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Number theory Difficulty 8.4 Shortlist Prove it China

Show that there are no 2-tuples (x,y)(x, y) of positive integers satisfying the equation
(x+1)(x+2)(x+2014)=(y+1)(y+2)(y+4028). (x+1)(x+2)\cdots(x+2014) = (y+1)(y+2)\cdots(y+4028).

Solution

Proof For n=2kmn = 2^k \cdot m (kk is a non-negative integer, mm is odd), let v(n)=2kv(n) = 2^k.
We prove by contradiction: assume (x,y)(x, y) is one positive integer solution of the equation. Let
v(x+i)=max1j2014{v(x+j)}, v(x + i) = \max_{1 \le j \le 2014} \{v(x + j)\},
then if 1j2014, ji1 \le j \le 2014,\ j \ne i,
v(x+j)=v(x+i+(ji))=v(ji), v(x + j) = v(x + i + (j - i)) = v(j - i),
so
v(1j2014, ji(x+j))=v((2014j)!(j1)!)v(2013!) v\left(\prod_{1 \le j \le 2014,\ j \ne i} (x + j)\right) = v\left((2014 - j)! \cdot (j - 1)!\right) \le v(2013!)
since j=12014(x+j)=j=14028(y+j)\prod_{j=1}^{2014} (x + j) = \prod_{j=1}^{4028} (y + j) is a multiple of 4028!4028!, thus
x+iv(x+i)v(4028!2013!)>21007, x + i \geq v(x + i) \geq v\left(\frac{4028!}{2013!}\right) > 2^{1007},
therefore x>21006x > 2^{1006}. So (y+4028)4028>j=14028(y+j)=j=12014(x+j)>210062014(y + 4028)^{4028} > \prod_{j=1}^{4028} (y + j) = \prod_{j=1}^{2014} (x + j) > 2^{1006 \cdot 2014}, we have y+4028>2503y + 4028 > 2^{503}, y>2502y > 2^{502}.

Lemma: Let 0xi<12 (1in)0 \le x_i < \frac{1}{2}\ (1 \le i \le n), if x=1ni=1nxix = \frac{1}{n} \sum_{i=1}^{n} x_i, y=2max1in{xi2}y = 2 \max_{1 \le i \le n} \{x_i^2\}, then
1x(i=1n(1xi))1n1xy. 1 - x \ge \left( \prod_{i=1}^{n} (1 - x_i) \right)^{\frac{1}{n}} \ge 1 - x - y.
Proof of lemma: By AM-GM inequality, the inequality on the left is easy to prove. The inequality on the right holds, since
(i=1n(1xi))1nni=1n11xini=1n(1+xi+2xi2)=nn+nx+2i=1nxi211+x+y1xy, \begin{aligned} \left( \prod_{i=1}^{n} (1-x_i) \right)^{\frac{1}{n}} &\ge \frac{n}{\sum_{i=1}^{n} \frac{1}{1-x_i}} \\ &\ge \frac{n}{\sum_{i=1}^{n} (1+x_i + 2x_i^2)} \\ &= \frac{n}{n+nx+2\sum_{i=1}^{n} x_i^2} \\ &\ge \frac{1}{1+x+y} \\ &\ge 1-x-y, \end{aligned}
the lemma is proved.

Back to the problem, Let w=x+2015>21007w = x + 2015 > 2^{1007}, z=y+40292>2502z = y + \frac{4029}{2} > 2^{502}, then the equation is equivalent to
w((11w)(12w)(12014w))12014=z2((114z2)(194z2)(1402724z2))12014. w \cdot \left( \left(1 - \frac{1}{w}\right) \left(1 - \frac{2}{w}\right) \cdots \left(1 - \frac{2014}{w}\right) \right)^{\frac{1}{2014}} = z^2 \cdot \left( \left(1 - \frac{1}{4z^2}\right) \left(1 - \frac{9}{4z^2}\right) \cdots \left(1 - \frac{4027^2}{4z^2}\right) \right)^{\frac{1}{2014}}.
By lemma,
w(120152w)>w((11w)(12w)(12014w))12014 w\left(1 - \frac{2015}{2w}\right) > w \cdot \left(\left(1 - \frac{1}{w}\right)\left(1 - \frac{2}{w}\right)\cdots\left(1 - \frac{2014}{w}\right)\right)^{\frac{1}{2014}}
\begin{aligned}
&> w \left( 1 - \frac{2015}{2w} - \frac{2 \cdot 2014^2}{w^2} \right) \\
&> w \left( 1 - \frac{2015}{2w} \right) - \frac{1}{8},
\end{aligned}
therefore the decimal of $w \cdot \left( \left( 1 - \frac{1}{w} \right) \left( 1 - \frac{2}{w} \right) \cdots \left( 1 - \frac{2014}{w} \right) \right)^{\frac{1}{2014}}$ belongs to $\left( \frac{3}{8}, \frac{1}{2} \right)$. On the other hand, by lemma
\begin{aligned}
z^2 \left( 1 - \frac{1^2 + 3^2 + \cdots + 4027^2}{4z^2 \cdot 2014} \right) &> z^2 \cdot \left( \left( 1 - \frac{1}{4z^2} \right) \left( 1 - \frac{9}{4z^2} \right) \cdots \left( 1 - \frac{4027^2}{4z^2} \right) \right)^{\frac{1}{2014}} \\
&> z^2 \left( 1 - \frac{1^2 + 3^2 + \cdots + 4027^2}{4z^2 \cdot 2014} - \frac{2 \cdot 4027^4}{(4z^2)^2} \right),
\end{aligned}
thus thus
\begin{aligned}
z^2 - \frac{4 \cdot 2014^2 - 1}{12} &> z^2 \cdot \left( \left( 1 - \frac{1}{4z^2} \right) \left( 1 - \frac{9}{4z^2} \right) \cdots \left( 1 - \frac{4027^2}{4z^2} \right) \right)^{\frac{1}{2014}} \\
&> z^2 - \frac{4 \cdot 2014^2 - 1}{12} - \frac{4027^4}{8z^2} \\
&> z^2 - \frac{4 \cdot 2014^2 - 1}{12} - \frac{1}{8}.
\end{aligned}
Since $z^2 - \frac{4 \cdot 2014^2 - 1}{12}$ is an integer, so the decimal of $z^2 \cdot \left( \left( 1 - \frac{1}{4z^2} \right) \left( 1 - \frac{9}{4z^2} \right) \cdots \left( 1 - \frac{4027^2}{4z^2} \right) \right)^{\frac{1}{2014}}$ belongs to $\left( \frac{7}{8}, 1 \right)$, this is a contradiction. Therefore, there are no 2-tuples $(x, y)$ of positive integers satisfying
\prod_{j=1}^{2014} (x + j) = \prod_{j=1}^{4028} (y + j).

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