An odd coincidence?

Algebra Level 1

1 1 is divisible by 1 1 .
1 × 2 × 3 1\times2\times3 is divisible by 1 + 2 + 3 1+2+3 .
1 × 2 × 3 × 4 × 5 1\times2\times3\times4\times5 is divisible by 1 + 2 + 3 + 4 + 5 1+2+3+4+5 .
1 × 2 × 3 × 4 × 5 × 6 × 7 1\times2\times3\times4\times5\times6\times7 is divisible by 1 + 2 + 3 + 4 + 5 + 6 + 7 1+2+3+4+5+6+7 .

In general, is it true that 1 × 2 × × n 1\times2\times\cdots\times n is divisible by 1 + 2 + + n 1+2+\cdots+n for all odd n ? n?

Yes No

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3 solutions

Naren Bhandari
Feb 25, 2018

Let us denote the odd integer n n as O O . P P and S S as product and sum of n n respectively and we have ( P / S ) n O = 1 × 2 × 3 × n 1 + 2 + 3 + + n = n ! n ( n + 1 ) 2 ( P / S ) n O = 2 n ! n ( n + 1 ) = 2 ( n 1 ) ! n + 1 \begin{aligned} & (P/S)_{n\in\mathbb O}=\dfrac{1\times 2\times3 \times \cdots n}{1+2+ 3+\cdots+ n } = \dfrac{n!}{\frac{n(n+1)}{2}} \\& (P/S)_{n\in\mathbb O}=\dfrac{2n!}{n(n+1)} = \frac{2(n-1)!}{n+1} \end{aligned} We can cleary note that ( n 1 ) (n-1) and ( n + 1 ) (n+1) are even integers since n n is an odd integer that implies that ( n 1 ) ! = ( 2 m 1 ) ! (n-1)! = (2m_1)! and n + 1 = 2 m 2 n+1 = 2m_2 where ( m 1 , m 2 ) N (m_1,m_2)\in\mathbb N also m 2 m 1 = 2 m_2-m_1 =2 ( P / S ) n O = 2 ( n 1 ) ! n + 1 = 2 ( 2 m 1 ) ! 2 m 2 ( P / S ) n O = ( 2 m 1 ) ! m 2 = ( 2 ( m 2 2 ) ) ! m 2 \begin{aligned}& (P/S)_{n\in\mathbb O} = \dfrac{2(n-1)!}{n+1} = \frac{2(2m_1)!}{2m_2}\\& (P/S)_{n\in\mathbb O} =\dfrac{(2m_1)!}{m_2}= \dfrac{(2(m_2-2))!}{m_2}\end{aligned} which is always divisible. Hence the answer is Yes .

You have a typo on the 2nd line

Piero Sarti - 3 years, 3 months ago
Piero Sarti
Feb 25, 2018

We know that S ( n ) = k = 1 n k = n ( n + 1 ) 2 S(n) = \displaystyle\sum^{n}_{k = 1}k = \dfrac{n(n+1)}{2} .

If we now divide n ! n! by S ( n ) S(n) as such, n ! S ( n ) = 2 n ! n ( n + 1 ) = 2 ( n 1 ) ! n + 1 \dfrac{n!}{S(n)} = \dfrac{2n!}{n(n+1)} = \dfrac{2(n - 1)!}{n+1} . Note that n n is an odd integer and therefore n + 1 n + 1 is even. Because of this, we can express n + 1 n + 1 as 2 x 2x where x Z + x \in \mathbb{Z}^+ and therefore we can simplify our fraction to ( n 1 ) ! x \dfrac{(n - 1)!}{x} and since x < ( n 1 ) x < (n - 1) it is contained within ( n 1 ) ! (n - 1)! and thus will divide into it. Therefore the answer is Yes \boxed{\text{Yes}} .

Suresh Jh
Mar 3, 2018

Much easier solution that others.

The product will be equal to n! , then sum 1+2+..+n = n(n+1)/2. As n is odd (n+1) will be even, and when divided by 2 it, number comes less than 'n' and let it be 'x'.

Now n!/(n \times x), it will be always divisible.

Easy solution, especially for me

Aryan Sanghi - 3 years, 3 months ago

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Thanks....

suresh jh - 3 years, 3 months ago

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