I encourage Pythonistas into teaching Python in the context of generic computer science and mathematics, high school and above (and below for the precocious) to consider "indigs" an accessible example of recursion at work.
I'm testing it out in a warm up section students might visit if feeling shaky on their Python, or just wanting more background reading. https://github.com/4dsolutions/clarusway_data_analysis/blob/main/python_warm_up/warmup_recursion.ipynb The idea of an indig is very simple and inherits from numerology, a kind of primordial NLP: Take any positive integer, split it into individual digits, add those digits, if the sum is a single digit you're done, otherwise: repeat. Examples: 32986513 = 3+2+9+8+6+5+1+3 = 37 = 3+7 = 10 = 1+0 = 1 59865279171 = 5+9 = 14+8 = 22+6 = 28+5 = 33+2 = 35+7 = 42+9 = 51+1 = 52+7 = 59+1 = 60 = 6+0 = 6 I got it down to this, but it's no crime to take a few more lines. def indig(n: int): return n if len(str(n))==1 else indig(sum(map(int, list(str(n))))) It's not important that "taking the indig of a number" be considered "meaningful" in any way. What's useful about it is it introduces type conversion (int to string digits, back to single ints for summing) as well as recursion. This idea and algorithm may well have been proposed and used before in the literature. Sounds Martin Gardnery. Maybe it's in Knuth already. I'm not trying to establish priority for anything, only circling what looks like promising pedagogy going forward and sharing it with peers. Kirby
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