Here’s the opening of Alice’s Adventures in Wonderland:

Alice was beginning to get very tired of sitting by her sister on the bank, and of having nothing to do; once or twice she had peeped into the book her sister was reading.

Choose any of the first 12 words and tap your way forward in the sentence from that point, tapping one word for each letter. So, for example, if you choose the word Alice, which has five letters, you’d tap was, beginning, to, get, and land on very. Then do the same thing with that word, advancing four letters to land on by. If you keep this up you’ll always arrive at the word sister.

That’s from Martin Gardner; the same trick works with “Twinkle, Twinkle, Little Star,” the opening of the Bible, and countless other texts.

It’s less surprising than it seems — it’s based on a principle called the Kruskal count, proffered originally by Rutgers physicist Martin Kruskal as a card trick. In each case various tributaries merge into a common stream that arrives at a predictable destination. Here’s an analysis (PDF).

Mark Levi notes an interesting coincidence in Why Cats Land on Their Feet (2012): Dividing normal human body temperature (in Celsius) into 100 approximates e:

“The estimate will be on the low side if you run a fever, or on the high side if you have hypothermia,” Levi writes. “This observation makes the natural logarithm — the one with the base e — seem even more natural.”

When David Huffman died in 1999, the world lost a talented computer scientist — Huffman was best known for discovering the Huffman coding technique used in data compression.

But it also lost a pioneer in mathematical origami, an extension of the traditional art of paper folding that applies computational geometry, number theory, coding theory, and linear algebra. The field today is finding wide application, helping researchers to fold everything from proteins to automobile airbags and space-based telescopes.

Huffman was drawn to the work through his investigations into the mathematical properties of “zero curvature” surfaces, studying how paper behaves near creases and apices of cones. During the last two decades of his life he created hundreds of beautiful, perplexing paper models in which the creases were curved rather than straight.

But he kept his folding research largely to himself. He published only one paper on the subject (PDF), and much of what he discovered was lost at his death. “He anticipated a great deal of what other people have since rediscovered or are only now discovering,” laser physicist Robert Lang told the New York Times in 2004. “At least half of what he did is unlike anything I’ve seen.” MIT computer scientist Erik Demaine is working now with Huffman’s family to recover and document his discoveries (PDF).

“I don’t claim to be an artist. I’m not even sure how to define art,” Huffman told an audience in 1979. “But I find it natural that the elegant mathematical theorems associated with paper surfaces should lead to visual elegance as well.”

Here’s another interesting source of complementary sequences. Take any positive irrational number, say , and call it X. Call its reciprocal Y; in this case , or about 0.7. Add 1 to each of X and Y and we get

1 + X ≈ 2.4

1 + Y ≈ 1.7.

Now make a table of the approximate multiples of 1 + X and 1 + Y:

If we drop the fractional part of each number in the table, we’re left with two complementary sequences — every number 1, 2, 3, … appears in one sequence or the other, but never in both.

They’re called Beatty sequences, after Sam Beatty of the University of Toronto, who discovered them in 1926. A pretty proof by A. Ostrowski and J. Hyslop appears in the March 1927 issue of the American Mathematical Monthly and in Ross Honsberger’s Ingenuity in Mathematics (1970).

In 1934, Indian mathematician S.P. Sundaram proposed this “sieve” for finding prime numbers.

In the first row of a table, write the arithmetic progression 4, 7, 10, …, with the first term 4 and a common difference of 3.

Copy these values into the first column, and then complete each row with its own arithemetic progression, with common differences of 3, 5, 7, 9 …, in successive rows.

Now, remarkably, for any natural number N > 2, if N occurs in the table then 2N + 1 is not a prime number, and if N does not occur in the table, then 2N + 1 is a prime number. (For example, 17 appears in the table, so 35 is not prime; 23 does not appear in the table, so 47 is prime.)

(From Ross Honsberger, Ingenuity in Mathematics, 1970.)

A pleasing fact from David Wells’ Archimedes Mathematics Education Newsletter:

Draw two parallel lines. Fix a point on one line and move a second point along the other line. If an equilateral triangle is constructed with these two points as two of its vertices, then as the second point moves, the third vertex of the triangle will trace out a straight line.

Thanks to reader Matthew Scroggs for the tip and the GIF.

Theoretical physicist Paul Dirac offered this example to show that some objects return to their original state after two full rotations, but not after one.

Hold a cup water in one hand and rotate it through 360 degrees (in either direction). You’ll have to contort yourself to accomplish this without spilling any water, but if you continue rotating the cup another 360 degrees in the same direction, you’ll find that you return to your original state.

The same principle can be demonstrated using belts. In the video below, the square goes through two full rotations and we find that the belts have returned to their original state. This would not be the case after a single rotation. (Here two belts are attached to the square, but the trick works with any number of belts.)

Another interesting item from James Tanton’s Mathematics Galore! (2012):

Write down a sequence of positive integers that never decreases. The list can include duplicates. As an example, here’s a list of primes:

2, 3, 5, 7, 11, 13

Call the sequence p_n. Now, a “frequency sequence” records the number of members less than 1, less than 2, and so on. For the list of primes above, the frequency sequence is:

0, 0, 1, 2, 2, 3, 3, 4, 4, 4, 4, 5, 5, 6

Pleasingly, the frequency sequence of the frequency sequence of p_n is p_n. That is, if we take the frequency sequence of the list 0, 0, 1, 2, 2, 3, 3, 4, 4, 4, 4, 5, 5, 6 above, we get 2, 3, 5, 7, 11, 13 again.

Now add position numbers to each of the two lists, p_n and its frequency sequence — that is, add 1 to the first element of each, 2 to the second, and so on. With the primes that gives us:

P_n: 3, 5, 8, 11, 16, 19 …

Q_n: 1, 2, 4, 6, 7, 9, 10, 12, 13, 14, 15, 17, 18, 20 …

These two sequences will always be complementary — all the counting numbers appear, but they’re split between the two sequences, with no duplicates.