Maths Olympiad Prep

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Number theory Difficulty 6.6 National olympiad Find the answer

Determine all composite positive integers nn with the following property: If 1=d1<d2<<dk=n1 = d_1 < d_2 < \cdots < d_k = n are all the positive divisors of nn, then

(d2d1):(d3d2)::(dkdk1)=1:2::(k1)(d_2 - d_1) : (d_3 - d_2) : \cdots : (d_k - d_{k-1}) = 1:2: \cdots :(k-1)

(Walther Janous)

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

Solution

To determine all composite positive integers n n with the given property, we need to analyze the sequence of divisors and their differences. Let's break down the problem step by step.

1. Identify the divisors and their differences:
Let 1=d1<d2<<dk=n 1 = d_1 < d_2 < \cdots < d_k = n be all the positive divisors of n n . The given property states that:
(d2d1):(d3d2)::(dkdk1)=1:2::(k1) (d_2 - d_1) : (d_3 - d_2) : \cdots : (d_k - d_{k-1}) = 1 : 2 : \cdots : (k-1)
This means:
d2d1=1,d3d2=2,,dkdk1=k1 d_2 - d_1 = 1, \quad d_3 - d_2 = 2, \quad \ldots, \quad d_k - d_{k-1} = k-1

2. **Express the divisors in terms of k k :**
Since d1=1 d_1 = 1 and d2d1=1 d_2 - d_1 = 1 , we have d2=2 d_2 = 2 . Similarly, d3d2=2 d_3 - d_2 = 2 implies d3=4 d_3 = 4 , and so on. We can generalize this pattern:
di=di1+(i1) d_i = d_{i-1} + (i-1)
For example:
d3=d2+2=2+2=4 d_3 = d_2 + 2 = 2 + 2 = 4
d4=d3+3=4+3=7 d_4 = d_3 + 3 = 4 + 3 = 7
However, this pattern does not hold for all i i . Instead, we need to find a consistent pattern that fits the given ratio.

3. **Analyze the pattern for small values of k k :**
Let's consider small values of k k to find a consistent pattern:
- For k=3 k = 3 :
d1=1,d2=2,d3=4 d_1 = 1, \quad d_2 = 2, \quad d_3 = 4
The differences are:
d2d1=1,d3d2=2 d_2 - d_1 = 1, \quad d_3 - d_2 = 2
This matches the ratio 1:2 1 : 2 .

4. **Verify if n=4 n = 4 is a solution:**
For n=4 n = 4 , the divisors are 1,2,4 1, 2, 4 . The differences are:
21=1,42=2 2 - 1 = 1, \quad 4 - 2 = 2
This matches the ratio 1:2 1 : 2 .

5. **Check for larger values of k k :**
If k>3 k > 3 , the pattern di=i d_i = i for each integer i i does not hold because the differences d3d2 d_3 - d_2 would not be equal to 1. Therefore, k k cannot be greater than 3.

6. Conclusion:
The only composite positive integer n n that satisfies the given property is n=4 n = 4 .

The final answer is n=4 \boxed{ n = 4 } .

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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.