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The Monty Hall Problem, explained

posted by Jason Kottke   Sep 20, 2016

The Monty Hall Problem is one of those things that demonstrates just how powerful a pull common sense has on the human reasoning process. The problem itself is easily stated: there are three doors and behind one of them there is a prize and behind the other two, nothing. You choose a door in hopes of finding the prize and then one of the other two doors is opened to reveal nothing. You are offered the opportunity to switch your guess to the other door. Do you take it?

Common sense tells you that switching wouldn’t make any difference. There are two remaining doors, the prize is randomly behind one of them, why would switching yield any benefit? But as the video explains and this simulation shows, counterintuition prevails: you should switch every time.

America was introduced to the difficulty of the problem by Marilyn vos Savant in her column for Parade magazine in 1990.1 In a follow-up explanation of the question, vos Savant offered a quite simple “proof” of the always switch method (from Wikipedia). Let’s assume you pick door #1, here are the possible outcomes:

Door 1 Door 2 Door 3 Result if you stay Result if you switch
Car Goat Goat Wins car Wins goat
Goat Car Goat Wins goat Wins car
Goat Goat Car Wins goat Wins car

As you can see, staying yields success 33% of the time while if you switch, you win 2 times out of three (67%), a result verified by a properly written simulator. In his Straight Dope column, Cecil Adams explained it like so (after he had gotten it wrong in the first place):

A friend of mine did suggest another way of thinking about the problem that may help clarify things. Suppose we have the three doors again, one concealing the prize. You pick door #1. Now you’re offered this choice: open door #1, or open door #2 and door #3. In the latter case you keep the prize if it’s behind either door. You’d rather have a two-in-three shot at the prize than one-in-three, wouldn’t you? If you think about it, the original problem offers you basically the same choice. Monty is saying in effect: you can keep your one door or you can have the other two doors, one of which (a non-prize door) I’ll open for you.

See also the case of the plane and the conveyor belt (sorry not sorry, I couldn’t resist).

  1. I was a religious reader of Parade and remember this column and the resulting furor very clearly. Re: the furor, it’s so interesting to note in hindsight (being 16 and clueless at the time) how much of the response was men who clearly were trying to put the smackdown on a prominent, intelligent woman and they just got totally owned.

We Work Remotely

The infamous Collatz Conjecture

posted by Jason Kottke   Aug 09, 2016

For a recent episode of Numberphile, David Eisenbud explains the Collatz Conjecture, a math problem that is very easy to understand but has an entire book devoted to it and led famous mathematician Paul Erdős to say “this is a problem for which mathematics is perhaps not ready”.

The problem is easily stated: start with any positive integer and if it is even, divide it by 2 and if odd multiply it by 3 and add 1. Repeat the process indefinitely. Where do the numbers end up? Infinity? 1? Loneliness? Somewhere in-between? My favorite moment of the video:

16. Whoa, a very even number.

I love math and I love this video. (via df)

The fractal and geometric beauty of plants

posted by Jason Kottke   Jul 12, 2016

Plant Geometry

Plant Geometry

Plant Geometry

When you look at some plants, you can just see the mathematics behind how the leaves, petals, and veins are organized.

From Winnie Cooper to math whiz

posted by Jason Kottke   Jun 03, 2016

As a child, Danica McKellar played Winnie Cooper on The Wonder Years. After the show was over, McKellar had difficulty breaking away from other people’s perceptions of her. But in college, she discovered an aptitude for mathematics, went on to have a theorem named after her — not because she was famous but because she’d helped prove it — and forged a new identity. (via @stevenstrogatz)

P vs. NP and the Computational Complexity Zoo

posted by Jason Kottke   May 20, 2016

When Grade-A nerds get together and talk about programming and math, a popular topic is P vs NP complexity. There’s a lot to P vs NP, but boiled down to its essence, according to the video:

Does being able to quickly recognize correct answers [to problems] mean there’s also a quick way to find [correct answers]?

Most people suspect that the answer to that question is “no”, but it remains famously unproven.

In fact, one of the outstanding problems in computer science is determining whether questions exist whose answer can be quickly checked, but which require an impossibly long time to solve by any direct procedure. Problems like the one listed above certainly seem to be of this kind, but so far no one has managed to prove that any of them really are so hard as they appear, i.e., that there really is no feasible way to generate an answer with the help of a computer.

The rarified air of the NBA 7-footer

posted by Jason Kottke   May 20, 2016

Muggsey Yao

According to this 2011 article on Mark Eaton and other 7-footers who have played in the NBA, there are only 70 American men between 20 and 40 who are 7 feet tall…and that more than 1 in 6 of them get to play in the NBA.

The curve shaped by the CDC’s available statistics, however, does allow one to estimate the number of American men between the ages of 20 and 40 who are 7 feet or taller: fewer than 70 in all. Which indicates, by further extrapolation, that while the probability of, say, an American between 6’6” and 6’8” being an NBA player today stands at a mere 0.07%, it’s a staggering 17% for someone 7 feet or taller.

Being seven feet tall is absurdly tall and comes with a whole host of challenges, from bumping one’s head on door frames to difficulty finding clothes to health issues. Some of these difficulties arise out of simple geometry: as height and width increase, volume increases more quickly.1

  1. See also one of my favorite links ever, The Biology of B-Movie Monsters.

Some prime numbers are illegal in the United States

posted by Jason Kottke   May 06, 2016

The possession of certain prime numbers is illegal in the US. For instance, one of these primes can be used to break a DVD’s copyright encryption.

How many digits of pi does NASA use?

posted by Jason Kottke   Mar 18, 2016

Mathematicians have calculated pi out to more than 13 trillion decimal places, a calculation that took 208 days. NASA’s Marc Rayman explains that in order to send out probes and slingshot them accurately throughout the solar system, NASA needs to use only 15 decimal places, or 3.141592653589793. How precise are calculations with that number? This precise:

The most distant spacecraft from Earth is Voyager 1. It is about 12.5 billion miles away. Let’s say we have a circle with a radius of exactly that size (or 25 billion miles in diameter) and we want to calculate the circumference, which is pi times the radius times 2. Using pi rounded to the 15th decimal, as I gave above, that comes out to a little more than 78 billion miles. We don’t need to be concerned here with exactly what the value is (you can multiply it out if you like) but rather what the error in the value is by not using more digits of pi. In other words, by cutting pi off at the 15th decimal point, we would calculate a circumference for that circle that is very slightly off. It turns out that our calculated circumference of the 25 billion mile diameter circle would be wrong by 1.5 inches. Think about that. We have a circle more than 78 billion miles around, and our calculation of that distance would be off by perhaps less than the length of your little finger.

When was humanity’s calculation of pi accurate enough for NASA? In 1424, Persian astronomer and mathematician Jamshid al-Kashi calculated pi to 17 digits.

Space filling curves

posted by Jason Kottke   Feb 19, 2016

From 3Blue1Brown, a quick video showing some space-filling curves.

17 equations that changed the world

posted by Jason Kottke   Jan 28, 2016

17 Equations

In the book In Pursuit of the Unknown, Ian Stewart discusses how equations from the likes of Pythagoras, Euler, Newton, Fourier, Maxwell, and Einstein have been used to build the modern world.

I love how as time progresses, the equations get more complicated and difficult for the layperson to read (much less understand) and then Boltzmann and Einstein are like, boom!, entropy is increasing and energy is proportional to mass, suckas!

The beauty of mathematics

posted by Jason Kottke   Dec 18, 2015

The “hidden” mathematics and order behind everyday objects & phenomenon like spinning tops, dice, magnifying glasses, and airplanes. (via @stevenstrogatz)

Einstein’s first proof

posted by Jason Kottke   Nov 19, 2015

Steven Strogatz walks us through the first mathematical proof Albert Einstein did when he was a boy: a proof of the Pythagorean theorem.

Einstein, unfortunately, left no such record of his childhood proof. In his Saturday Review essay, he described it in general terms, mentioning only that it relied on “the similarity of triangles.” The consensus among Einstein’s biographers is that he probably discovered, on his own, a standard textbook proof in which similar triangles (meaning triangles that are like photographic reductions or enlargements of one another) do indeed play a starring role. Walter Isaacson, Jeremy Bernstein, and Banesh Hoffman all come to this deflating conclusion, and each of them describes the steps that Einstein would have followed as he unwittingly reinvented a well-known proof.

Twenty-four years ago, however, an alternative contender for the lost proof emerged. In his book “Fractals, Chaos, Power Laws,” the physicist Manfred Schroeder presented a breathtakingly simple proof of the Pythagorean theorem whose provenance he traced to Einstein.

Of course, that breathtaking simplicity later became a hallmark of Einstein’s work in physics. See also this brilliant visualization of the Pythagorean theorem

P.S. I love that two of the top three most popular articles on the New Yorker’s web site right now are about Albert Einstein.

Brilliant visualization of the Pythagorean Theorem

posted by Jason Kottke   Oct 26, 2015

Everyone knows that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides. What this video presupposes is, fuck yeah math!

PhotoMath iOS app can do your homework for you

posted by Jason Kottke   Oct 15, 2015


Some iOS apps still seem like magic. Case in point: PhotoMath. Here’s how it works. You point your camera at a math problem and PhotoMath shows the answer. It’ll even give you a step-by-step explanation and solution.

The mathematical secrets of Pascal’s triangle

posted by Jason Kottke   Sep 16, 2015

Pascal’s triangle1 is a simple arrangement of numbers in a triangle…rows are formed by the successive addition of numbers in previous rows. But out of those simple rows comes deep and useful mathematical relationships related to probability, fractals, squares, and binomial expansions. (via digg)

  1. As the video says, Pascal was nowhere near the discoverer of this particular mathematical tool. By the time he came along in 1653, the triangle had already been described in India (possibly as early as the 2nd century B.C.) and later in Persia and China.

Cool furniture alert: the Fibonacci Shelf

posted by Jason Kottke   Jul 29, 2015

The Fibonacci Shelf by designer Peng Wang might not be the most functional piece of furniture, but I still want one.

Fibonacci Shelf

Fibonacci Shelf

The design of the shelf is based on the Fibonacci sequence of numbers (0, 1, 1, 2, 3, 5, 8, 13, 21, …), which is related to the Golden Rectangle. When assembled, the Fibonacci Shelf resembles a series of Golden Rectangles partitioned into squares. (via ignant)

Fibonacci sequence hidden in ordinary division problem

posted by Jason Kottke   Jul 06, 2015

If you divide 1 by 999,999,999,999,999,999,999,998,999,999,999,999,999,999,999,999 (that’s 999 quattuordecillion btw), the Fibonacci sequence neatly pops out. MATH FTW!

Fibonacci division

At the end of Carl Sagan’s Contact (spoilers!), the aliens give Ellie a hint about something hidden deep in the digits of π. After a long search, a circle made from a sequence of 1s and 0s is found, providing evidence that intelligence was built into the fabric of the Universe. I don’t know if this Fibonacci division thing is on quite the same level, but it might bake your noodle if you think about it too hard. (via @stevenstrogatz)

Update: From svat at Hacker News, an explanation of the magic behind the math.

It’s actually easier to understand if you work backwards and arrive at the expression yourself, by asking yourself: “If I wanted the number that starts like 0.0…000 0…001 0…001 0…002 0…003 0…005 0…008 … (with each block being 24 digits long), how would I express that number?”

(thx, taylor)

Web Mandelbrot

posted by Jason Kottke   May 04, 2015


This web app allows you to explore the Mandelbrot set interactively…just click and zoom. I had an application like this on my computer in college, but it only went a few zooms deep before crashing though. There was nothing quite like zooming in a bunch of times on something that looked like a satellite photo of a river delta and seeing something that looks exactly like when you started. (via @stevenstrogatz)

Finding Zero

posted by Jason Kottke   Apr 23, 2015

Finding Zero

The latest book from Amir Aczel, who has written previously about the compass, the Large Hadron Collider, and Fermat’s Last Theorem, is Finding Zero: A Mathematician’s Odyssey to Uncover the Origins of Numbers…in particular, the number zero.

Finding Zero is an adventure filled saga of Amir Aczel’s lifelong obsession: to find the original sources of our numerals. Aczel has doggedly crisscrossed the ancient world, scouring dusty, moldy texts, cross examining so-called scholars who offered wildly differing sets of facts, and ultimately penetrating deep into a Cambodian jungle to find a definitive proof.

The NY Times has a review of the book, written by another Amir, Amir Alexander, who wrote a recent book on infinitesimals, aka very nearly zero. (via @pomeranian99)

The beauty of pi puts infinity within reach

posted by Jason Kottke   Mar 13, 2015

I’m dreading it. No hope of solving any equations that day, what with the pie-eating contests, the bickering over the merits of pi versus tau (pi times two), and the throwdowns over who can recite more digits of pi. Just stay off the streets at 9:26:53, when the time will approximate pi to ten places: 3.141592653.

The New Yorker’s Steven Strogatz on why pi matters.

Dancing mathematics

posted by Jason Kottke   Mar 02, 2015

Dancing Math

Mathematical functions depicted as stick figure dance moves. (via @mulegirl)

The Infinite Hotel Paradox

posted by Jason Kottke   Feb 19, 2015

In a lecture given in 1924, German mathematician David Hilbert introduced the idea of the paradox of the Grand Hotel, which might help you wrap your head around the concept of infinity. (Spoiler alert: it probably won’t help…that’s the paradox.) In his book One Two Three… Infinity, George Gamow describes Hilbert’s paradox:

Let us imagine a hotel with a finite number of rooms, and assume that all the rooms are occupied. A new guest arrives and asks for a room. “Sorry,” says the proprietor, “but all the rooms are occupied.” Now let us imagine a hotel with an infinite number of rooms, and all the rooms are occupied. To this hotel, too, comes a new guest and asks for a room.

“But of course!” exclaims the proprietor, and he moves the person previously occupying room N1 into room N2, the person from room N2 into room N3, the person from room N3 into room N4, and so on…. And the new customer receives room N1, which became free as the result of these transpositions.

Let us imagine now a hotel with an infinite number of rooms, all taken up, and an infinite number of new guests who come in and ask for rooms.

“Certainly, gentlemen,” says the proprietor, “just wait a minute.”

He moves the occupant of N1 into N2, the occupant of N2 into N4, and occupant of N3 into N6, and so on, and so on…

Now all odd-numbered rooms became free and the infinite of new guests can easily be accommodated in them.

This TED video created by Jeff Dekofsky explains that there are similar strategies for finding space in such a hotel for infinite numbers of infinite groups of people and even infinite amounts of infinite numbers of infinite groups of people (and so on, and so on…) and is very much worth watching:

(via brain pickings)

A regular expression for finding prime numbers

posted by Jason Kottke   Feb 12, 2015

Given that there’s so much mathematicians don’t know about prime numbers, you might be surprised to learn that there’s a very simple regular expression for detecting prime numbers:


If you’ve got access to Perl on the command line, try it out with some of these (just replace [number] with any integer):

perl -wle 'print "Prime" if (1 x shift) !~ /^1?$|^(11+?)\\1+$/' [number]

An explanation is here which I admit I did not quite follow. A commenter at Hacker News adds a bit more context:

However while cute, it is very slow. It tries every possible factorization as a pattern match. When it succeeds, on a string of length n that means that n times it tries to match a string of length n against a specific pattern. This is O(n^2). Try it on primes like 35509, 195341, 526049 and 1030793 and you can observe the slowdown.

Parable of the Polygons

posted by Jason Kottke   Dec 08, 2014

Using only squares, triangles, and the condition that each shape wants to move if less than 1/3 of its neighbors are like it, watch how extreme segregation appears in even the most random mixing of shapes.

These little cuties are 50% Triangles, 50% Squares, and 100% slightly shapist. But only slightly! In fact, every polygon prefers being in a diverse crowd. You can only move them if they’re unhappy with their immediate neighborhood. Once they’re OK where they are, you can’t move them until they’re unhappy with their neighbors again. They’ve got one, simple rule: “I wanna move if less than 1/3 of my neighbors are like me.”

Harmless, right? Every polygon would be happy with a mixed neighborhood. Surely their small bias can’t affect the larger shape society that much? Well… And… our shape society becomes super segregated. Daaaaang. Sometimes a neighborhood just becomes square, and it’s not their fault if no triangles wanna stick around. And a triangular neighborhood would welcome a square, but they can’t help it if squares ain’t interested.

Super super fascinating. Take your time and go through and play with all the interactive widgets. (via @ftrain)

The “power tower feeding frenzy” and hyper-large numbers

posted by Jason Kottke   Dec 04, 2014

What’s a large number? A billion? A billion times a billion? A billion to the billionth power? A googol? A googolplex? A googolplex is 10^googol, BTW:

So a googol is 1 with just 100 zeros after it, which is a number 10 billion times bigger than the grains of sand that would fill the universe. Can you possibly imagine what kind of number is produced when you put a googol zeros after the 1?

That’s pretty big, right? Not. Even. It turns out you can construct numbers that are so much larger than a googolplex, that it’s gonna light your head on fire just to read about them. Put on your asbestos hat and feast your eyes on Graham’s Number.

Moving up another level, exponentiation is iterated multiplication. Instead of saying 3 x 3 x 3 x 3, exponentiation allows me to bundle that string into the more concise 3^4.

Now, the thing is, this is where most people stop. In the real world, exponentiation is the highest operation we tend to ever use in the hyperoperation sequence. And when I was envisioning my huge googolplex^googolplex number, I was doing the very best I could using the highest level I knew — exponentiation. On Level 3, the way to go as huge as possible is to make the base number massive and the exponent number massive. Once I had done that, I had maxed out.

The key to breaking through the ceiling to the really big numbers is understanding that you can go up more levels of operations — you can keep iterating up infinitely. That’s the way numbers get truly huge.

You might get lost around the “power tower feeding frenzy” bit or the “power tower feeding frenzies psycho festival” bit, but persist…the end result is really just beyond superlatives. (via @daveg)

Update: In this video, you can listen to the inventor of Graham’s number, Ron Graham, explain all about it.

(via @eightohnine)

The Simpsons and their mathematical secrets

posted by Jason Kottke   Nov 13, 2014

Simpsons Math

Acclaimed science and math writer Simon Singh has written a book on the mathematics of The Simpsons, The Simpsons and Their Mathematical Secrets. Boing Boing has an excerpt.

The principles of rubber sheet geometry can be extended into three dimensions, which explains the quip that a topologist is someone who cannot tell the difference between a doughnut and a coffee cup. In other words, a coffee cup has just one hole, created by the handle, and a doughnut has just one hole, in its middle. Hence, a coffee cup made of a rubbery clay could be stretched and twisted into the shape of a doughnut. This makes them homeomorphic.

By contrast, a doughnut cannot be transformed into a sphere, because a sphere lacks any holes, and no amount of stretching, squeezing, and twisting can remove the hole that is integral to a doughnut. Indeed, it is a proven mathematical theorem that a doughnut is topologically distinct from a sphere. Nevertheless, Homer’s blackboard scribbling seems to achieve the impossible, because the diagrams show the successful transformation of a doughnut into a sphere. How?

Although cutting is forbidden in topology, Homer has decided that nibbling and biting are acceptable. After all, the initial object is a doughnut, so who could resist nibbling? Taking enough nibbles out of the doughnut turns it into a banana shape, which can then be reshaped into a sphere by standard stretching, squeezing, and twisting. Mainstream topologists might not be thrilled to see one of their cherished theorems going up in smoke, but a doughnut and a sphere are identical according to Homer’s personal rules of topology. Perhaps the correct term is not homeomorphic, but rather Homermorphic.

Doing math like a girl

posted by Jason Kottke   Aug 12, 2014

The Fields Medal is viewed as the greatest honor in mathematics; the Nobel of math. Today, Iranian mathematician Maryam Mirzakhani became the first woman (and Iranian) to win a Fields Medal.

Maryam Mirzakhani has made stunning advances in the theory of Riemann surfaces and their moduli spaces, and led the way to new frontiers in this area. Her insights have integrated methods from diverse fields, such as algebraic geometry, topology and probability theory.

In hyperbolic geometry, Mirzakhani established asymptotic formulas and statistics for the number of simple closed geodesics on a Riemann surface of genus g. She next used these results to give a new and completely unexpected proof of Witten’s conjecture, a formula for characteristic classes for the moduli spaces of Riemann surfaces with marked points.

In dynamics, she found a remarkable new construction that bridges the holomorphic and symplectic aspects of moduli space, and used it to show that Thurston’s earthquake flow is ergodic and mixing.

Most recently, in the complex realm, Mirzakhani and her coworkers produced the long sought-after proof of the conjecture that - while the closure of a real geodesic in moduli space can be a fractal cobweb, defying classification - the closure of a complex geodesic is always an algebraic subvariety.

Get all that? Adolescent math fans, you have a new role model. She does math like a girl. Here’s more on Mirzakhani from Quanta Magazine.

Möbius Bagel

posted by Tim Carmody   Jul 31, 2014

What if you wanted to cut a bagel in half not for toasting or sandwich purposes, but to explore its topology and mildly astonish your friends?

bagel cutting pattern.jpg

If you cut a bagel along a möbius strip pattern, you end up with two separate halves that form interlocking rings, as shown below.


Geoge Hart, who cut this bagel and made this video, is an engineering professor at SUNY-Stony Brook and “mathematical sculptor. On his web site, he offers two bagel-derived math problems: What is the ratio of the surface area of this linked cut to the surface area of the usual planar bagel slice? and Modify the cut so the cutting surface is a one-twist Mobius strip.

Via @mark_e_evans and The Onion A/V Club.

Fantastic recursive book cover

posted by Jason Kottke   Jun 23, 2014

The book cover for Naive Set Theory by Paul Halmos is so so good:

Naive Set Theory Cover

The cover is a riff on, I think, Russell’s Paradox, a problem with naive set theory described by Bertrand Russell in 1901 about whether sets can contain themselves.

Russell’s paradox is based on examples like this: Consider a group of barbers who shave only those men who do not shave themselves. Suppose there is a barber in this collection who does not shave himself; then by the definition of the collection, he must shave himself. But no barber in the collection can shave himself. (If so, he would be a man who does shave men who shave themselves.)

Reminds me of David Pearson’s genius cover for Benjamin’s The Work of Art in the Age of Mechanical Reproduction.

Cheese charts

posted by Jason Kottke   Jun 18, 2014

Camembert chart

In France, pie charts are called “le camembert” after the cheese. Or sometimes “un diagramme en fromage” (cheese diagram). In Brazil, they are pizza charts. (via numberphile & reddit)