The remarkable polymath Christopher Wren died in March 1723, just 300 years ago. Sarah Hart, Professor of Geometry at Gresham College, recently presented a lecture, *The Mathematical Life of Sir Christopher Wren*; a video of her presentation in available online (see sources below). The illustration above is from the Gresham College website.

## Archive for the 'Occasional' Category

### Christopher Wren and the Cycloid

Published March 23, 2023 Occasional Leave a CommentTags: Analysis, History

### Bach and Euler chat in Frederick’s Court

Published March 16, 2023 Occasional Leave a CommentTags: History, Music

Frederick the Great of Prussia, a devoted patron of the arts, had a particular interest in music, and admired the music of Johann Sebastian Bach. In 1747, Bach visited Potsdam, where his son Carl Philipp Emanuel was the Kapellmeister in Frederick’s court. When Frederick learned of this, he summoned ‘Old Bach’ to the palace and invited him to try out his collection of pianofortes. As they went from room to room, Bach improvised a new piece of music on each instrument [TM243 or search for “thatsmaths” at irishtimes.com].

### Sets that are Elements of Themselves: Verboten

Published March 9, 2023 Occasional Leave a CommentTags: Logic, Set Theory

Can a set be an element of itself? A simple example will provide an answer to this question. Continue reading ‘Sets that are Elements of Themselves: Verboten’

### Benford’s Law Revisited

Published March 2, 2023 Occasional Leave a CommentTags: Probability, Statistics

Several researchers have observed that, in a wide variety of collections of numerical data, the leading — or most significant — decimal digits are not uniformly distributed, but conform to a logarithmic distribution. Of the nine possible values, occurs more than of the time while is found in less than of cases (see Figure above). Specifically, the probability distribution is

A more complete form of the law gives the probabilities for the second and subsequent digits. A full discussion of Benford’s Law is given in Berger and Hill (2015).

### A Puzzle: Two-step Selection of a Digit

Published February 23, 2023 Occasional ClosedTags: Number Theory

Here is a simple problem in probability.

(1) Pick a number *k* between 1 and 9. Assume all digits are equally likely.

(2) Pick a number *m* in the range from 1 to *k*.

**What is the probability distribution for the number m?**

A graph of the probability distribution is shown in the figure here.

Can you derive a formula for this probability distribution?

Can you generalise it to the range from 1 to 10^n?

Can you relate this problem to Benford’s Law [described here]?

Solution, and more on Benford’s Law, next week.

### Ford Circles & Farey Series

Published February 9, 2023 Occasional 1 CommentTags: Geometry, Number Theory

American mathematician Lester Randolph Ford Sr. (1886–1967) was President of the Mathematical Association of America from 1947 to 1948 and editor of the American Mathematical Monthly during World War II. He is remembered today for the system of circles named in his honour.

For any rational number in reduced form ( and coprime), a Ford circle is a circle with center at and radius . There is a Ford circle associated with every rational number. Every Ford circle is tangent to the horizontal axis and each two Ford circles are either tangent or disjoint from each other.

Mathematics has an amazing capacity to help us to understand the physical world. Just consider the profound implications of Einstein’s simple equation . Another example is the wave equation derived by Scottish mathematical physicist James Clerk Maxwell. Our modern world would not exist without the knowledge encapsulated in Maxwell’s equations. Continue reading ‘From Wave Equations to Modern Telecoms’

Curvature is of critical importance in numerous contexts. An example is shown in the figure above, a map of the Silverstone Formula 1 racetrack. The sharp bends (high curvature) force drivers to reduct speed drastically.

The real line is an example of a locally compact Hausdorff space. In a Hausdorff space, two distinct points have disjoint neighbourhoods. As the old joke says, “any two points can be *housed off* from each other”. We will define local compactness below. The one-point compactification is a way of embedding a locally compact Hausdorff space in a compact space. In particular, it is a way to “make the real line compact”.

### Summing the Fibonacci Sequence

Published January 5, 2023 Occasional ClosedTags: Arithmetic, Number Theory

### Spiric curves and phase portraits

Published December 29, 2022 Occasional ClosedTags: Mechanics, Topology

*Spiric sections*, formed by the intersections of a torus by planes parallel to its axis. Like the conics, they come in various forms, depending upon the distance of the plane from the axis of the torus (see Figure above). We examine how spiric curves may be found in the phase-space of a dynamical system.

### Closeness in the 2-Adic Metric

Published December 22, 2022 Occasional ClosedTags: Algebra, Number Theory

*When is 144 closer to 8 than to 143?*

The usual definition of the *norm* of a real number is its modulus or absolute value . We measure the “distance” between two real numbers by means of the absolute value of their difference. This gives the Euclidean metric and, using it, we can define the usual topology on the real numbers .

The standard arrangement of the real numbers on a line automatically ensures that numbers with small Euclidean difference between them are geometrically close to each other. It may come as a surprise that there are other ways to define norms and distances, which provide other topologies, leading us to a radically different concept of closeness, and to completely new number systems, the p-adic numbers.

### Curvature and Geodesics on a Torus

Published December 8, 2022 Occasional ClosedTags: Geometry, Topology

We take a look at the curvature on a torus, and the various forms that geodesics can have. These are compared to the geodesics on a “flat torus”.

Continue reading ‘Curvature and Geodesics on a Torus’

### Low-pass Filtering and the Remarkable Integrals of Borwein and Borwein

Published December 1, 2022 Occasional ClosedTags: Analysis, Numerical Analysis

In last week’s post we looked at aspects of puzzles of the form *“What is the next number”.* We are presented with a short list of numbers, for example and asked for the next number in the sequence. Arguments were given indicating why *any* number might be regarded as the next number.

In this article we consider a sequence of seven ones: . Most people would agree that the next number in the sequence is . We will show how the number could be the “correct” answer. Continue reading ‘Low-pass Filtering and the Remarkable Integrals of Borwein and Borwein’

### What’s the Next Number?

Published November 24, 2022 Occasional ClosedTags: Arithmetic, Numerical Analysis

We are all familiar with simple mathematical puzzles that give a short sequence and ask “What is the next number in the sequence”. Simple examples would be

the sequence of odd numbers, the sequence of squares and the Fibonacci sequence.

### Mercury’s Mercurial Orbit

Published November 10, 2022 Occasional ClosedTags: Astronomy, Mechanics, Relativity

The tiny deviation of the orbit of Mercury from a pure ellipse might seem to be of no consequence. Yet the minute precession of this planet was one of the factors leading to a revolution in our world view. Attempts to explain the anomaly in the context of Newtonian mechanics were unsatisfactory. It was only with the emergence of general relativity that we were able to understand the observed phenomenon. Continue reading ‘Mercury’s Mercurial Orbit’

### The Power of the 2-gon: Extrapolation to Evaluate Pi

Published November 3, 2022 Occasional ClosedTags: Algorithms, Numerical Analysis, Pi

Richardson’s extrapolation procedure yields a significant increase in the accuracy of numerical solutions of differential equations. We consider his elegant illustration of the technique, the evaluation of , and show how the estimates improve dramatically with higher order extrapolation.

[This post is a condensed version of a paper in *Mathematics Today* (Lynch, 2003).]

Continue reading ‘The Power of the 2-gon: Extrapolation to Evaluate Pi’

### Dropping Pebbles down a Mine-shaft

Published October 27, 2022 Occasional ClosedTags: Geophysics, Mechanics

If you drop a pebble down a mine-shaft, it will not fall vertically, but will be deflected slightly to the East by the Coriolis force, an effect of the Earth’s rotation. We can solve the equations to calculate the amount of deflection; for a ten-second drop, the pebble falls about 500 metres (air resistance is neglected) and is deflected eastward by about 25 cm. The figure on the left shows the trajectory in the vertical xz-plane (scales are not the same).

We derive the equations after making some simplifying assumptions. We assume the mine-shaft is at the Equator; we assume the meridional or north-south motion is zero; we neglect variations in the gravitational force; we neglect the sphericity of the Earth; we neglect air resistance. We can still get accurate estimates provided the elapsed time is short. However, carrying the analysis to the extreme, we obtain results that are completely unrealistic. The equations predict that the pebble will reach a minimum altitude and then rise up again to its initial height a great distance east of its initial position. Then this up-and-down motion will recur indefinitely.

### From Sub-atomic to Cosmic Strings

Published October 20, 2022 Irish Times , Occasional ClosedTags: Astronomy, Physics

The two great pillars of modern physics are quantum mechanics and general relativity. These theories describe small-scale and large-scale phenomena, respectively. While quantum mechanics predicts the shape of a hydrogen atom, general relativity explains the properties of the visible universe on the largest scales.

It is a tricky matter to find the area of a field that has irregular or meandering boundaries. The standard method is to divide the field into triangular parts. If the boundaries are linear, this is simple. If they twist and turn, then a large number of triangles may be required.

### CND Functions: Curves that are Continuous but Nowhere Differentiable

Published October 6, 2022 Occasional ClosedTags: Analysis

A function that is differentiable at a point is continuous there, and if differentiable on an interval , is continuous on that interval. However, the converse is not necessarily true: the continuity of a function at a point or on an interval does not guarantee that it is differentiable at the point or on the interval.

Continue reading ‘CND Functions: Curves that are Continuous but Nowhere Differentiable’

### Topological Calculus: away with those nasty epsilons and deltas

Published September 29, 2022 Occasional ClosedTags: Analysis

### The 3-sphere: Extrinsic and Intrinsic Forms

Published September 22, 2022 Occasional ClosedTags: Relativity, Topology

The circle in two dimensions and the sphere in three are just two members of an infinite family of hyper-surfaces. By analogy with the circle in the plane and the sphere in three-space , we can consider hyper-spheres in higher dimensional spaces. In particular, we will consider the 3-sphere which can be embedded in but can also be envisaged as a non-Euclidean manifold in .

Continue reading ‘The 3-sphere: Extrinsic and Intrinsic Forms’

### Dynamic Equations for Weather and Climate

Published September 8, 2022 Occasional ClosedTags: Analysis, Geometry, Relativity

*“I could have done it in a much more complicated way”,
said the Red Queen, immensely proud. — Lewis Carroll.*

Books on dynamic meteorology and oceanography usually have a full chapter devoted to the basic dynamical equations. Since the Earth’s fluid envelop is approximately a thin spherical shell, spherical coordinates are convenient. Here is the longitude and the latitude. In Figure 1 we show the momentum equations as presented in the monograph of Lorenz (1967):

Continue reading ‘Dynamic Equations for Weather and Climate’

Many of us have struggled with the vector differential operators, **grad**, div and **curl**. There are several ways to represent vectors and several expressions for these operators, not always easy to remember. We take another look at some of their properties here.

How can mathematicians grapple with abstruse concepts that are, for the majority of people, beyond comprehension? What mental processes enable a small proportion of people to produce mathematical work of remarkable creativity? In particular, is there a connection between mathematical creativity and autism? We revisit a book and a film that address these questions.

### Space-Filling Curves, Part II: Computing the Limit Function

Published August 11, 2022 Occasional ClosedTags: Analysis

It is simple to define a mapping from the unit interval into the unit square . Georg Cantor found a one-to-one map from ** onto** , showing that the one-dimensional interval and the two-dimensional square have the same cardinality. Cantor’s map was not continuous, but Giuseppe Peano found a continuous surjection from onto , that is, a

*curve that fills the entire unit square.*Shortly afterwards, David Hilbert found an even simpler space-filling curve, which we discussed in Part I of this post.

Continue reading ‘Space-Filling Curves, Part II: Computing the Limit Function’

### Space-Filling Curves, Part I: “I see it, but I don’t believe it”

Published August 4, 2022 Occasional ClosedTags: Analysis

We are all familiar with the concept of dimension: a point is zero-dimensional, a line is one-dimensional, a plane is two-dimensional and the space around us is three-dimensional. A position on a line can be specified by a single number, such as the distance from a fixed origin. In the plane, a point can be located by giving its Cartesian coordinates , or its polar coordinates . In space, we may specify the location by giving three numbers .

Continue reading ‘Space-Filling Curves, Part I: “I see it, but I don’t believe it”’

### Poincare’s Square and Unbounded Gomoku

Published July 28, 2022 Occasional ClosedTags: Games, Geometry, Relativity

Henri Poincar’e was masterful in presenting scientific concepts and ideas in an accessible way. To explain that the Universe might be bounded and yet infinite, he imagined that the temperature of space decreased from the centre to the periphery in such a way that everything contracted with the distance from the centre. As travellers moved outward from the centre, everything got smaller in such a way that it would take an infinite time to reach the boundary.

### Fields Medals presented at IMC 2022

Published July 21, 2022 Occasional ClosedTags: Combinatorics, Geometry, Number Theory

Every four years, at the International Congress of Mathematicians, the Fields Medal is awarded to two, three, or four young mathematicians. To be eligible, the awardees must be under forty years of age. For the chosen few, who came from England, France, Korea and Ukraine, the award, often described as the Nobel Prize of Mathematics, is the crowning achievement of their careers [TM235 or search for “thatsmaths” at irishtimes.com].

The congress, which ran from 6th to 14th July, was originally to take place in St Petersburg. When events made that impossible, the action shifted to Helsinki and the conference presentations were moved online. The International Mathematical Union generously allowed participants to register at no cost.

### Goldbach’s Conjecture and Goldbach’s Variation

Published July 14, 2022 Occasional ClosedTags: Logic, Number Theory

Goldbach’s Conjecture is one of the great unresolved problems of number theory. It simply states that** every even natural number greater than two is the sum of two prime numbers.** It is easily confirmed for even numbers of small magnitude.

The conjecture first appeared in a letter dated 1742 from German mathematician Christian Goldbach to Leonhard Euler. The truth of the conjecture for all even numbers up to four million million million () has been demonstrated. There is essentially no doubt about its validity, but no proof has been found.

Continue reading ‘Goldbach’s Conjecture and Goldbach’s Variation’

** Cardinals and Ordinals **

The cardinal number of a set is an indicator of the size of the set. It depends only on the elements of the set. Sets with the same cardinal number — or cardinality — are said to be equinumerate or (with unfortunate terminology) to be the same size. For finite sets there are no problems. Two sets, each having the same number of elements, both have cardinality . But an infinite set has the definitive property that it can be put in one-to-one correspondence with a proper subset of itself.

Atmospheric motions are chaotic: a minute perturbation can lead to major changes in the subsequent evolution of the flow. How do we know this? There is just one atmosphere and, if we perturb it, we can never know how it might have evolved if left alone.

We know, from simple nonlinear models that exhibit chaos, that the flow is very sensitive to the starting conditions. We can run “identical twin” experiments, where the initial conditions for two runs are almost identical, and watch how the two solutions diverge. This — and an abundance of other evidence — leads us to the conclusion that the atmosphere behaves in a similar way.

### The Arithmetic Triangle is Analytical too

Published June 23, 2022 Occasional ClosedTags: Arithmetic, Euler

Pascal’s triangle is one of the most famous of all mathematical diagrams. It is simple to construct and rich in mathematical patterns. There is always a chance of finding something never seen before, and the discovery of new patterns is very satisfying.

Not too long ago, Harlan Brothers found Euler’s number in the triangle (Brothers, 2012(a),(b)). This is indeed surprising. The number is ubiquitous in analysis but it is far from obvious why it should turn up in the arithmetic triangle.

Continue reading ‘The Arithmetic Triangle is Analytical too’

The swinging spring, or elastic pendulum, exhibits some fascinating dynamics. The bob is free to swing like a spherical pendulum, but also to bounce up and down due to the stretching action of the spring. The behaviour of the swinging spring has been described in a previous post on this blog [Reference 1 below].

In some recent posts, here and here we discussed the extension of the concept of parity (Odd v. Even) from the integers to the rational numbers. We found that it is natural to consider three parity classes, determined by the parities of the numerator and denominator of a rational number (in reduced form):

**q Odd:**odd and odd.**q Even:**even and odd.**q None:**odd and even.

or, in symbolic form,

Here, stands for “Neither Odd Nor Even”.

The rational numbers are dense in the real numbers . The cardinality of rational numbers in the interval is . We cannot list them in ascending order, because there is no least rational number greater than .

### Parity and Partition of the Rational Numbers. Part II: Density of the Three Parity Classes

Published May 12, 2022 Occasional ClosedTags: Number Theory

In last week’s post, we defined an extension of parity from the integers to the rational numbers. Three parity classes were found — even, odd and *none*. This week, we show that, with an appropriate ordering or enumeration of the rationals, the three classes are not only equinumerate (having the same cardinality) but of equal density in . This article is a condensation of part of a paper [Lynch & Mackey, 2022] recently posted on arXiv.

### Parity and Partition of the Rational Numbers. Part I: The Three Parity Classes

Published May 5, 2022 Occasional ClosedTags: Number Theory

We define an extension of parity from the integers to the rational numbers. Three parity classes are found — even, odd and *none*. Using the 2-adic valuation, we partition the rationals into subgroups with a rich algebraic structure.

Continue reading ‘Parity and Partition of the Rational Numbers. Part I: The Three Parity Classes’

Henri Poincaré described a beautiful geometric model with some intriguing properties. He envisioned a circular disk in the Euclidean plane, where distances were distorted to give it geometric properties quite different from those of Euclid’s *Elements*. He supposed that the temperature varied linearly from a fixed value at the centre of the disk to absolute zero on the boundary, and that lengths varied in proportion to the temperature.

### Following the Money around the Eurozone

Published April 14, 2022 Occasional ClosedTags: modelling, Numerical Analysis

Take a fistful of euro coins and examine the obverse sides; you may be surprised at the wide variety of designs. The eurozone is a monetary union of 19 member states of the European Union that have adopted the euro as their primary currency. In addition to these countries, Andorra, Monaco, San Marino and Vatican City use euro coins so, from 2015, there have been 23 countries, each with its own national coin designs. For the €1 and €2 coins, there are 23 distinct national patterns; for the smaller denominations, there are many more. Thus, there is a wide variety of designs in circulation.

### Mamikon’s Visual Calculus and Hamilton’s Hodograph

Published April 7, 2022 Occasional ClosedTags: Analysis, Mechanics

[This is a condensed version of an article [5] in *Mathematics Today*]

A remarkable theorem, discovered in 1959 by Armenian astronomer Mamikon Mnatsakanian, allows problems in integral calculus to be solved by simple geometric reasoning, without calculus or trigonometry. Mamikon’s Theorem states that *`The area of a tangent sweep of a curve is equal to the area of its tangent cluster’.* We shall illustrate how this theorem can help to solve a range of integration problems.

Continue reading ‘Mamikon’s Visual Calculus and Hamilton’s Hodograph’

### Infinitesimals: vanishingly small but not quite zero

Published March 31, 2022 Occasional ClosedTags: Analysis, History

A few weeks ago, I wrote about Hyperreals and Nonstandard Analysis , promising to revisit the topic. Here comes round two.

Continue reading ‘Infinitesimals: vanishingly small but not quite zero’

### The Chromatic Number of the Plane

Published March 24, 2022 Occasional ClosedTags: Analysis, Combinatorics, Topology

To introduce the problem in the title, we begin with a quotation from the Foreword, written by Branko Grünbaum, to the book by Alexander Soifer (2009): *The Mathematical Coloring Book: Mathematics of Coloring and the Colorful Life of its Creators*:

*If each point of the plane is to be given a color, how many colors do we need if every two points at unit distance are to receive distinct colors?*

About 70 years ago it was shown that the least number of colours needed for such a colouring is one of 4, 5, 6 and 7. But which of these is the correct number? Despite efforts by many very clever people, some of whom had solved problems that appeared to be much harder, no advance has been made to narrow the gap

.

### Hyperreals and Nonstandard Analysis

Published March 10, 2022 Occasional ClosedTags: Analysis, Logic

Following the invention of calculus, serious concerns persisted about the mathematical integrity of the method of infinitesimals. Leibniz made liberal use of infinitesimals, with great effect, but his reasoning was felt to lack rigour. The Irish bishop George Berkeley criticised the assumptions underlying calculus, and his objections were not properly addressed for several centuries. In the 1800s, Bolzano, Cauchy and Weierstrass developed the – definition of limits and continuity, which allowed derivatives and integrals to be defined without recourse to infinitesimal quantities. Continue reading ‘Hyperreals and Nonstandard Analysis’

### Why Waffle when One Wordle Do?

Published February 24, 2022 Occasional ClosedTags: Algorithms, Recreational Maths

Hula hoops were all the rage in 1958. Yo-yos, popular before World War II, were relaunched in the 1960s. Rubik’s Cube, invented in 1974, quickly became a global craze. Sudoku, which had been around for years, was wildly popular when it started to appear in American and European newspapers in 2004.

### Where is the Sun?

Published February 10, 2022 Occasional ClosedTags: Astronomy, Spherical Trigonometry

*ecliptic*. The observer’s position on Earth is given by the geographic latitude and longitude. The path of the Sun depends on the latitude and the date, while the time when the Sun crosses the local meridian is determined by the longitude.

We are all familiar with splitting natural numbers into prime components. This decomposition is unique, except for the order of the factors. We can apply the idea of prime components to many more general sets of numbers.

The **Gaussian integers** are all the complex numbers with integer real and imaginary parts, that is, all numbers in the set

The set forms a two-dimensional lattice in the complex plane. For any element we consider the four numbers as *associates*. The associates of are known as units: .

### Letters to a German Princess: Euler’s Blockbuster Lives On

Published January 20, 2022 Irish Times , Occasional ClosedTags: Euler, History

The great Swiss mathematician Leonhard Euler produced profound and abundant mathematical works. Publication of his *Opera Omnia* began in 1911 and, with close to 100 volumes in print, it is nearing completion. Although he published several successful mathematical textbooks, the book that attracted the widest readership was not a mathematical work, but a collection of letters [TM227 or search for “thatsmaths” at irishtimes.com].

For several years, starting in 1760, Euler wrote a series of letters to Friederike Charlotte, Princess of Brandenburg-Schwedt, a niece of Frederick the Great of Prussia. The collection of 234 letters was first published in French, the language of the nobility, as *Lettres à une Princesse d’Allemagne*. This remarkably successful popularisation of science appeared in many editions, in several languages, and was widely read. Subtitled “On various subjects in physics and philosophy”, the first two of three volumes were published in 1768 by the Imperial Academy of Sciences in St. Petersburg, with the support of the empress, Catherine II.

Continue reading ‘Letters to a German Princess: Euler’s Blockbuster Lives On’

### Euler’s Journey to Saint Petersburg

Published January 13, 2022 Occasional ClosedTags: Euler, History

It all began with an invitation to Leonhard Euler to accept a chair of mathematics at the new Imperial Academy of Science in the city founded by Peter the Great. Euler’s journey from Basel to Saint Petersburg was a highly influential factor for the development of the mathematical sciences. The journey is described in detail in a full-length biography of Euler by Ronald Calinger (2016). The account below is heavily dependent on Calinger’s book.