Maths Olympiad Prep

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Number theory Difficulty 5.7 AIME, harder Find the answer

Find all quadruples of positive integers (p,q,a,b)(p, q, a, b), where pp and qq are prime numbers and a>1a > 1, such that pa=1+5qb.p^a = 1 + 5q^b.

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

Solution

We are tasked with finding all quadruples of positive integers (p,q,a,b)(p, q, a, b), where pp and qq are prime numbers, a>1a > 1, and they satisfy the equation:

pa=1+5qb. p^a = 1 + 5q^b.

Given the integers involved, we will first check small values for pp, and test if they can satisfy the equation.

### Case 1: p=2p = 2

If p=2p = 2, the equation becomes:

2a=1+5qb. 2^a = 1 + 5q^b.

Rearranging gives:

2a1=5qb. 2^a - 1 = 5q^b.

Check small values of aa starting from a=2a = 2:

- **For a=2a = 2:**
221=35qb. 2^2 - 1 = 3 \neq 5q^b.

- **For a=3a = 3:**
231=75qb. 2^3 - 1 = 7 \neq 5q^b.

- **For a=4a = 4:**
241=15=531. 2^4 - 1 = 15 = 5 \cdot 3^1.

Here we find a solution: (p,q,a,b)=(2,3,4,1)(p, q, a, b) = (2, 3, 4, 1).

### Case 2: p=3p = 3

If p=3p = 3, the equation becomes:

3a=1+5qb. 3^a = 1 + 5q^b.

Rearranging gives:

3a1=5qb. 3^a - 1 = 5q^b.

Check small values of aa:

- **For a=2a = 2:**
321=85qb. 3^2 - 1 = 8 \neq 5q^b.

- **For a=3a = 3:**
331=265qb. 3^3 - 1 = 26 \neq 5q^b.

- **For a=4a = 4:**
341=80=524. 3^4 - 1 = 80 = 5 \cdot 2^4.

Here we find another solution: (p,q,a,b)=(3,2,4,4)(p, q, a, b) = (3, 2, 4, 4).

### Verification for Other Possible Primes

Beyond p=3p = 3, the exponential growth of pap^a becomes too large rapidly compared to the relatively small possibilities of 5qb+15q^b + 1, thus unlikely to satisfy the equation. Therefore, we focus on small primes and small values for aa.

### Conclusion

The solutions (p,q,a,b)(p, q, a, b) that satisfy the equation are:

(2,3,4,1) and (3,2,4,4). \boxed{(2, 3, 4, 1) \text{ and } (3, 2, 4, 4)}.

These are the only solutions that meet the conditions of the given problem.

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