# Divisibility factorial

Prove that $\cfrac { (3n)! }{ { (3!) }^{ n } }$ is integer for all $n\ge 0$.

This is the solution I tried

${(3!)}^{ n }={(6) }^{ n }$.We have to prove ${(6) }^{ n }|(3n)!$.We have product of 3 consecutive numbers is divisible by $6$.Now there are $n$ pairs of $3$ consecutive numbers.Therefore ${ 6 }^{ n }y=(3n)!$ for some $y$..Therefore $\cfrac { (3n)! }{ { (3!) }^{ n } }$ is integer for all $n\ge 0$.

Is it correct?

Note by Shivamani Patil
5 years, 1 month ago

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I believe I have a brilliantly simple solution to your question - I will use induction. Let f(n) = $\frac{(3n)!}{(3!)^{n} }$. Base test: for n=1, f(1) = 1 and f(2) = $\frac{6!}{3! * 3!}$ = $\frac{4 * 5 * 6}{6}$ = 20. Now for the inductive step: $\frac{(3n+3)!}{(3!)^{n} * 3!}$, and by breaking up the fraction we see that f(n+1) is a product of $\frac{(3n)!}{(3!)^{n} }$ and $\frac{(3n+1)(3n+2)(3n+3)}{3!}$. We already know that the former is already an integer by our inductive hypothesis, so we need to prove that the latter is also an integer. This is simple as 3 or more consecutive integers are divisible by 3 and 2 (In fact, n consecutive integers are always divisible by n!). Hence, our proof is complete. I hope that helps :D

- 5 years, 1 month ago

Note that $\dfrac{(3n)!}{(3!)^n}$ represents the number of ways to arrange $3n$ objects with $n$ triplets of object being identical, in a row. For instance, at $n = 2$ , you are actually arranging 6 objects with 2 triplets, such as arranging AAABBB in a row, yielding $\dfrac{6!}{3!3!} = \dfrac{(2(3))!}{(3!)^2}$

Note that the number of objects = number of objects limited by the requirement of the permutation. (That is from the previous example, if I have $6$ objects , and there must be $2$ sets of $3$ objects, it is physically possible.) Hence, it is actually possible to arrange $3n$ objects with $n$ triplets of object being identical in a row, thus causing the number of ways to be an integer. Henceforth, $\dfrac{(3n)!}{(3!)^n}$ that represents the number of ways to arrange $3n$ objects with $n$ triplets of object being identical, in a row, will thus be an integer.

P.S : If however, the number of objects $<$ number of objects limited by the requirement of the permutation, the scenario will not physically happens. Hence, the expression that evaluates the number of ways might not be an integer. However, since we are not interested in it, we can hence ignore it.

- 5 years, 1 month ago

(3!)^n=3^n×2^n.now 3n> 3 (n-1)> 3 (n-2)......> 3 so 3n×3 (n-1)×3 (n-2).........×3=3^n×n! contains in (3n)!.again 3n> 2n> 2 (n-1).......> 2 so 2n×2 (n-1)......×2=2^n×n! contains in (3n)!.so (3n)! is is divisible by (3!)^n

- 5 years, 1 month ago

Try formatting your maths so it's easier to read - I know it's tricky so I'll give you a few pointers. For (3)!^n place curly brackets around the n (i.e. {n} ), then place normal brackets around the whole expression, before putting a '\' before you 1st and last bracket - $...$. This should give $(3!)^{n}$. Also, (3n)! = 3n(3n-1)(3n-2)...2*1 as it is inside the brackets, rather than outside.

- 5 years, 1 month ago

The given expression can be written as $\dfrac{(3n)!}{6^{n}}$ which can be written as $\dfrac{(3n)!}{3^{n}\times2^{n}}.$Now,we have that the maximum power of $3$ that divides $(3n!)$$=\lfloor{\dfrac{3n}{3}}\rfloor+\lfloor{\dfrac{3n}{9}}\rfloor+...$Similarly the maximum power of $2$ that divides $3n!$=$\lfloor{\dfrac{3n}{2}}\rfloor+\lfloor{\dfrac{3n}{4}}\rfloor+.....$Now,for$n\geq3$ the expressions above are $>3^{n}$ and $2^{n}$ respectively.Thu for all$n\geq3$ the given expression is an integer.Checking cases for $n=0,1,2$ reveals that the given expression is an integer for all three.Hence proved.

- 5 years, 1 month ago

There are $n(3n-1)!$ pairs of 3 consecutive numbers.

How did get this? Since there are $3n$ numbers being multiplied, there should be $\frac{3n}{3} = n$ pairs of consecutive number. For example, $6! =(6*5*4)(3*2*1)$ can be partitioned into 2 pairs.

- 5 years, 1 month ago

But we have to take $(3n)!$ numbers in pairs of 3 .Therefore dividing by 3 we get $n(3n-1)!$ pairs.

- 5 years, 1 month ago

$(3n)!$ is the product of $3n$ numbers,i.e $1*2*...*(3n-1)*(3n)$. Not $(3n)!$ numbers. Also, you can check smaller cases. The highest power of $6$ dividing $(3*2)!$ is $6^2$, not $6^{2(5!)} = 6^{240}$.

- 5 years, 1 month ago

Ohh yes I missed it in hurry .I have updated solution now is it correct?

- 5 years, 1 month ago

Yes, now it is, though you could also mention the fact that the product of 3 consecutive integers is also divisible by 6.

- 5 years, 1 month ago

I have mentioned that na.

- 5 years, 1 month ago

- 5 years, 1 month ago

That's ok. u study in which class?

- 5 years, 1 month ago

11th.

- 5 years, 1 month ago

Ohhh

- 5 years, 1 month ago

Using some combinatorics:

(3n)! / (3!)^n = (3n)! / (3!)(3!) ... (3!)(3!) n factors of (3!). One can think of this as the number of identical permutations with 3n letters and n different letters with 3 as frequency in the letter. It remains to prove that the number of identical permutations is an integer.

- 5 years, 1 month ago

Your solution is great.But can you give elementary number theory proof.

- 5 years, 1 month ago