Divisibility of Product of Consecutive Integers

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Theorem

The product of $n$ consecutive positive integers is divisible by the product of the first $n$ consecutive positive integers.

That is:

$\ds \forall m, n \in \Z_{>0}: \exists r \in \Z: \prod_{k \mathop = 1}^n \paren {m + k} = r \prod_{k \mathop = 1}^n k$


Proof

\(\ds \prod_{k \mathop = 1}^n \paren {m + k}\) \(=\) \(\ds \frac {\paren {m + n}!} {m!}\)
\(\ds \) \(=\) \(\ds n! \frac {\paren {m + n}!} {m! \, n!}\)
\(\ds \) \(=\) \(\ds n! \binom {m + n} m\) Definition of Binomial Coefficient
\(\ds \) \(=\) \(\ds \binom {m + n} m \prod_{k \mathop = 1}^n k\)


Hence the result, and note that for a bonus we have identified exactly what the divisor is:

$\dbinom {m + n} m$

$\blacksquare$


Examples

Product of $5$ to $8$

\(\ds 5 \times 6 \times 7 \times 8\) \(=\) \(\ds 1680\)
\(\ds \) \(=\) \(\ds 70 \times 24\)
\(\ds \) \(=\) \(\ds \binom 8 4 \times 1 \times 2 \times 3 \times 4\)


Product of $10$ to $13$

\(\ds 10 \times 11 \times 12 \times 13\) \(=\) \(\ds 17 \, 160\)
\(\ds \) \(=\) \(\ds 715 \times 24\)
\(\ds \) \(=\) \(\ds \binom {13} 4 \times 1 \times 2 \times 3 \times 4\)


Product of $4$ to $8$

\(\ds 4 \times 5 \times 6 \times 7 \times 8\) \(=\) \(\ds 6720\)
\(\ds \) \(=\) \(\ds 56 \times 120\)
\(\ds \) \(=\) \(\ds \binom 8 5 \times 1 \times 2 \times 3 \times 4 \times 5\)


Product of $11$ to $15$

\(\ds 11 \times 12 \times 13 \times 14 \times 15\) \(=\) \(\ds 360 \, 360\)
\(\ds \) \(=\) \(\ds 3003 \times 120\)
\(\ds \) \(=\) \(\ds \binom {15} 5 \times 11 \times 12 \times 13 \times 14 \times 15\)


Sources

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