(x,y)=1+4\cos x\sin y=0\\ f_{y}(x,y)=1+4\sin x\cos y=0 \end{cases} \Rightarrow \begin{cases} 2\sin(x+y)-2\sin(x-y)+1=0\\ 2\sin(x+y)+2\sin(x-y)+1=0 \end{cases} \end{equation*}

Treat this system as linear and solve for $\sin(x+y)$ and $\sin(x-y)$, we obtain

$$\begin{cases} \sin(x-y)=0\\ \sin(x+y)=-\frac{1}{2} \end{cases}$$

The first equation gives $x-y=n\pi$, where $n\in\Z$. The second equation gives

$$x+y=-\dfrac{\pi}{6}+m2\pi\quad\text{and}\quad x+y=\dfrac{7\pi}{6}+m2\pi,\quad m\in\Z$$

To make things a bit easer, we can combine the two solutions of $x+y$ and obtain

$$ x+y=(-1)^{m+1}\dfrac{\pi}{6}+m\pi $$

We now have a simplified system of equations

$$\begin{cases} x - y = n\pi\\ x+y=(-1)^{m+1}\dfrac{\pi}{6}+m\pi \end{cases}$$

Continuing solving for $x$ and $y$ as a linear system, we obtain

$$ \begin{cases} x=(-1)^{m+1}\dfrac{\pi}{12}+(m+n)\dfrac{\pi}{2}\\[1em] y=(-1)^{m+1}\dfrac{\pi}{12}+(m-n)\dfrac{\pi}{2} \end{cases} ,\quad\text{where}\; (m,n)\in\Z^2. $$

Follow the standard procedure again and find the second partial derivatives of the function,

$$f_{xx}=-4\sin x\sin y,\;f_{xy}=4\cos x\cos y,\;\text{and}\;f_{yy}=-4\sin x\sin y.$$

By then,

\begin{align*} \Delta_2(x,y)&=f_{xx}(x,y)f_{yy}(x,y)-f_{xy}^2(x,y)\\[0.5em] &=-(16\cos^2 x\cos^2 y-16\sin^2 x\sin^2 y)\\[0.5em] &=-16(\cos x\cos y+\sin x\sin y)(\cos x\cos y-\sin x\sin y)\\[0.5em] &=-16\cos(x-y)\cos(x+y) \end{align*}

Substituting

$$x-y=n\pi \quad\text{and}\quad x+y=(-1)^{m+1}\dfrac{\pi}{6}+m\pi$$

derived from the equations above into $\Delta_2(x,y)$, we obtain

$$ \Delta_2(x,y) \Big|^{x-y=n\pi}_{x+y=(-1)^{m+1}\frac{\pi}{6}+m\pi}=-16\cos(n\pi)\cos\left((-1)^{m+1}\frac{\pi}{6}+m\pi\right)$$

Since $m$ and $n$ are integers, and therefore the signs of $\Delta_2(x,y)$ depend on the even and odd properties of $m$ and $n$. Thus, we set up an appropriate table according to their odd and even properties.

$m$	$n$	$\Delta_2(x,y)$	$f_{xx}(x_0,y_0)$	Conclusion
Even	Even	$\Delta_2=-16\cos\left(-\dfrac{\pi}{6}\right)=-8\sqrt{3}<0$	~	Saddle Point
Odd	Odd	$\Delta_2=16\cos\left(\dfrac{7\pi}{6}\right)=-8\sqrt{3}<0$	~	Saddle Point
Even	Odd	$\Delta_2=16\cos\left(-\dfrac{\pi}{6}\right)=8\sqrt{3}>0$	$\sqrt{3}+2>0$	Local Minimum, where $$f_{\min}=m\pi-\frac{\pi}{6}-2-\sqrt{3}$$
Odd	Even	$\Delta_2=-16\cos\left(\dfrac{7\pi}{6}\right)=8\sqrt{3}>0$	$-\sqrt{3}-2<0$	Local maximum, where $$f_{\max}=m\pi+\frac{\pi}{6}+2+\sqrt{3}$$

In 4D Space

Find all local maximum, minimum, and saddle points of the following function $$f(x,y,z)=x^2+y^2+z^2-xy+x-2z$$

Solution

We proceed with the standard procedure: finding the domain of $f$: $D=\R^3$. Next, we solve the equation $\nabla f(x,y,z)=\mathbf{0}$.

\begin{equation*} \begin{cases} f_{x}=2x-y+1=0\\[0.5em] f_{y}=2y-x=0\\[0.5em] f_{z}=2z-2=0 \end{cases} \;\Rightarrow\; \begin{cases} x=-2/3\\[0.5em] y=-1/3\\[0.5em] z=1 \end{cases} \end{equation*}

From the equations, we see that this equation only has one stationary point $M=\left(-\dfrac{2}{3},-\dfrac{1}{3},1\right)$. We then proceed to find all the second derivatives of $f$:

$$f_{xx}=2,\;f_{yy}=2,\;f_{zz}=2,\;f_{xy}=-1,f_{xz}=0,\;\text{and}\; f_{yz}=0$$

The Hessian matrix at $\left(-\dfrac{2}{3},-\dfrac{1}{3},1\right)$:

$$H_f(M)= \begin{bmatrix} f_{xx} & f_{xy} & f_{xz}\\ f_{yx} & f_{yy} & f_{yz}\\ f_{zx} & f_{zy} & f_{zz} \end{bmatrix}= \begin{bmatrix} 2 & -1 & 0\\ -1 & 2 & 0\\ 0 & 0 & 2 \end{bmatrix}$$

Apply the Sylvester's criterion on $H_f(M)$ by calculating the major sub-matrices of $H_f(M)$:

$$\Delta_1=2>0,\;\; \Delta_2= \begin{vmatrix} 2 & -1\\ -1 & 2 \end{vmatrix}=3>0 \;\;\text{and}\;\; \Delta_3= \begin{vmatrix} 2 & -1 & 0\\ -1 & 2 & 0\\ 0 & 0 & 2 \end{vmatrix}= (-1)^{3+3}\begin{vmatrix} 2 & -1\\ -1 & 2 \end{vmatrix}=6>0,$$

we see that the matrix $H_f(M)$ is positive-definite, therefore $M=\left(-\dfrac{2}{3},-\dfrac{1}{3},1\right)$ is a minimum of $f$ and $$f_{\min}=f\left(-\dfrac{2}{3},-\dfrac{1}{3},1\right)=-\dfrac{4}{3}$$

Find all local maximum, minimum, and saddle points of the following function

$$f(x,y,z)=x^3+xy+y^2-2xz+2z^2+3y-1$$

Solution

We first need to find the domain of $f$: $D=\R^3$. Next, we solve the equation $\nabla f(x,y,z)=\mathbf{0}$.

\begin{equation*} \begin{cases} f_{x}=3x^2+y-2z=0\\ f_{y}=x+2y+3=0\\ f_{z}=-2x+4z=0 \end{cases} \;\Rightarrow\; \begin{cases} 3x^2+y-2z=0\\[0.5em] y=-\dfrac{x+3}{2}\\[0.5em] z=\dfrac{1}{2}x \end{cases} \;\Rightarrow\; \begin{cases} 6x^2-3x-3=0\\[0.5em] y=-\dfrac{x+3}{2}\\[0.5em] z=\dfrac{1}{2}x \end{cases} \end{equation*}

Solve for $x$ from the quadratic equation and plug it back into $y$ and $z$, we obtain two stationary points,

$$M=\left(1,-2,\dfrac{1}{2}\right),\;\text{and}\;N=\left(-\dfrac{1}{2},-\dfrac{5}{4},-\dfrac{1}{4}\right).$$

The partial derivatives of $f$:

$$f_{xx}=6x,\;f_{yy}=2,\;f_{zz}=4,\;f_{xy}=1,f_{xz}=-2,\;\text{and}\; f_{yz}=0$$

At $M=\left(1,-2,\dfrac{1}{2}\right)$,

$$H_f(M)= \begin{bmatrix} f_{xx} & f_{xy} & f_{xz}\\ f_{yx} & f_{yy} & f_{yz}\\ f_{zx} & f_{zy} & f_{zz} \end{bmatrix}= \begin{bmatrix} 6 & 1 & -2\\ 1 & 2 & 0\\ -2 & 0 & 4 \end{bmatrix}$$

The major sub-matrices of $H_f(M)$:

$$\Delta_1=6>0,\;\; \Delta_2= \begin{vmatrix} 6 & 1\\ 1 & 2 \end{vmatrix}=7>0 \;\;\text{and}\;\; \Delta_3= \begin{vmatrix} 6 & 1 & -2\\ 1 & 2 & 0\\ -2 & 0 & 4 \end{vmatrix} \;\xrightarrow{-2R_1+R_2\to R_2}\; \begin{vmatrix} 6 & 1 & -2\\ -11 & 0 & 4\\ -2 & 0 & 4 \end{vmatrix}=36>0.$$

We conclude that $H_f(M)$ is positive definite, therefore $M$ is a minimum and $f_{\min}=-\frac{3}{2}$.
At $N=\left(-\dfrac{1}{2},-\dfrac{5}{4},-\dfrac{1}{4}\right)$,

$$H_f(M)= \begin{bmatrix} f_{xx} & f_{xy} & f_{xz}\\ f_{yx} & f_{yy} & f_{yz}\\ f_{zx} & f_{zy} & f_{zz} \end{bmatrix}= \begin{bmatrix} -3 & 1 & -2\\ 1 & 2 & 0\\ -2 & 0 & 4 \end{bmatrix}$$

The major sub-matrices of $H_f(N)$:

$$\Delta_1=-3<0,\;\; \Delta_2=\begin{vmatrix} -3 & 1\\ 1 & 2 \end{vmatrix}=7>0 \;\;\text{and}\;\; \Delta_3= \begin{vmatrix} -3 & 1 & -2\\ 1 & 2 & 0\\ -2 & 0 & 4 \end{vmatrix} \;\xrightarrow{-2R_1+R_2\to R_2}\; \begin{vmatrix} -3 & 1 & -2\\ 7 & 0 & 4\\ -2 & 0 & 4 \end{vmatrix}=36>0.$$

We conclude that $H_f(N)$ is neither positive nor negative definite, therefore $N$ is a saddle point.