Questions tagged [recreational-mathematics]

Math motivated by external "recreational" factors such as forms of entertainment including: games, logic puzzles, magic squares, Rubik's cube, chess problems, mazes, ciphers, cryptarithmetic, calendrical calculation, magic tricks, juggling, fractals (but numerology is off-topic). Also see the [puzzle] and [contest-math] tags.

Recreational mathematics is a fuzzy term for mathematics motivated by reasons mostly external to mainstream mathematical investigations. e.g. entertainment such as games, logic puzzles, magic squares, Rubik's cube, chess problems, mazes, ciphers, cryptarithmetic, calendrical calculation, magic tricks, juggling, fractals (but numerology is off-topic)

Recreational mathematics problems are often easy to understand even for people without an extensive mathematical education, even if the theory they lead to may turn out to be surprisingly deep. Thus, recreational mathematics can serve to attract the curiosity of non-mathematicians and to inspire them to develop their mathematical skills further.

Many typical recreational mathematics problems fall into the fields of discrete mathematics (combinatorics, elementary number theory, etc.), probability theory and geometry. Some important contributors to recreational mathematics are John Horton Conway, Martin Gardner, Richard K. Guy and Sam Loyd

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Visually stunning math concepts which are easy to explain

Since I'm not that good at (as I like to call it) 'die-hard-mathematics', I've always liked concepts like the golden ratio or the dragon curve , which are easy to understand and explain but are mathematically beautiful at the same time. Do you know…
RBS
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"The Egg:" Bizarre behavior of the roots of a family of polynomials.

In this MO post , I ran into the following family of polynomials: $$f_n(x)=\sum_{m=0}^{n}\prod_{k=0}^{m-1}\frac{x^n-x^k}{x^m-x^k}.$$ In the context of the post, $x$ was a prime number, and $f_n(x)$ counted the number of subspaces of an $n$…
Alexander Gruber
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Calculating the length of the paper on a toilet paper roll

Fun with Math time. My mom gave me a roll of toilet paper to put it in the bathroom, and looking at it I immediately wondered about this: is it possible, through very simple math, to calculate (with small error) the total paper length of a toilet…
user266764
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Can't argue with success? Looking for "bad math" that "gets away with it"

I'm looking for cases of invalid math operations producing (in spite of it all) correct results (aka "every math teacher's nightmare"). One example would be "cancelling" the 6s in $$\frac{64}{16}$$ Another one would be something like $$\frac{9}{2} -…
kjo
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The Mathematics of Tetris

I am a big fan of the old-school games and I once noticed that there is a sort of parity associated to one and only one Tetris piece, the $\color{purple}{\text{T}}$ piece. This parity is found with no other piece in the game. Background: The Tetris…
Eric Naslund
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"Integral milking": working backward to construct nontrivial integrals

I begin this post with a plea: please don't be too harsh with this post for being off topic or vague. It's a question about something I find myself doing as a mathematician, and wonder whether others do it as well. It is a soft question about…
Franklin Pezzuti Dyer
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What is the maximum volume that can be contained by a sheet of paper?

I was writing some exercises about the AM-GM inequality and I got carried away by the following (pretty nontrivial, I believe) question: Q: By properly folding a common $210mm\times 297mm$ sheet of paper, what is the maximum amount of water such a…
Jack D'Aurizio
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Can you answer my son's fourth-grade homework question: Which numbers are prime, have digits adding to ten and have a three in the tens place?

My son Horatio (nine years old, fourth grade) came home with some fun math homework exercises today. One of his problems was the following little question: I am thinking of a number... It is prime. The digits add up to $10.$ It has a $3$ in the tens…
JDH
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How can a piece of A4 paper be folded in exactly three equal parts?

This is something that always annoys me when putting an A4 letter in a oblong envelope: one has to estimate where to put the creases when folding the letter. I normally start from the bottom and on eye estimate where to fold. Then I turn the letter…
Nicky Hekster
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How many fours are needed to represent numbers up to $N$?

The goal of the four fours puzzle is to represent each natural number using four copies of the digit $4$ and common mathematical symbols. For example, $165=\left(\sqrt{4} + \sqrt{\sqrt{{\sqrt{4^{4!}}}}}\right) \div .4$. If we remove the restriction…
David Bevan
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Proving you *can't* make $2011$ out of $1,2,3,4$: nice twist on the usual

An undergraduate was telling me about a puzzle he'd found: the idea was to make $2011$ out of the numbers $1, 2, 3, 4, \ldots, n$ with the following rules/constraints: the numbers must stay in order, and you can only use $+$, $-$, $\times$, $/$, ^…
Kevin Buzzard
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Are $14$ and $21$ the only "interesting" numbers?

The numbers $14$ and $21$ are quite interesting. The prime factorisation of $14$ is $2\cdot 7$ and the prime factorisation of $14+1$ is $3\cdot 5$ . Note that $3$ is the prime after $2$ and $5$ is the prime before $7$ . Similarly, the prime…
Simon Parker
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There are apparently $3072$ ways to draw this flower. But why?

This picture was in my friend's math book: Below the picture it says: There are $3072$ ways to draw this flower, starting from the center of the petals, without lifting the pen. I know it's based on combinatorics, but I don't know how to show that…
user265554
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How do people perform mental arithmetic for complicated expressions? (In particular, $\frac{10^2+11^2+12^2+13^2+14^2}{365}$)

This is the famous picture " Mental Arithmetic. In the Public School of S. Rachinsky. " by the Russian artist Nikolay Bogdanov-Belsky . The problem on the blackboard is: $$ \dfrac{10^{2} + 11^{2} + 12^{2} + 13^{2} + 14^{2}}{365} $$ The answer is…
Vlad
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Deleting any digit yields a prime... is there a name for this?

My son likes his grilled cheese sandwich cut into various numbers, the number depends on his mood. His mother won't indulge his requests, but I often will. Here is the day he wanted 100: But today he wanted the prime 719, which I obliged. When…
Fixee
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