# Category Archives: Teaching

Teaching

## Walsh functions III: The convergence of the Walsh coefficients and pointwise convergence

In a previous post, we discussed the orthogonality properties of Walsh functions and showed that they form a complete orthonormal system in $L_2([0,1])$. In this post we discuss the rate of decay of the Walsh coefficients when the function has bounded variation of fractional order $0 \textless \alpha \le 1$ and we investigate pointwise convergence of the Walsh series and pointwise convergence of the Walsh series to the function. We consider only Walsh functions in base $2,$ although the results can be generalized to Walsh functions over groups. Information on Walsh functions over groups can be found in this post. For the necessary background information see the previous post on Walsh functions here. A table of contents for the posts on Walsh functions can be found here. Continue reading

## Walsh functions II: Walsh functions in general base and Walsh functions over groups

In a previous post we introduced Walsh functions. We showed that this set of functions is orthogonal and complete. In this post we generalize the Walsh function system such that the orthonormality and completeness of the new Walsh function system still holds. A table of contents for the posts on Walsh functions can be found here. Continue reading

## Math2111: Chapter 5: Recommended reading: The fundamental theorems

In this blog entry you can find lecture notes for Math2111, several variable calculus. See also the table of contents for this course. This blog entry printed to pdf is available here.

In this entry we show that all the fundamental theorems (fundamental theorem of calculus, fundamental theorem of line integrals, Green’s theorem, Stokes’ theorem and the divergence theorem) are based on the same principle. Further, we will see that those theorems are all the fundamental theorems in $\mathbb{R},$ $\mathbb{R}^2$ and $\mathbb{R}^3.$ Continue reading

## Math2111: Chapter 5: Additional Material: Differential forms and the general Stokes’ theorem

In this blog entry you can find lecture notes for Math2111, several variable calculus. See also the table of contents for this course. This blog entry printed to pdf is available here.

We have studied a variety of generalisations of the fundamental theorem of calculus:

Now we show how all of these formulae concisely fit into one approach. Differential forms provide the underlying theory to present all formulae in one framework. This also allows us to generalise the theorems above to arbitrary dimensions. Continue reading

## Math2111: Chapter 3: Additional Material: Rectifiable parameterised curves

In this blog entry you can find lecture notes for Math2111, several variable calculus. See also the table of contents for this course. This blog entry printed to pdf is available here.

We now discuss curves and their lengths in more detail. Let $n\in\mathbb{N}$ be an arbitrary natural number which is fixed throughout this post. Note that in general one needs to distinguish between a parameterised curve, which is a continuous mapping $\boldsymbol{c}:[a,b]\to\mathbb{R}^n,$ and the curve $\mathcal{C},$ which is the image of $\boldsymbol{c}$ given by $\{\boldsymbol{c}(t) \in \mathbb{R}^n: t\in [a,b]\}.$ Here we shall discuss parameterised curves. Hence, for instance, the parameterised curve $\boldsymbol{c}(t)= \cos t \widehat{\boldsymbol{i}} + \sin t \widehat{\boldsymbol{j}}$ with $0 \le t \le 4\pi$ is a circle traversed twice and has therefore length $4\pi,$ whereas its image is just a circle which has length $2\pi.$ Continue reading

## Math2111: Chapter 3: Additional Material: For which curves and regions does Green’s theorem apply?

In this blog entry you can find lecture notes for Math2111, several variable calculus. See also the table of contents for this course. This blog entry printed to pdf is available here.

Here we discuss the conditions under which Green’s theorem (see also here) applies, more precisely for which curves and regions enclosed by the curve.

The formula in Green’s theorem states that

$\displaystyle \oint_{\mathcal{C}} \boldsymbol{F} \cdot \mathrm{d} \boldsymbol{s} = \iint_R \left(\frac{\partial Q}{\partial x}-\frac{\partial P}{\partial y}\right)\,\mathrm{d} A, \qquad\qquad\qquad (1)$

where $\boldsymbol{F}:\mathbb{R}^2\to\mathbb{R}^2,$ $\boldsymbol{F}=P\widehat{\boldsymbol{i}}+Q\widehat{\boldsymbol{j}},$ is a continuously differentiable vector field, $\mathcal{C}$ is a curve which is simple, closed, smooth and oriented positively, and ${}R$ is the region enclosed by the curve $\mathcal{C}.$ Continue reading

## Math2111: Chapter 3: Recommended reading: Line integrals and orientation

In this blog entry you can find lecture notes for Math2111, several variable calculus. See also the table of contents for this course. This blog entry printed to pdf is available here.

We discuss in what sense the scalar line integrals do not depend on the orientation of the curve.

Let us start with integrals $\int_a^b f(t)\,\mathrm{d} t$ where $f$ is a Riemann integrable function. Let $a \textless b$ and let $P_n=\{x_0,\ldots, x_n\}$ be such that $a=x_0 \textless x_1 \textless x_2 \textless \cdots \textless x_{n-1} \textless x_n =b,$ let $x_{i-1} \le t_i \le x_{i}$ for $1\le i \le n$ and $\delta(P_n)=\max_{1\le i\le n} x_{i+1}-x_i.$ We can now form the Riemann sum

$\displaystyle \mathcal{S}(P_n) = \sum_{i=1}^n f(t_i) (x_i-x_{i-1}).$

By considering the limit $\lim \mathcal{S}(P_n)$ where $n\to\infty$ such that $\delta(P_n) \to 0,$ we obtain the Riemann integral $\int_a^b f(t)\,\mathrm{d} t.$ Continue reading