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Number theory Difficulty 6.6 National olympiad Find the answer

Determine all even numbers nn, nNn \in \mathbb N such that 1d1+1d2++1dk=16201003,{ \frac{1}{d_{1}}+\frac{1}{d_{2}}+ \cdots +\frac{1}{d_{k}}=\frac{1620}{1003}},
where d1,d2,,dkd_1, d_2, \ldots, d_k are all different divisors of nn.

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

Solution

1. We start with the given equation:
1d1+1d2++1dk=16201003 \frac{1}{d_{1}} + \frac{1}{d_{2}} + \cdots + \frac{1}{d_{k}} = \frac{1620}{1003}
where d1,d2,,dkd_1, d_2, \ldots, d_k are all different divisors of nn.

2. Notice that 1003=17×591003 = 17 \times 59. This implies that for the sum of the reciprocals of the divisors to have a denominator of 10031003, nn must be divisible by both 1717 and 5959. Since nn is even, nn must also be divisible by 22. Therefore, nn must be divisible by 2×17×59=20062 \times 17 \times 59 = 2006.

3. Next, we need to verify that the sum of the reciprocals of all divisors of 20062006 equals 16201003\frac{1620}{1003}. The divisors of 20062006 are 1,2,17,34,59,118,1003,20061, 2, 17, 34, 59, 118, 1003, 2006.

4. We calculate the sum of the reciprocals of these divisors:
d20061d=11+12+117+134+159+1118+11003+12006 \sum_{d|2006} \frac{1}{d} = \frac{1}{1} + \frac{1}{2} + \frac{1}{17} + \frac{1}{34} + \frac{1}{59} + \frac{1}{118} + \frac{1}{1003} + \frac{1}{2006}

5. To find a common denominator, we use 2006=2×17×592006 = 2 \times 17 \times 59:
11=20062006,12=10032006,117=1182006,134=592006 \frac{1}{1} = \frac{2006}{2006}, \quad \frac{1}{2} = \frac{1003}{2006}, \quad \frac{1}{17} = \frac{118}{2006}, \quad \frac{1}{34} = \frac{59}{2006}
159=342006,1118=172006,11003=22006,12006=12006 \frac{1}{59} = \frac{34}{2006}, \quad \frac{1}{118} = \frac{17}{2006}, \quad \frac{1}{1003} = \frac{2}{2006}, \quad \frac{1}{2006} = \frac{1}{2006}

6. Summing these fractions:
2006+1003+118+59+34+17+2+12006=32402006 \frac{2006 + 1003 + 118 + 59 + 34 + 17 + 2 + 1}{2006} = \frac{3240}{2006}

7. Simplifying 32402006\frac{3240}{2006}:
32402006=3240÷22006÷2=16201003 \frac{3240}{2006} = \frac{3240 \div 2}{2006 \div 2} = \frac{1620}{1003}

8. Therefore, the sum of the reciprocals of the divisors of 20062006 is indeed 16201003\frac{1620}{1003}.

9. Since nn must be a multiple of 20062006 to satisfy the given equation, all solutions for nn are multiples of 20062006.

The final answer is n=2006k \boxed{ n = 2006k } for kNk \in \mathbb{N}.

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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.