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Here's the same thing for a 2×2 grid. The triangle in the middle has to go one way or the other, so you have to break the vertical line of symmetry

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I've been thinking about the fact that an n×n square grid has as many cells as a triangular lattice with n triangles on each side.
Is there a nice continuous bijection between them? When n is odd, you can at least keep one line of symmetry throughout.

I'm not sure I've seen this puzzle before, and I really like it:

We have n keys and n boxes. Each key fits only one box. We shuffle the keys and put one in each box. Then we randomly break open 1≤k≤n boxes. What is the probability that we can unlock all the other boxes?

accessibility, color contrast 

Are you kidding.

Also, bold ploy by someone in MS to recommend Chrome over their own browser.

Currently embroiled in an all-Chris email conversation. Have had to resort to using surnames to have any chance of knowing who we're talking about

Consider the algorithm "M(x): if x<0 return -x, else return M(x-M(x-1))/2". This algorithm terminates for all real x, though this is not so easy to prove. In fact, Peano Arithmetic cannot prove the statement "M(x) terminates for all natural x". Paper to come! Joint work with @jeffgerickson and @alreadydone

Came across the fact that \[ \frac{1}{96} = \sum_{k=1}^{\infty} \frac{k^2}{(k+1)(k+2)(k+3)(k+4)(k+5)} \], without any references.
How do I go about proving that? I hope the answer doesn't involve the Riemann ζ function.

kickstarter, math circle book 

@christianp Nice. I wish there were a way to do the same at arXiv, but things are a bit too dense there.

What I use most often at arXiv does work at MathWorld though: mathworld.wolfram.com/search/?

Just noticed that the LaTeX → unicode replacement on mathstodon.xyz wasn't there - it was on the mathjax-only feature branch!
Now turned back on. Enjoy ∀!

Is anyone planning anything for the next palindrome day, 2020-02-02?

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