MathDB
Highly divisible positive integers

Source: IMO Shortlist 2005, N5

March 19, 2007
number theoryprime numbersprime factorizationIMO Shortlist

Problem Statement

Denote by d(n)d(n) the number of divisors of the positive integer nn. A positive integer nn is called highly divisible if d(n)>d(m)d(n) > d(m) for all positive integers m<nm < n. Two highly divisible integers mm and nn with m<nm < n are called consecutive if there exists no highly divisible integer ss satisfying m<s<nm < s < n. (a) Show that there are only finitely many pairs of consecutive highly divisible integers of the form (a,b)(a, b) with aba\mid b. (b) Show that for every prime number pp there exist infinitely many positive highly divisible integers rr such that prpr is also highly divisible.