We all have learned the Euclidean Algorithm. So what's the worst case of it?
You need output a progression from its very front, for example:
An arithmetic progression 1, 2, 3, 4, ... or 1, 3, 5, 7, ...
A geometric progression 1, 2, 4, 8, ... or 1, 4, 16, 64, ...
You need to output the first 20 numbers of the progression.