How can you arrange seven points in a plane so that if any three are chosen, at least two of them will be a unit distance apart?

The solution is shown here — it’s known as a Moser spindle.

In 1998, the BBC reported that four card players at a whist club in Bucklesham, Suffolk, had each been dealt one suit from a shuffled deck. Hilda Golding found herself holding 13 clubs, Hazel Ruffles held 13 diamonds, and Alison Chivers held 13 hearts (and won, as this was trumps). The dummy hand, face down on the table, held 13 spades.

Though there were 55 people in the village hall at the time, some of whom claimed to have witnessed the event, it’s vastly more likely that this was some misunderstanding or a false report. In 1939 Horace Norton of University College London calculated the odds of such a deal arising naturally to be 1 in 2,235,197,406,895,366,368,301,599,999.

In The Mathematics of Games (2013), John D. Beasley writes, “A typical evening’s bridge comprises perhaps twenty deals, so a once-a-week player must play for over one hundred million years to have an even chance of receiving a thirteen-card suit. If ten million players are active once a week, a hand containing a thirteen-card suit may be expected about once every fifteen years, but it is still extremely unlikely that a genuine deal will produce four such hands.”

(Martin Gardner points out somewhere that anecdotal reports of four perfect hands are strangely more frequent than reports of two perfect hands, which is more likely — though, I guess, less newsworthy.)

(Via Martin Cohen, 101 Philosophy Problems, 2002.)

Male side-blotched lizards compete for mates using a three-sided strategy that resembles a game of rock-paper-scissors. Orange-throated males, the strongest, don’t form strong pair bonds but establish large territories and fight blue-throated males outright for females. The blue-throated males, middle-sized, are less aggressive and tend to pair strongly with individual females. Yellow-throated males, the smallest, have a coloration that resembles that of females; this allows them to approach females in the territories of orange-throated males — though this won’t work with females that have formed strong pair bonds with blue-throated males.

So, broadly speaking, orange beats blue, blue beats yellow, and yellow beats orange, an equilibrium of sorts in which each variety has an advantage over another but not over the third.

A pleasing little theorem by Dutch mathematician Frans van Schooten:

Inscribe equilateral triangle ABC in a circle. Now, from a point P on that circle, the length of the longest of segments PA, PB, PC equals the sum of the lengths of the other two segments (in this example, the length of segment PA equals the sum of the lengths of PB and PC).

From Lee Sallows:

A recent contribution to Futility Closet showed an atypical type of 3×3 geometric magic square in which the 4 pieces occupying each of its nine 2×2 subsquares are able to tile the same rectangle. A different square with the same property is seen in the figure here shown, where the nine tiled rectangles appear at right.

As in the earlier example, the square is to be interpreted as if drawn on a torus, the relations among its peripheral cells then being the same as those that result if the square is surrounded with copies of itself, as seen in the following figure showing four such copies, one in each quadrant:

The figure makes it easier to identify the different 2 × 2 subsquares, exactly nine distinct examples of which can be identified. A brief commentary on the square pointed out that the number of ‘magic’ conditions it satisfies is one greater than the eight conditions demanded by a conventional 3 × 3 magic square. Hence the title of the piece, ‘Extra Magic.’

It was while perusing this diagram that an alternative division of the cells into sets of 4 suggested itself. Instead of 2 × 2 subsquares, consider the four cells defined by a cross that can be centered on any chosen cell. The above figure shows a yellow-shaded example, along with a rectangle tiled by its four associated shapes. It is interesting to note that, as before, there are just nine distinct crosses of this kind to be found in a 3 × 3 square. An obvious question thereby prompted was whether or not a new 3 × 3 magic square could be found based upon such crosses rather than 2×2 subsquares? The answer turned out to be yes, but in the process of scrutinizing an initial specimen I noticed that although it embodied nine cross-based sets of 4 rectangle-tiling pieces, as required, it also included a couple of additional rectangle-tiling sets contained within 2 × 2 subsquares. Clearly the maximum number of such surplus sets would be nine, one for each cross, but could a specimen showing nine cross-based and nine subsquare-based rectangle-tiling sets really exist? I lost no time in seeking an answer.

Regrettably, I was unable to find one. However, the figure below shows a close approach to perfection. It is the same 3 × 3 square with which we started, but now shown alongside no less than eight additional rectangles, each of them tiled with a set of 4 pieces belonging to a cross. Note that the missing rectangle is the one belonging to the non-magic central cross, a show of symmetry that seems appropriate.

So whereas a 3 × 3 magic square, numerical or geometric, satisfies at least 8 separate conditions (3 rows + 3 columns + 2 diagonals), the square here shown satisfies no less than eight more.

(Thanks, Lee.)

P. Anning noted this curiosity in Scripta Mathematica in 1956 — if the middle digit 1 in both the numerator and denominator of 101010101/110010011 is replaced with any odd number of 1s, then the proportion remains the same. And all of these numbers are palindromes!

Waclaw Sierpinski gives a proof in 250 Problems in Elementary Number Theory (1970).

In 1983, University of British Columbia physicist Lorne Whitehead noted “a simple and dramatic demonstration of exponential growth, as in a nuclear chain reaction.” He determined that one domino can knock down another that’s about half again as large in all dimensions; since the gravitational potential energy of an upright domino is proportional to the fourth power of its size, this means that one tiny domino can set off a graduated chain reaction with impressively thunderous results.

Whitehead’s first domino was less than 10 mm high; he nudged it with a piece of cotton. The resulting chain reaction brought down a 13th domino that was 64 times as tall; an investment of 0.024 microjoules at one end had released 51 joules of energy at the other, an amplification factor of about 2 billion.

Of course, it’s possible to construct impressive chains of graduated dominoes even if they grow less dramatically than this one. Here’s a world record set in the Netherlands in 2009:

(Lorne A. Whitehead, “Domino ‘Chain Reaction,'” American Journal of Physics 51:2 [February 1983], 183.)

To multiply this 58-digit number by 6, just move its last digit to its head.

Surprisingly, no smaller number displays this behavior when multiplied by 6. It’s called a parasitic number.