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Number theory Difficulty 6.5 National olympiad Find the answer

How many ordered triples (a,b,c)(a, b, c) of positive integers are there such that none of a,b,ca, b, c exceeds 20102010 and each of a,b,ca, b, c divides a+b+ca + b + c?

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

Solution

To find the number of ordered triples (a,b,c)(a, b, c) of positive integers such that none of a,b,ca, b, c exceeds 20102010 and each of a,b,ca, b, c divides a+b+ca + b + c, we need to analyze the conditions given.

1. Condition Analysis:
- Each of a,b,ca, b, c divides a+b+ca + b + c.
- None of a,b,ca, b, c exceeds 20102010.

2. Rewriting the Problem:
Let abca \leq b \leq c. This implies a,b,ca+b+ca, b, c \mid a + b + c. We need to consider the possible values of a,b,ca, b, c under these constraints.

3. Case Analysis:
- Case 1: a+b=ca + b = c
- Since a,b,ca, b, c are positive integers and abca \leq b \leq c, we have a+b=ca + b = c.
- This implies ca+b+c=2cc \mid a + b + c = 2c, which is always true.
- Therefore, a+b=ca + b = c is a valid condition.

- Case 2: a+bca + b \neq c
- If a+bca + b \neq c, then a+b+ca + b + c must be divisible by a,b,ca, b, c.
- This implies ca+bc \mid a + b, ba+cb \mid a + c, and ab+ca \mid b + c.

4. Detailed Analysis:
- Subcase 1: a+b=ca + b = c
- For each aa and bb, c=a+bc = a + b.
- Since a,b,c2010a, b, c \leq 2010, we have a+b2010a + b \leq 2010.
- The number of such pairs (a,b)(a, b) is the number of pairs (a,b)(a, b) such that a+b2010a + b \leq 2010.

- Subcase 2: a+bca + b \neq c
- We need to find the number of triples (a,b,c)(a, b, c) such that a,b,ca+b+ca, b, c \mid a + b + c and a+bca + b \neq c.

5. Counting the Triples:
- Case 1: a+b=ca + b = c
- For each aa from 11 to 20102010, bb can range from aa to 2010a2010 - a.
- The number of such pairs is a=12010(2010a)\sum_{a=1}^{2010} (2010 - a).

- Case 2: a+bca + b \neq c
- This case is more complex and requires further analysis.

6. Summing Up:
- The total number of ordered triples (a,b,c)(a, b, c) is the sum of the valid triples from both cases.

The final answer is 9045\boxed{9045}

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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.