Pick an integer greater than 1 (say, 14). List its prime factors in order from smallest to greatest (2 7), and then “paste” those factors together to create a new number (27). Apply the same procedure to that number, and keep going until you reach a prime number:

27 = 3 × 3 × 3 → 333
333 = 3 × 3 × 37 → 3337
3337 = 47 × 71 → 4771
4771 = 13 × 367 → 13367

13367 is prime, so that’s the end of that sequence: 13367 is the home prime of 14.

A home prime should exist for every positive integer, but finding them can be stupendously hard. The sequence starting with 49 has been pressed through 118 steps now without producing a prime; the search continues. Details are maintained at Patrick De Geest’s website World of Numbers.

At age 7, F.W. de Bruijn found himself unable to pack a box measuring 6 × 6 × 6 quite completely with bricks measuring 1 × 2 × 4. The box had volume 216, so it might be expected to accommodate exactly 27 bricks, but he found there was no way to pack more than 26.

He mentioned this to his father, who happened to be mathematician Nicolaas Govert de Bruijn, and Nicolaas found that a “harmonic brick” (one in which the length of each side is a multiple of the next smaller side length) can be packed efficiently only into a box whose dimensions are multiples of the brick’s dimensions.

This can seen intuitively by imagining the 6 × 6 × 6 box filled with small colored cubes as shown here. No matter where it’s placed, each 1 × 2 × 4 brick must now displace an equal number of white and black cubes. But the box contains 112 white cubes and 104 black ones. So the task is impossible.

(Nicholas G. de Bruijn, “Filling Boxes With Bricks,” American Mathematical Monthly 76:1 [1969], 37-40.)

Multiply the first n prime numbers:

2 × 3 × 5 × 7 × 11 × 13 = 30030

Now find the smallest integer greater than 1 that will produce a prime number when it’s added to that product. In this example it’s 17:

30030 + 17 = 30047,

which is prime. This makes 17 a Fortunate number, named for Reo Fortune, the social anthropologist who first studied this. The first few Fortunate numbers are

3, 5, 7, 13, 23, 17, 19, 23, 37, 61, 67, 61, 71, 47, 107, 59, 61, 109, 89, 103, 79, 151 …

Are all Fortunate numbers prime? Fortune conjectured so, but whether it’s true remains an open problem.

When I give talks on factoring, I often repeat an incident that happened to me long ago in high school. I was involved in a math contest, and one of the problems was to factor the number 8051. A time limit of five minutes was given. It is not that we were not allowed to use pocket calculators; they did not exist in 1960, around when this event occurred! Well, I was fairly good at arithmetic, and I was sure I could trial divide up to the square root of 8051 (about 90) in the time allowed. But on any test, especially a contest, many students try to get into the mind of the person who made it up. Surely they would not give a problem where the only reasonable approach was to try possible divisors frantically until one was found. There must be a clever alternate route to the answer. So I spent a couple of minutes looking for the clever way, but grew worried that I was wasting too much time. I then belatedly started trial division, but I had wasted too much time, and I missed the problem. …

The trick is to write 8051 as 8100 – 49, which is 90² – 7², so we may use algebra, namely, factoring a difference of squares, to factor 8051. It is 83 × 97.

— Carl Pomerance, “A Tale of Two Sieves,” Notices of the AMS 43:12 (December 1996), 1473-1485

This is surprising: A laminar flow induced in a viscous fluid confined in the gap between two rotating cylinders can be (to a large extent) reversible. The dyes here appear to mix, but in fact they’re being stretched into distinct spirals that can then be “unmade” by reversing the direction of the rotation.

“A Polish girl, desperate and vengeful after failing her examination, took a gun, concealed herself in [Alfred] Werner’s garden, and awaited his return. He arrived home. She fired and missed. Werner calmly turned to her and remarked, ‘Your aim is no better than your knowledge of chemistry.'”

— George B. Kauffman, Alfred Werner: Founder of Coordination Chemistry, 2013

(“The Polizeiinspektorat der Stadt Zürich reports that neither the Stadtpolizei nor the Kantonspolizei have any record of the incident. Se non è vero, è ben trovato!”)

When a friend showed him a 16 × 16 magic square devised by Michel Stifelius, Ben Franklin went home and composed the square above, “not willing to be outdone.” An admirer describes its properties:

1. The sum of the sixteen numbers in each column or row, vertical or horizontal, is 2,056. — 2. Every half column, vertical or horizontal, makes 1,028, or just one half of the same sum, 2,056. — 3. Any half vertical row added to any half horizontal, makes 2,056. — 4. Half a diagonal ascending added to half a diagonal descending, makes 2,056, taking these half diagonals from the ends of any side of the square to the middle of it, and so reckoning them either upward, or downward, or sideways. — 5. The same with all the parallels to the half diagonals, as many as can be drawn in the great square: for any two of them being directed upward and downward, from the place where they begin to that where they end, make the sum 2,056; thus, for example, from 64 up to 52, then 77 down to 65, or from 194 up to 204, and from 181 down to 191; nine of these bent rows may be made from each side. — 6. The four corner numbers in the great square added to the four central ones, make 1,028, the half of any column. — 7. If the great square be divided into four, the diagonals of the little squares united, make, each, 2,056. — 8. The same number arises from the diagonals of an eight sided square taken from any part of the great square. — 9. If a square hole, equal in breadth to four of the little squares or cells, be cut in a paper, through which any of the sixteen little cells may be seen, and the paper be laid on the great square, the sum of all the sixteen numbers seen through the hole is always equal to 2,056.

Franklin wrote, “This I sent to our friend the next morning, who, after some days, sent it back in a letter with these words: ‘I return to thee thy astonishing or most stupendous piece of the magical square, in which’ — but the compliment is too extravagant, and therefore, for his sake as well as my own, I ought not to repeat it. Nor is it necessary; for I make no question but you will readily allow this square of 16 to be the most magically magical of any magic square ever made by any magician.”

(“Clavis,” “Magic Squares,” The Mirror of Literature, Amusement, and Instruction 4:109 [Oct. 23, 1824], 293-294.) (Thanks, Walker.)

12/21/2020 UPDATE: The square appeared originally in 1767 in James Ferguson’s Tables and Tracts, Relative to Several Arts and Sciences and was reprinted a year later in the Gentleman’s Magazine. Only the second publication credits Franklin. I don’t have a date for Franklin’s purported composition, so I don’t know what to make of this. (Thanks, Tom.)

Astronomer Thomas Dick believed that every planet in the solar system was inhabited. In his 1838 book Celestial Scenery he worked out that they contain 21 trillion inhabitants in all — assuming “280 inhabitants to a square mile, which is the rate of population in England.”