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Dear Dirichlet, Issue 23

Moonlighting agony uncle Professor Dirichlet answers your personal problems. Want the prof’s help? Send your problems to deardirichlet@chalkdustmagazine.com.

Dear Dirichlet,

I run one of the many pubs in my village, and I’ve been thinking about adding more potato options to the menu. The problem is, I think the landlord down the road has similar plans. If only one of us makes the change, it will undoubtedly attract more customers (everyone living here is a big fan of potatoes). But I’m worried that if we both do it, customers will think we’re in cahoots, and stop coming to either of our pubs. What do we do?!

— Christian D, Eynsham

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What’s hot and what’s not, Issue 23

Maths is a fickle world. Stay à la mode with our guide to the latest trends.

HOT Aliens did my homework

Obama has confirmed they’re real. How to cite in your dissertation: ‘private correspondence’.

NOT Gemini Pro did my homework

Inventing notation you’ve never used since 2025. Good luck when the free trial runs out.

HOT Dressing up as a Springer graduate text for World Book Day

Also doubles as the sun for those of us who have forgotten what it looks like.

NOT Dressing up as the A-level formula book for World Book Day

We don’t need to see where you keep your double angle identities.

HOT Tikzmaxxing

I mean, you could plot that graph in Matplotlib, but then the font size wouldn’t quite match and the colours wouldn’t be consistent…

NOT TK Maxxing

You don’t need another woven basket anyway.

HOT Doomscrolling maths reels on Instagram

Ten-second videos really are the best way to learn any idea. Then you can go watch golden retrievers being cute.

NOT Finding someone talking about your stuff in a reel without credit

Tut tut.

NOT Impact

The only impact we care about is the one with Bruce Willis and the asteroid. What do you mena, that was a different film?

HOT Rewatching TV shows

Will Meredith and McDreamy make it this time? Only one way to find out.

NOT Rewatching lecture capture videos

You weren’t there, man.

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The big argument: 6 or 7?

6, definitely, argues moustache Sam

Let’s get one thing right off the bat: 6 is just perfect. No, seriously, it is literally a perfect number! We say a number is perfect if its proper divisors add up to that number. The divisors of 6 are 1, 2 and 3… and, sure enough, $6=1+2+3$. Perfect!

Since 6 is perfect, that means it has an associated Mersenne prime; those of the form $2^p-1$. If you take any perfect number and extract all of its powers of 2, you will always get yourself a Mersenne prime. In the case of 6, $6=2^1\times3=2^1(2^2-1)$.

On the topic of primes… a pair of consecutive primes that are a distance of 6 away like {23, 29} or {6961, 6967} are called sexy primes, named aptly for their stunning appearance in the naturals and definitely not because the Latin for six is sex.

6 is a highly composite number, having more divisors than any number that precedes it. It’s also $3!$, which can be interpreted as the number of ways to arrange three distinct objects in a row. Knowing that $3!=6$ is really useful for the next time your friend asks you how many ways you can arrange three distinct objects in a row.

The final thing to say is that 6ft is an excellent height. You would definitely not want to be 7ft. And 6 is even.

Lucky #7, argues circus Sam

Not only is everyone’s lucky number a prime number, it’s a Mersenne prime: $7=2^3-1$. This means it has an associated perfect number; if you take any Mersenne prime $2^p-1$ and multiply it by $p-1$ twos, you get a perfect number. In the case of 7, $2^2(2^3-1)=28$. Perfect!

7 also appears just about everywhere. Consider the easy-to-remember rhyme I made to help you remember many occurrences of 7 in the world:

7 for the dwarfs in Snow White’s tale,
7 for the days in the week’s trail.
7 for the sins, deadly and vile,
7 for Samurai, who smites you with a smile.
7 for the wonders, both ancient and new,
7 for… red, orange, yellow, green, indigo, violet and blue.

Division by 7 is also extremely easy to do because the recurrence $0.\overline{142857}$ repeats. Just don’t ask me to test whether you can do it whole.

In chemistry, 7 remains the neutral, nay, nonchalant, constant of pH. I’m pretty sure a glass of pH 6 water would not taste good.

All jokes aside, 7 is obviously the more dominant number. It just shows strength. Why else would 6 be afraid? And 7 is seven.

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The Chalkdust guide to: Citation metrics

$h$-index: Largest $h$ such that an author has at least $h$ papers with at least $h$ citations.

i10-index: Number of papers by an author with at least 10 citations.

self-$h$-index: Largest $h$ such that an author has at least $h$ papers that have been cited at least $h$ times in other papers by the author.

$y$-index: Largest $y$ such that an author published a paper in year $y$ with at least $y$ citations.

$d$-index: Largest $d$ such that for some $n$, an author has a paper with $n$ citations and a paper with $n+d$ citations and no paper with a number of citations between $n$ and $n+d$.

$g$-index: Largest $g$ such that an author’s $g$ most cited papers have at least $g^2$ citations.

$k$-index: Largest $k$ such that for some $n$, an author has papers with $n, n+1, \dots, n+k-1$ citations.

ii-index: Number of papers by an author with at least $\sqrt{-1}$ citations.

$m$-index: Author’s $h$-index divided by the number of years since their first paper.

$o$-index: Geometric mean of an author’s $h$-index and the number of citations of their most cited paper.

$p$-index: Largest $p$ such that an author has a paper with at least $p$ pages and at least $p$ citations.

$z$-index: Number of papers where an author is last alphabetically.

\$h\$-index: Number of papers with {\LaTeX} in their title that’s not parsed on the journal website.

$\mathbb{N}$-index: Smallest $n\in\mathbb{N}$ such that an author doesn’t have a paper with exactly $n$ citations.

$\mathbb{N}$-index: As above, but using the convention that $0\not\in\mathbb{N}$.

nepo-index: An author’s $h$-index multiplied by the sum of their parents’ $h$-indices.

Runge-index: Maximum value of the interpolation polynomial through the points $(\text{year},\text{number of citations})$ in the range $[\text{year of first publication},\text{this year}]$.

uv-index: Number of papers that were published on days when it was sunny.

Benford-index: Number of an author’s publications where the number of citations starts with a 1.

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Cryptic crossword, Issue 23

Cryptic #11, set by Humbug: Download as a PDF or solve below!

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Prize crossnumber, Issue 23

Our original prize crossnumber is featured on pages 64 and 65 of Issue 23.

Rules

  • The digits 0 to 9 (inclusive) each appear exactly once in every row and column of the completed crossnumber.
  • There is only one solution to the completed crossnumber. Solvers may wish to use the OEIS, Python, Microsoft Encarta, etc to (for example) obtain a list of prime numbers, but no programming should be necessary to solve the puzzle. As usual, no entries begin with 0.
  • One randomly selected correct answer will win a £100 Maths Gear goody bag, including non-transitive dice, a Festival of the Spoken Nerd DVD, and much, much more. Three randomly selected runners up will win a Chalkdust T-shirt. Maths Gear is a website that sells nerdy things worldwide, with free UK shipping.
  • To enter, submit the sum of all the digits in the row marked by arrows using this form by 18 August 2026. Only one entry per person will be accepted. Winners will be notified by email and announced in issue 24.

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