A numerical coincidence is an equality or near-equality between different mathematical quantities which has no known theoretical explanation. Sometimes such equalities remain mysterious and intriguing, and sometimes theory advances to the point where they can be explained and are no longer regarded as surprising.

### Numerical Coincidences

Published March 23, 2017 Occasional Leave a CommentTags: Number Theory, Recreational Maths

### A Life-saving Whirligig

Published March 16, 2017 Irish Times Leave a CommentTags: Mechanics, medicine

Modern science is big: the gravitational wave detector (LIGO) cost over a billion dollars, and the large hadron collider (LHC) in Geneva took decades to build and cost almost five billion euros. It may seem that scientific advances require enormous financial investment. So, it is refreshing to read in *Nature Biomedical Engineering *(Vol 1, Article 9) about the development of an ultra-cheap centrifuge that costs only a few cents to manufacture [TM111 or search for “thatsmaths” at irishtimes.com].

### Brun’s Constant and the Pentium Bug

Published March 9, 2017 Occasional Leave a CommentTags: Arithmetic, Euler, Number Theory

Euclid showed by a deliciously simple argument that the number of primes is infinite. In a completely different manner, Euler confirmed the same result. Euler’s conclusion followed from his demonstration that the sum of the reciprocals of the primes diverges:

Obviously, this could not happen if there were only finitely many primes.

### Enigmas of Infinity

Published March 2, 2017 Irish Times Leave a CommentTags: Analysis, History, Logic

Children sometimes amuse themselves searching for the biggest number. After trying millions, billions and trillions, they realize that there is no end to the game: however big a number may be, we can always add 1 to produce a bigger number: the set of counting numbers is infinite. The concept of infinity has intrigued philosophers since antiquity, and it leads to many surprises and paradoxical results [TM110 or search for “thatsmaths” at irishtimes.com].

### Topology in the Oval Office

Published February 23, 2017 Occasional Leave a CommentTags: Graph Theory, Recreational Maths, Topology

Imagine a room – the Oval Office for example – that has three electrical appliances:

• An air-conditioner ( a ) with an American plug socket ( A ),

• A boiler ( b ) with a British plug socket ( B ),

• A coffee-maker ( c ) with a Continental plug socket ( C ).

The problem is to connect each appliance to the correct socket, **avoiding any crossings of the connecting wires.**

### The Spire of Light

Published February 16, 2017 Irish Times Leave a CommentTags: Applied Maths, Mechanics

Towering over O’Connell Street in Dublin, the Spire of Light, at 120 metres, is about three times the height of its predecessor [TM109 or search for “thatsmaths” at irishtimes.com]. The Spire was erected in 2003, filling the void left by the destruction in 1966 of Nelson’s Pillar. The needle-like structure is a slender cone of stainless steel, the diameter tapering from 3 metres at the base to 15 cm at its apex. The illumination from the top section shines like a beacon throughout the city.

### Metallic Means

Published February 9, 2017 Occasional Leave a CommentTags: Algebra, Arithmetic, Recreational Maths

with the value

.

There is no doubt that is significant in many biological contexts and has also been an inspiration for artists. Called the *Divine Proportion*, it was described in a book of that name by Luca Pacioli, a contemporary and friend of Leonardo da Vinci.