It took me a while to realize what this picture on the cover of the book "Introduction to Analytic Number Theory" (Apostol, 1976) really means.

Special thanks to @LavaboMouille for helping me interpret this picture correctly. On the horizontal axis we have x = 0, 1, 2, ..., 17. On the vertical axis we have y = 0, 1, 2, ..., 17. A cell exists at (x, y) if and only if gcd(x, y) ≠ 1.

In other words, a cell exists at (x, y) if and only if x and y are not relatively prime to each other.

How do you feel about all the viral brain teasers being circulated on social media? Here is how I feel about them: xkcd.com/169/ But here is how I really react to them: xkcd.com/356/