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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 .
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)