# Pointwise convergence and properties of the limit (part 1)

We look here at the continuity of a sequence of functions that converges pointwise and give some counterexamples of what happens versus uniform convergence.

### Recalling the definition of pointwise convergence

We consider here real functions defined on a closed interval $$[a,b]$$. A sequence of functions $$(f_n)$$ defined on $$[a,b]$$ converges pointwise to the function $$f$$ if and only if for all $$x \in [a,b]$$ $$\displaystyle \lim\limits_{n \to +\infty} f_n(x) = f(x)$$. Pointwise convergence is weaker than uniform convergence.

### Pointwise convergence does not, in general, preserve continuity

Suppose that $$f_n \ : \ [0,1] \to \mathbb{R}$$ is defined by $$f_n(x)=x^n$$. For $$0 \le x <1$$ then $$\displaystyle \lim\limits_{n \to +\infty} x^n = 0$$, while if $$x = 1$$ then $$\displaystyle \lim\limits_{n \to +\infty} x^n = 1$$. Hence the sequence $$f_n$$ converges to the function equal to $$0$$ for $$0 \le x < 1$$ and to $$1$$ for $$x=1$$. Although each $$f_n$$ is a continuous function of $$[0,1]$$, their pointwise limit is not. $$f$$ is discontinuous at $$1$$. We notice that $$(f_n)$$ doesn't converge uniformly to $$f$$ as for all $$n \in \mathbb{N}$$, $$\displaystyle \sup\limits_{x \in [0,1]} \vert f_n(x) - f(x) \vert = 1$$. That's reassuring as uniform convergence of a sequence of continuous functions implies that the limit is continuous! Continue reading Pointwise convergence and properties of the limit (part 1)

# A nowhere locally bounded function

In that article, I described some properties of Thomae’s function$$f$$. Namely:

• The function is discontinuous on $$\mathbb{Q}$$.
• Continuous on $$\mathbb{R} \setminus \mathbb{Q}$$.
• Its right-sided and left-sided limits vanish at all points.

Let’s modify $$f$$ to get function $$g$$ defined as follow:
$g: \left|\begin{array}{lrl} \mathbb{R} & \longrightarrow & \mathbb{R} \\ x & \longmapsto & 0 \text{ if } x \in \mathbb{R} \setminus \mathbb{Q} \\ \frac{p}{q} & \longmapsto & q \text{ if } \frac{p}{q} \text{ in lowest terms and } q > 0 \end{array}\right.$ $$f$$ and $$g$$ both vanish on the set of irrational numbers, while on the set of rational numbers, $$g$$ is equal to the reciprocal of $$f$$. We now consider an open subset $$O \subset \mathbb{R}$$ and $$x \in O$$. As $$f$$ right-sided and left-sided limits vanish at all points, we have $$\lim\limits_{n \to +\infty} f(x_n) = 0$$ for all sequence $$(x_n)$$ of rational numbers converging to $$x$$ (and such a sequence exists as the rational numbers are everywhere dense in the reals). Hence $$\lim\limits_{n \to +\infty} g(x_n) = + \infty$$ as $$f$$ is positive.

We can conclude that $$g$$ is nowhere locally bounded. The picture of the article is a plot of function $$g$$ on the rational numbers $$r = \frac{p}{q}$$ in lowest terms for $$0 < r < 1$$ and $$q \le 50$$.

# A function continuous at all irrationals and discontinuous at all rationals

Let’s discover the beauties of Thomae’s function also named the popcorn function, the raindrop function or the modified Dirichlet function.

Thomae’s function is a real-valued function defined as:
$f: \left|\begin{array}{lrl} \mathbb{R} & \longrightarrow & \mathbb{R} \\ x & \longmapsto & 0 \text{ if } x \in \mathbb{R} \setminus \mathbb{Q} \\ \frac{p}{q} & \longmapsto & \frac{1}{q} \text{ if } \frac{p}{q} \text{ in lowest terms and } q > 0 \end{array}\right.$

### $$f$$ is periodic with period $$1$$

This is easy to prove as for $$x \in \mathbb{R} \setminus \mathbb{Q}$$ we also have $$x+1 \in \mathbb{R} \setminus \mathbb{Q}$$ and therefore $$f(x+1)=f(x)=0$$. While for $$y=\frac{p}{q} \in \mathbb{Q}$$ in lowest terms, $$y+1=\frac{p+q}{q}$$ is also in lowest terms, hence $$f(y+1)=f(y)=\frac{1}{q}$$. Continue reading A function continuous at all irrationals and discontinuous at all rationals

# A function whose derivative at 0 is one but which is not increasing near 0

From the mean value theorem, a real function whose derivative is strictly positive at every point of an interval is strictly increasing. In particular, a continuously differentiable function $$f$$ defined in a non-degenerate interval $$I$$ with a strictly positive derivative at a point $$a$$ of the interval is strictly increasing near that point. For the proof, we just have to notice that as $$f^\prime$$ is continuous and $$f^\prime(a) > 0$$, $$f^\prime$$ is strictly positive within an interval $$J \subset I$$ containing $$a$$. By the mean value theorem, $$f$$ is strictly increasing on $$J$$.

We now suppose that $$f$$ is differentiable on an interval $$I$$ containing $$0$$ with $$f^\prime(0)>0$$. For $$x>0$$ sufficiently close to zero we have $$\displaystyle \frac{f(x)-f(0)}{x-0} > \frac{f^\prime(0)}{2}>0$$, hence $$f(x)>f(0)$$. But that doesn’t imply that $$f$$ is strictly increasing in a neighborhood of zero. Let’s prove it with a counterexample. Continue reading A function whose derivative at 0 is one but which is not increasing near 0

# A decreasing function converging to zero whose derivative diverges (part2)

In that article, I gave examples of real valued functions defined on $$(0,+\infty)$$ that converge to zero and whose derivatives diverge. But those functions were not monotonic. Here I give an example of a decreasing real valued function $$g$$ converging to zero at $$+\infty$$ and whose derivative is unbounded.

We first consider the polynomial map:
$P(x)=(1+2x)(1-x)^2=1-3x^2+2x^3$ on the segment $$I=[0,1]$$. $$P$$ derivative equals $$P^\prime(x)=-6x(1-x)$$. Therefore $$P$$ is decreasing on $$I$$. Moreover we have $$P(0)=1$$, $$P(1)=P^\prime(0)=P^\prime(1)=0$$ and $$P^\prime(1/2)=-3/2$$. Continue reading A decreasing function converging to zero whose derivative diverges (part2)

# Differentiable functions converging to zero whose derivatives diverge (part1)

In this article, I consider real valued functions $$f$$ defined on $$(0,+\infty)$$ that converge to zero, i.e.:
$\lim\limits_{x \to +\infty} f(x) = 0$ If $$f$$ is differentiable what can be the behavior of its derivative as $$x$$ approaches $$+\infty$$?

Let’s consider a first example:
$\begin{array}{l|rcl} f_1 : & (0,+\infty) & \longrightarrow & \mathbb{R} \\ & x & \longmapsto & \frac{1}{x} \end{array}$ $$f_1$$ derivative is $$f_1^\prime(x)=-\frac{1}{x^2}$$ and we also have $$\lim\limits_{x \to +\infty} f_1^\prime(x) = 0$$. Let’s consider more sophisticated cases! Continue reading Differentiable functions converging to zero whose derivatives diverge (part1)

# Counterexamples on real sequences (part 2)

In that article, I provide basic counterexamples on sequences convergence. I follow on here with some additional and more advanced examples.

#### If $$(u_n)$$ converges then $$(\vert u_n \vert )$$ converge?

This is true and the proof is based on the reverse triangle inequality: $$\bigl| \vert x \vert – \vert y \vert \bigr| \le \vert x – y \vert$$. However the converse doesn’t hold. For example, the sequence $$u_n=(-1)^n$$ is such that $$\lim \vert u_n \vert = 1$$ while $$(u_n)$$ diverges.

#### If for all $$p \in \mathbb{N}$$ $$\lim\limits_{n \to +\infty} (u_{n+p} – u_n)=0$$ then $$(u_n)$$ converges?

The assertion is wrong. A simple counterexample is $$u_n= \ln(n+1)$$. It is well known that $$(u_n)$$ diverges. However for any $$p \in \mathbb{N}$$ we have $$\lim\limits_{n \to +\infty} (u_{n+p} – u_n) =\ln(1+\frac{p}{n+1})=0$$.
The converse proposition is true. Assume that $$(u_n)$$ is a converging sequence with limit $$l$$ and $$p \ge 0$$ is any integer. We have $$\vert u_{n+p}-u_n \vert = \vert (u_{n+p}-l)-(u_n-l) \vert \le \vert u_{n+p}-l \vert – \vert u_n-l \vert$$ and both terms of the right hand side of the inequality are converging to zero. Continue reading Counterexamples on real sequences (part 2)

# Counterexamples on real sequences (part 1)

The purpose of this article is to provide some basic counterexamples on real sequences. Counterexamples are provided as answers to questions.

I will come back later on with more complex cases. Unless otherwise stated, $$(u_n)_{n \in \mathbb{N}}$$ and $$(v_n)_{n \in \mathbb{N}}$$ are two real sequences. Continue reading Counterexamples on real sequences (part 1)

# A solution of a differential equation not exploding in finite time

In this post, I mention that Peano existence theorem is valid for finite dimensional vector spaces, but not for Banach spaces of infinite dimension. I highlight here a second property of ordinary differential equations which is valid for finite dimensional vector spaces but not for infinite dimensional Banach spaces. Continue reading A solution of a differential equation not exploding in finite time

# A continuous function not differentiable at the rationals but differentiable elsewhere

We build here a continuous function of one real variable whose derivative exists on $$\mathbb{R} \setminus \mathbb{Q}$$ and doesn’t have a left or right derivative on each point of $$\mathbb{Q}$$.

As $$\mathbb{Q}$$ is (infinitely) countable, we can find a bijection $$n \mapsto r_n$$ from $$\mathbb{N}$$ to $$\mathbb{Q}$$. We now reuse the function $$f$$ defined here. Recall $$f$$ main properties: Continue reading A continuous function not differentiable at the rationals but differentiable elsewhere