# Consecutive integers

• Apr 12th 2009, 02:17 PM
jamix
Consecutive integers
Prove that for any set of six consecutive integers (all greater than 5), at least one integer is not divisible by 2,3 or 5.
• Apr 12th 2009, 05:53 PM
NonCommAlg
Quote:

Originally Posted by jamix

Prove that for any set of six consecutive integers (all greater than 5), at least one integer is not divisible by 2,3 or 5.

Hint: look at your set modulo 6 and note that for any integer $k,$ either $6k+1$ or $6k+5$ is not divisible by 5.

by the way, do we really need the condition in red?
• Apr 12th 2009, 06:24 PM
jamix
Oops. I guess we don't need the condition in red.

Too finish off the hint, I'd show that if 6k+1 is divisible by 5, then 6k+5 = 4mod5. Similarily if 6k+5 = 0mod5, then 6k+1 = (-4) = 1mod5. As both of these are clearly not divisible by 2 or 3, it follows that one of them is not divisible by 2,3, or 5.

Question #2:

Show that any set of 8 consecutive integers have the property that at least one member in the set is not divisible by 2,3,5 or 7.

I guess maybe for this one, we would look at things modulo 30, yes?

Question #3:

Show by counterexample that the following statement is false:

Any set of 2k consecutive integers (all greater than p_k) have the property that at least one member of the set is not divisible by the any of the primes up to p_k (inclusive).