# BMO1, Q6 Number Theory

Consecutive positive integers $m$, $m+1$, $m+2$ and $m+3$ are divisible by consecutive odd positive integers $n$, $n+ 2$, $n+ 4$ and $n+ 6$ respectively. Determine the smallest possible $m$ in terms of $n$.

So, I already tried solving this using the fact that this is a system of linear congruences. We have:

$$m\equiv 0 \pmod{n}$$ $$m+1 \equiv 0 \pmod{n + 2}$$ $$m+2 \equiv 0 \pmod{n + 4}$$ $$m+3 \equiv 0 \pmod{n + 6}$$

My approach revolved around this:

From the first congruence, we know that $$m=nk$$ where $k$ is an integer. Then we take this and substitute it into the second congruence: $$nk+1 \equiv 0 \pmod{n + 2}$$

And then solve for $k$, introduce a new variable, and substitute it back to the original equation for $m$. Needless to say, this approach gets extremely long and ugly. My teacher told me there is a way to do this problem using the Chinese Remainder theorem, however how do we prove that $n$, $n+2$, $n+4$ and $n+6$ are pairwise relatively prime?

• $n,n+2,n+4,n+6$ might not be pairwise coprime. Indeed, if $n$ is divisibile by $3$, then so is $n+6$. But, apart from this exception, these are actually coprime. That's because every common prime divisor should divide their difference as well: this may be $2,4$ or $6$. But these are odd numbers, hence only $3$ may be a common divisor. – Crostul Dec 5 '16 at 23:55
• So the approach may be to take three of the congruences and apply the CRT to them? – user14513462563 Dec 6 '16 at 5:24

Begin by showing that, for $n$ relatively prime to $3$, that $$m=\frac{n(n+2)(n+4)(n+6)+n}{2}$$ works, and for $n$ divisible by $3$, that $$m=\frac{n(n+2)(n+4)(n+6)+3n}{6}$$ works.
Then use the Chinese Remainder Theorem to argue that no smaller $m$ will do.