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Number theory Difficulty 6.2 National olympiad Find the answer

Find all positive integers aa, bb, nn and prime numbers pp that satisfy

a2013+b2013=pn. a^{2013} + b^{2013} = p^n\text{.}

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

Solution

We are tasked with finding all positive integers a a , b b , n n , and prime numbers p p that satisfy:

a2013+b2013=pn. a^{2013} + b^{2013} = p^n.

Given the reference answer (a,b,p,n)=(2k,2k,2,2013k+1)(a, b, p, n) = (2^k, 2^k, 2, 2013k + 1), we can derive a solution as follows:

1. **Assume a=b a = b :**
If a=b a = b , the equation simplifies to:
a2013+a2013=2a2013. a^{2013} + a^{2013} = 2a^{2013}.
This should be equal to some power of a prime p p :
2a2013=pn. 2a^{2013} = p^n.

2. **Set a=2k a = 2^k :**
Assume a=2k a = 2^k . Then:
2(2k)2013=22013k+1. 2(2^k)^{2013} = 2^{2013k + 1}.
For this to be a power of a prime, p=2 p = 2 .

3. **Verification with p=2 p = 2 :**
The equation then becomes:
22013k+1=2n. 2^{2013k + 1} = 2^n.
Therefore, for equality, we must have:
n=2013k+1. n = 2013k + 1.

4. **Check that other configurations ab a \neq b or other p2 p \neq 2 do not work:**
- If ab a \neq b , it'd be hard to express the sum as a single power of a prime unless a a and b b both are specific powers of 2 and equal.
- Attempting to use a different prime p2 p \neq 2 would not satisfy integer constraints on powers since we already simplified with the assumption that a=b=2k a = b = 2^k .

5. Conclude and box the solution:
Thus, parameterization in terms of k k provides the only solution:
(a,b,p,n)=(2k,2k,2,2013k+1) \boxed{(a, b, p, n) = (2^k, 2^k, 2, 2013k + 1)}

This solution encompasses all positive integer solutions for the given equation, as validated by the reference answer.

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