Because 2 ** 2 % 8 == 4. But it should be 0.

guys... there are no spoiler flags here...

Here is some explanations regarding the equation you mention: https://math.stackexchange.com/questions/192720/automorphic-numbers

Thanks!

I like your approach; it's much more Pythonic, scaleable, and elegant also - for me, I used this kata as an opportunity to learn about these numbers, so my approach is basically just a copy of my "reading notes" as I worked through the various articles/web pages!

This problem has two attractors f and s for each magnitude of ten 10 ** k
They are defined as follows: f, s = f**2 % 10**k, (1 - (s-1)**2) % 10 ** k (a) (b)
They are also linked together with a simpler formula: 10 ** k = f + s - 1 (2)
So you need to calculate only the simpler attractor with (a) and compute the other with (2) before adding them to the solution set