Skip to main content
OlympiadHQ

Browse · MathNet

Print

APMO

number theory

Problem

Determine the largest of all integers with the property that is divisible by all positive integers that are less than .
Solution
Observation from that is divisible by every integer less than or equal to and that is not divisible by . One may guess is the required integer.

Let be the required integer and suppose . Put . Then

Since , should divide and hence , which implies . But then should divide , which implies .

Observe that any four consecutive integers are divisible by and that any two out of four consecutive integers have gcd either , or . So, we have divides and in particular,



From above follows



Since ,



which is a contradiction.

Therefore, the largest such integer is .
Final answer
420

Techniques

Least common multiples (lcm)Greatest common divisors (gcd)