The guy in figure 24 is a French mathematician called Étienne Ghys and somewhat known to the public for his commitment to the popularisation of mathematics. I guess he would love this sort of things. I wonder if this picture that chosen randomly on the internet or if this is some kind of homage.

I guess they know each other as they have a lot of collaborations, such as this one [1].

[1] http://www.chaos-math.org/en

If you have enjoyed the article and you have macOS, you will probably like the LotsaEscher [1] screensaver. Too bad it does not work since Sierra---the source code [2] is in the public domain, though, so hopefully someone can work around.

[1] http://wakaba.c3.cx/sup/kareha.pl/1118971388/ (the original URL is gone, I guess that's because the program does not work now)

[2] https://bitbucket.org/WAHa_06x36/lotsaescher/

I love this article. Years ago I tried to find the formulas on my own and implement them in my own Complex function visualizator but failed :( I have to retry !

Someone who succeeded : http://2008.sub.blue/projects/droste.html