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Number theory Difficulty 8.7 Shortlist Find the answer

For a finite set AA of positive integers, a partition of AA into two disjoint nonempty subsets A1A_1 and A2A_2 is good\textit{good} if the least common multiple of the elements in A1A_1 is equal to the greatest common divisor of the elements in A2A_2. Determine the minimum value of nn such that there exists a set of nn positive integers with exactly 20152015 good partitions.

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

Solution

Given a finite set A A of positive integers, we need to determine the minimum value of n n such that there exists a set A A with exactly 2015 good partitions. A partition of A A into two disjoint nonempty subsets A1 A_1 and A2 A_2 is termed as \textit{good} if:

lcm(A1)=gcd(A2). \text{lcm}(A_1) = \gcd(A_2).

To find the minimum n n , we shall analyze and derive the connection between the number of elements and the number of good partitions.

### Strategy
Consider A={a1,a2,,an} A = \{ a_1, a_2, \ldots, a_n \} . According to the definition of a good partition:

1. Least Common Multiple (LCM) and Greatest Common Divisor (GCD):
- lcm(A1) \text{lcm}(A_1) should equal gcd(A2) \gcd(A_2) .
- This implies that for a chosen subset A1 A_1 , there needs to be a complementary A2 A_2 such that their respective LCM and GCD equality holds.

2. Exploring Possible Configurations:
- Let us explore the structural properties required for the existence of exactly 2015 good partitions using different number sets.
- Specifically, if A A consists of powers of a particular integer or well-known small integers, we can derive conditions under which the LCM equals the GCD.

3. Utilize Mathematical Properties:
- Since LCM and GCD have known mathematical relationships, we shall employ them to construct the set A A efficiently.

Given that 2015 2015 factors as 2015=5×13×31 2015 = 5 \times 13 \times 31 , we need a configuration that supports exactly 2015 ways to achieve lcm(A1)=gcd(A2) \text{lcm}(A_1) = \gcd(A_2) .

### Construction of the Set

A known viable construction involves using a set of integers forming a highly structured presentation of LCM and GCD calculations:

Example construction employs:
- Selecting large enough n n such that the number of combinatorial partitions yields exactly 2015 solutions for the equality criterion.
- Leverage mathematical properties by careful choice of numbers like highly composite numbers or structured factor arrangements.
- Apply the relations and assess when count reaches the target threshold of 2015.

### Result

By systematically following through this approach and trying constructions suited by factors of interest:

n=3024 n = 3024

is the minimal number satisfying the exact number of good partitions condition.

Thus, the minimum value of n n is:
3024 \boxed{3024}

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