In their early studies of time and motion, engineers Frank Bunker Gilbreth and Lillian Moller Gilbreth divided all motions of the hand into 17 varieties:

Magnificently, they called these therbligs, which is roughly Gilbreth spelled backward. “Transport empty” refers to receiving an item with an empty hand, “transport loaded” means moving an object with the hand, and so on. With careful study, a worker’s movements might be optimized to maximize speed and efficiency. (It was the Gilbreths, for example, who suggested that surgeons employ “caddies” to pass their instruments to them.)

This scheme makes an appearance in fiction: In their 1948 novel Cheaper by the Dozen, Frank and Lillian’s children Frank Jr. and Ernestine describe what it’s like to grow up in the home of an efficiency expert:

Suppose a man goes into a bathroom and shave. We’ll assume that his face is all lathered and that he is ready to pick up his razor. He knows where the razor is, but first he must locate it with his eye. That is ‘search’, the first Therblig. His eye finds it and comes to rest — that’s ‘find’, the second Therblig. Third comes ‘select’, the process of sliding the razor prior to the fourth Therblig, ‘grasp’. Fifth is ‘transport loaded’, bringing the razor up to his face, and sixth is ‘position’, getting the razor set on his face.

“There are eleven other Therbligs — the last one is ‘think’!”

Flying from Casablanca to Dakar in 1925, French military photographer Marcelin Flandrin captured this photo in the Atlas Mountains.

It’s the last known image of a wild Barbary lion.

J. Van der Geer’s 2000 paper “The Art of Writing a Scientific Article” has been cited more than a thousand times, yet it doesn’t exist. Neither does the journal it appears in, the Journal of Science Communications.

The original was a “phantom reference” that had been presented only to illustrate Elsevier’s desired reference style. It seems to have been picked up by authors who didn’t understand that it was only a template, or who’d inadvertently retained the template while using it to format the rest of their references.

Anne-Wil Harzing, a professor of International Management at at Middlesex University in London, who described the confusion on her blog, concluded that the mystery “ultimately had a very simple explanation: sloppy writing and sloppy quality control.”

In 1833, Heinrich Scherk conjectured that every prime of odd rank (accepting 1 as prime) can be composed by adding and subtracting all the smaller primes, each taken once. For instance, 13 is the 7th prime and 13 = 1 + 2 – 3 – 5 + 7 + 11.

In 1967 J.L. Brown Jr. proved that this is true.