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Christian Lawson-Perfect

@christianp (1/2) Here's a fun one that my advisor gave me a while back. There is a building with two floors, and a shaft containing N electrical wires running from the top floor to the bottom floor. Each floor has an electrical panel containing a sequence of ports that the ends of the wires are connected to. However, the wires may be permuted between the two panels, so port #1 on the top floor may not be connected to port #1 on the bottom floor.

59. A PUZZLING WATCH. A friend pulled out his watch and said, "This watch of mine does not keep perfect time; I must have it seen to. I have noticed that the minute hand and the hour hand are exactly together every sixty-five minutes." Does that watch gain or lose, and how much per hour?

@christianp This one may be too easy: prove that in the set \( [2n] := {1, 2, \ldots, 2n} \), any subset of size \(n + 1 \) contains a coprime pair.

Hi! I'm a maths student at Helsinki University. For the past year or so I've been writing a popular (not as in visitors) maths blog,, in Finnish. In addition to human languages (FIN/ENG/SWE/FRE) I respond to quite a few computer ones (esp. C#).

Interested in seeing if toots > tweets!

Hello, new followers!
I'm on paternity leave with a tiny sleepy baby in my arms. Can anyone give me a maths puzzle I can do entirely in my head, with no writing or drawing?

212. THE BURMESE PLANTATION. A short time ago I received an interesting communication from the British chaplain at Meiktila, Upper Burma, in which my correspondent informed me that he had found some amusement on board ship on his way out in trying to solve this little poser.

If he has a plantation of forty-nine trees, planted in the form of a square as shown in the accompanying illustration, he wishes to know how he may cut down twenty-seven of the trees so that the twenty-two left (1/2)

The hardness of the Lemmings game, or Oh no, more NP-completeness proofs
None by Cormode, Graham
In collections: Attention-grabbing titles, Basically computer science, Games to play with friends, Animals
Entry: read.somethingorotherwhatever.

New to Mastodon. Here's an image I made a few years ago. #math #mathart

140. FIND ADA'S SURNAME. This puzzle closely resembles the last one, my remarks on the solution of which the reader may like to apply in another case. It was recently submitted to a Sydney evening newspaper that indulges in "intellect sharpeners," but was rejected with the remark that it is childish and that they only published problems capable of solution! Five ladies,
accompanied by their daughters, bought cloth at the same shop. Each of (1/2)

Topology Explains Why Automobile Sunshades Fold Oddly
Article by Feist, Curtis and Naimi, Ramin
In collections: Easily explained, Basically physics
We use braids and linking number to explain why automobile shades fold into an odd number of loops.
Entry: read.somethingorotherwhatever.

I know the joke, but I've never known what Zorn's lemma is actually useful for.
The Tricki explains all!

To Dissect a Mockingbird: A Graphical Notation for the Lambda Calculus with Animated Reduction
Web page by David C Keenan
In collections: Animals, Basically computer science
The lambda calculus, and the closely related theory of combinators, are important in the foundations of mathematics, logic and computer science. This paper provides an informal and entertaining introduction by means of...
Entry: read.somethingorotherwhatever.

Odd: a colleague mentioned Shanks transformations; I said "that's probably local hero William Shanks!".
It isn't, but this one also set a π record: Daniel Shanks was the first to compute π to 100k decimal places:, knocking William Shanks' 527 into a cocked hat

New entry!
Article by Pierre Cartier
In collections: Notation and conventions, The act of doing maths
My thesis is:there is another way of doing mathematics, equally successful, and the two methods should supplement each other and not fight.
Entry: read.somethingorotherwhatever.

I've updated to Mastodon 2.0.0.
Release notes:

We can add custom emoji now. Any ideas for mathematical emoji?