What is O(log* N)?

Question

What is O(log* N) and how is it different from O(log N)?

Answer 1

O( log* N ) is "iterated logarithm":

In computer science, the iterated logarithm of n, written log* n (usually read "log star"), is the number of times the logarithm function must be iteratively applied before the result is less than or equal to 1.