Some of this complexity can be understood if we look at each number not in terms of its overall size, but rather in terms of its digit sequence or set of possible divisors.

Factoring integers The difficulty of factoring is presumably related to the irregularity of the pattern of divisors shown on page 909 .

Unsolved problems [in number theory] Problems in number theory that are simple to state (say in the notation of Peano arithmetic) but that so far remain unsolved include: • Is there any odd number equal to the sum of its divisors?