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Number theory Difficulty 6.4 National olympiad Find the answer

Find all integer sequences of the form xi,1i1997 x_i, 1 \le i \le 1997, that satisfy k\equal119972k\minus1xk1997\equal1996k\equal11997xk \sum_{k\equal{}1}^{1997} 2^{k\minus{}1} x_{k}^{1997}\equal{}1996\prod_{k\equal{}1}^{1997}x_k.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Understanding the problem:
We need to find all integer sequences xi x_i for 1i1997 1 \le i \le 1997 that satisfy the equation:
k=119972k1xk1997=1996k=11997xk \sum_{k=1}^{1997} 2^{k-1} x_k^{1997} = 1996 \prod_{k=1}^{1997} x_k

2. Using the hint:
The hint suggests that if (x1,x2,,x1997)(x_1, x_2, \ldots, x_{1997}) is a solution, then (x12,x22,,x19972)\left(\frac{x_1}{2}, \frac{x_2}{2}, \ldots, \frac{x_{1997}}{2}\right) is also a solution. This implies that the sequence can be scaled by a factor of 12\frac{1}{2} and still satisfy the equation.

3. Exploring the implications of the hint:
Let's assume that (x1,x2,,x1997)(x_1, x_2, \ldots, x_{1997}) is a solution. Then, substituting (x12,x22,,x19972)\left(\frac{x_1}{2}, \frac{x_2}{2}, \ldots, \frac{x_{1997}}{2}\right) into the equation, we get:
k=119972k1(xk2)1997=1996k=11997(xk2) \sum_{k=1}^{1997} 2^{k-1} \left(\frac{x_k}{2}\right)^{1997} = 1996 \prod_{k=1}^{1997} \left(\frac{x_k}{2}\right)
Simplifying the left-hand side:
k=119972k1(xk2)1997=k=119972k1xk199721997=k=119972k1xk199721997 \sum_{k=1}^{1997} 2^{k-1} \left(\frac{x_k}{2}\right)^{1997} = \sum_{k=1}^{1997} 2^{k-1} \frac{x_k^{1997}}{2^{1997}} = \sum_{k=1}^{1997} \frac{2^{k-1} x_k^{1997}}{2^{1997}}
=121997k=119972k1xk1997 = \frac{1}{2^{1997}} \sum_{k=1}^{1997} 2^{k-1} x_k^{1997}
Simplifying the right-hand side:
1996k=11997(xk2)=1996k=11997xk21997 1996 \prod_{k=1}^{1997} \left(\frac{x_k}{2}\right) = 1996 \frac{\prod_{k=1}^{1997} x_k}{2^{1997}}

4. Equating both sides:
121997k=119972k1xk1997=1996k=11997xk21997 \frac{1}{2^{1997}} \sum_{k=1}^{1997} 2^{k-1} x_k^{1997} = 1996 \frac{\prod_{k=1}^{1997} x_k}{2^{1997}}
Multiplying both sides by 219972^{1997}:
k=119972k1xk1997=1996k=11997xk \sum_{k=1}^{1997} 2^{k-1} x_k^{1997} = 1996 \prod_{k=1}^{1997} x_k
This is the original equation, confirming that the hint is valid.

5. Considering integer solutions:
Since the sequence can be scaled by 12\frac{1}{2} and still be a solution, and we are looking for integer solutions, the only possibility is that all xix_i must be zero. This is because any non-zero integer scaled by 12\frac{1}{2} would not remain an integer.

6. Verifying the solution:
If xi=0x_i = 0 for all ii, then:
k=119972k1xk1997=k=119972k10=0 \sum_{k=1}^{1997} 2^{k-1} x_k^{1997} = \sum_{k=1}^{1997} 2^{k-1} \cdot 0 = 0
1996k=11997xk=19960=0 1996 \prod_{k=1}^{1997} x_k = 1996 \cdot 0 = 0
Both sides of the equation are equal, confirming that xi=0x_i = 0 for all ii is indeed a solution.

The final answer is xi=0 \boxed{ x_i = 0 } for all ii.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.