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I found this problem in a book, I can't solve it unfortunately. Prove that for all integer values $n$, $n^9 - 6n^7 + 9n^5 - 4n^3$ is divisible by $8640.$ So far I've noticed that $8460 = 6! \times 12$, also I've tried to simplify that expression and I've found that it's equal to this $n^3(n^3-3n-2)(n^3-3n+2)$, but I can't move on after that.

wythagoras
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Kareem
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4 Answers4

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$$ \begin{align} &n^9-6n^7+9n^5-4n^3\\ &\small=362880\binom{n}{9}+1451520\binom{n}{8}+2298240\binom{n}{7}+1814400\binom{n}{6}\\ &\small+734400\binom{n}{5}+138240\binom{n}{4}+8640\binom{n}{3}\\ &=\small8640\left[42\binom{n}{9}+168\binom{n}{8}+266\binom{n}{7}+210\binom{n}{6}+85\binom{n}{5}+16\binom{n}{4}+\binom{n}{3}\right] \end{align} $$

robjohn
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    That is an interesting technique. Is there a general reason to use ${n \choose k}$ rather than any other polynomial? – DanielV Feb 26 '16 at 11:07
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    Could you explain where the first equality comes from? Repeated differences? – lhf Feb 26 '16 at 11:23
  • Your answer is very interesting, How did you come up with it? I would never think of something like that before I've seen your answer. – Kareem Feb 26 '16 at 12:51
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    @Kareem: every polynomial that sends $\mathbb{Z}$ to $\mathbb{Z}$ can be written as an integral combination of binomial coefficients. It is very easy to compute a degree $n$ polynomial by evaluating at $0,1,2,\dots n$. Thus, $\frac{n^9-6n^7+9n^5-4n^3}{8640}$ must be in this form. – robjohn Feb 26 '16 at 14:30
  • @DanielV: These polynomials have particularly nice properties. See this answer. It is also very easy to compute the coefficients given the values of the polynomial at $0\dots9$. – robjohn Feb 26 '16 at 15:31
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    @lhf: we can compute $a_0$, the coefficient of $\binom{n}{0}$, by $$a_0=P(0)$$ then $$a_1=P(1)-a_0\binom{1}{0}$$ and $$a_2=P(2)-a_0\binom{2}{0}-a_1\binom{2}{1}$$ and $$a_3=P(3)-a_0\binom{3}{0}-a_1\binom{3}{1}-a_2\binom{3}{2}$$ etc. – robjohn Feb 26 '16 at 15:37
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    @robjohn Fantastic, thank you for referring me to that explanation. – DanielV Feb 26 '16 at 15:44
  • @lhf, these same coefficients $a_k$ arise from the leftmost column of the difference table. – RobPratt Mar 14 '20 at 20:44
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We can further note that $-1$ is a root of $n^3-3n-2$, and that $1$ is a root of $n^3-3n+2$.

You can find these roots by the Rational Root theorem:

Rational root theorem. All rational roots have the form $\frac{p}{q}$,with $p$ a divisior of the constant term and $q$ a divisior of the first coefficient.

Other techniques can also be found in this question and its answers.

Then we simplify it to \begin{align*} n^3(n+1)(n^2-n-2)(n-1)(n^2+n-2) &= n^3(n+1)(n-2)(n+1)(n-1)(n+2)(n-1)\\ &=n^3(n+1)^2(n-1)^2(n-2)(n+2) \end{align*}

There are 6 factors two in 8640.

  • If $n$ is even, $n, n-2$ and $n+2$ are even, and furthermore at least one of them is divisible by $4$, which gives 6 factors 2.
  • If $n$ is odd, $n-1$ and $n+1$ are even, and furthermore at least one of them is divisible by $4$, which gives 6 factors 2.

There are 3 factors three in 8640.

  • If $n \equiv 0 \mod 3$, then $n$ is divisible by 3, and then $27 \mid n^3$.
  • If $n \equiv 1 \mod 3$, then $n-1$ and $n+2$ are divisible by 3.
  • If $n \equiv 2 \mod 3$, then $n+1$ and $n-2$ are divisible by 3.

The factor 5 can be taken care of since the product is a multiple of five consecutive integers, namely a product of $(n-2)(n-1)n(n+1)(n+2)$.

Community
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wythagoras
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If you do complete factorisation you get $$n^9−6n^7+9n^5−4n^3=n^3(n-2)(n-1)^2(n+1)^2(n+2)$$ and $$8640=2^6\cdot 3^3\cdot 5$$ Now just look at different cases. For $2\mid n$ we have $2^3\mid n^3$, $2\mid n-2$, $2\mid n+2$ and either $n-2$ or $n+2$ has the factor $2^2$, so we get $2^6\mid n^9−6n^7+9n^5−4n^3$. Do it for the case $n$ odd and for the other factors in a similar style.

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Hints:

  • Note for example $n^3-3n+2 = (n-1)(n^2-n-2) = (n-1)(n+1)(n-2)$. You can factorise completely to a product of terms like $(n-1)$ and $(n+2)$. Do that
  • How many powers of $5$ must the product have?
  • How many powers of $3$? How many powers of $2$? Note for example that one of two consecutive even numbers is a multiple of $4$
Henry
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