For instance, I had

'ootoguprkfwwyrtpaorss oghipretpdaagfgkauiluiptisesulqlue w' should equal ' ootoguprkfwwyrtpaorss oghipretpdaagfgkauiluiptisesulqlue w'

which doesn't make sense: should the return string be stripped or not?

You don't need to import math, you can just use i**j instead of math.pow(i, j).

It is weird that your solution worked, I had a really similar one, and it always timed out. I had to add a break in my for loop to quit when I already reached i and j such that i**j>n.

I think the description should mention that add('0', '0'), for instance, is supposed to return '0' (it could be None, nill, [], or whatever…)