Choose a prime number p, draw a p×p array, and fill it with integers like so:
Now: Can we always find p cells that contain prime numbers such that no two occupy the same row or column? (This is somewhat like arranging rooks on a chessboard so that every rank and file is occupied but no rook attacks another.)
The example above shows one solution for p=11. Does a solution exist for every prime number? No one knows.
What is the smallest integer that’s not named on this blog?
Suppose that the smallest integer that’s not named (explicitly or by reference) elsewhere on the blog is 257. But now the phrase above refers to that number. And that instantly means that it doesn’t refer to 257, but presumably to 258.
But if it refers to 258 then actually it refers to 257 again. “If it ‘names’ 257 it doesn’t, so it doesn’t,” writes J.L. Mackie, “but if it doesn’t, then it does, so it does.”
(Adduced by Max Black of Cornell.)
Mr. X, who thinks Mr. Y a complete idiot, walks along a corridor with Mr. Y just before 6 p.m. on a certain evening, and they separate into two adjacent rooms. Mr. X thinks that Mr. Y has gone into Room 7 and himself into Room 8, but owing to some piece of absent-mindedness Mr. Y has in fact entered Room 6 and Mr. X Room 7. Alone in Room 7 just before 6, Mr. X thinks of Mr. Y in Room 7 and of Mr. Y‘s idiocy, and at precisely 6 o’clock reflects that nothing that is thought by anyone in Room 7 at 6 o’clock is actually the case. But it has been rigorously proved, using only the most general and certain principles of logic, that under the circumstances supposed Mr. X just cannot be thinking anything of the sort.
— A.N. Prior, “On a Family of Paradoxes,” Notre Dame Journal of Formal Logic, 1961
Dick and Jane are playing a game. Each holds up one or two fingers. If the total number of fingers is odd, then Dick pays Jane that number of dollars. If it’s even, then Jane pays Dick:
At first blush this looks fair, but in fact it’s distinctly favorable for Jane. Let p be the proportion of times that Jane holds up one finger. Her average winnings when Dick holds up one finger are -2p + 3(1 – p), and her average winnings when he holds up two fingers are 3p – 4(1 – p). If she sets those equal to one another she gets p = 7/12. This means that if she raises one finger with probability 7/12, then on average she’ll win -2(7/12) + 3(5/12) = 1/12 dollar every round, no matter what Dick does. Dick’s best strategy is also to raise one finger 7/12 of the time, but the best this can do is to restrict his loss to 1/12 dollar on average. It’s not a fair game.
Why do mathematicians confuse Halloween with Christmas?
Because 31 Oct = 25 Dec.
(Thanks, Jan.)
In 1976, Queen Elizabeth College chemist Leslie Hough asked graduate researcher Shashikant Phadnis to test a certain chlorinated sugar compound. Phadnis, whose English was imperfect, “thought I needed to taste it! … So I took a small quantity of the sample on a spatula and tasted it with the tip of my tongue.”
To his surprise, Phadnis found the compound intensely and pleasantly sweet. When he reported his discovery to Hough, “‘Are you crazy or what?’ he asked me. ‘How could you taste compounds without knowing anything about their toxicity?'” After some further cautious tasting, Hough dubbed the compound Serendipitose. It became the artificial sweetener Splenda.
“Later on, Les even had a cup of coffee sweetened with a few particles of Serendipitose. When I reminded him that it could be toxic (as it contained a high proportion of chlorine), he simply said, ‘Oh, forget it, we’ll survive!'”
Suppose I switch on my reading lamp at time zero. After one minute I switch it off again. Then I switch it on after a further 30 seconds, off after 15 seconds, and so on.
James Thomson asks: “At the end of two minutes, is the lamp on or off? … It cannot be on, because I did not ever turn it on without at once turning it off. It cannot be off, because I did in the first place turn it on, and thereafter I never turned it off without at once turning it on.”
What is the answer? Would the final state be different if I had switched the lamp off at time zero, rather than on? What if I carry out the experiment twice in succession?
See The Before-Effect.
A “curious paradox” presented by Raymond Smullyan at the first Gathering for Gardner: Consider two positive integers, x and y. One is twice as great as the other, but we’re not told which is which.
The two conclusions contradict one another, so something is amiss. But what?