Show that there are no 2-tuples (x,y) of positive integers satisfying the equation (x+1)(x+2)⋯(x+2014)=(y+1)(y+2)⋯(y+4028).
Solution
Proof For n=2k⋅m (k is a non-negative integer, m is odd), let v(n)=2k. We prove by contradiction: assume (x,y) is one positive integer solution of the equation. Let v(x+i)=1≤j≤2014max{v(x+j)}, then if 1≤j≤2014,j=i, v(x+j)=v(x+i+(j−i))=v(j−i), so v1≤j≤2014,j=i∏(x+j)=v((2014−j)!⋅(j−1)!)≤v(2013!) since ∏j=12014(x+j)=∏j=14028(y+j) is a multiple of 4028!, thus x+i≥v(x+i)≥v(2013!4028!)>21007, therefore x>21006. So (y+4028)4028>∏j=14028(y+j)=∏j=12014(x+j)>21006⋅2014, we have y+4028>2503, y>2502.
Lemma: Let 0≤xi<21(1≤i≤n), if x=n1∑i=1nxi, y=2max1≤i≤n{xi2}, then 1−x≥(i=1∏n(1−xi))n1≥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=1∏n(1−xi))n1≥∑i=1n1−xi1n≥∑i=1n(1+xi+2xi2)n=n+nx+2∑i=1nxi2n≥1+x+y1≥1−x−y, the lemma is proved.
Back to the problem, Let w=x+2015>21007, z=y+24029>2502, then the equation is equivalent to w⋅((1−w1)(1−w2)⋯(1−w2014))20141=z2⋅((1−4z21)(1−4z29)⋯(1−4z240272))20141. By lemma, w(1−2w2015)>w⋅((1−w1)(1−w2)⋯(1−w2014))20141 \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 \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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