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AMO-Bench 2025 Problem 2

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Let pp be a prime number and let aa and bb be non-negative integers. Determine all ordered triples (p,a,b)(p, a, b) for which the expression pa+pbp^a + p^b is a perfect power.

Dataset text AMO-Bench. Open licence.

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The solution set is the union of the following five sets: \begin{enumerate} \item {(2,n,n)∣n∈N+}\{(2, n, n) | n\in\mathbb{N}^{+}\} \item {(2,2n+3,2n)∣n∈N}\{(2, 2n+3, 2n) | n\in\mathbb{N}\} \item {(2,2n,2n+3)∣n∈N}\{(2, 2n, 2n+3) | n\in\mathbb{N}\} \item {(2q−1,kq+1,kq)∣q∈N+ such that 2q−1 is a prime (i.e., a “Mersenne prime”), and k∈N}\{(2^{q}-1, kq+1, kq) | q\in\mathbb{N}^{+} \text{ such that } 2^{q}-1 \text{ is a prime (i.e., a ``Mersenne prime''), and } k\in\mathbb{N}\} \item {(2q−1,kq,kq+1)∣q∈N+ such that 2q−1 is a prime (i.e., a “Mersenne prime”), and k∈N}\{(2^{q}-1, kq, kq+1) | q\in\mathbb{N}^{+} \text{ such that } 2^{q}-1 \text{ is a prime (i.e., a ``Mersenne prime''), and } k\in\mathbb{N}\} \end{enumerate} Here, N+\mathbb{N}^{+} denotes the set of positive integers, and N\mathbb{N} denotes the set of non-negative integers. Prime numbers of the form 2q−12^{q}-1 are also known as ``Mersenne primes''. (Note: For the cases where p=2q−1p=2^{q}-1, the sole requirement is that 2q−12^{q}-1 be a prime number; no further conditions are imposed on the exponent qq.)

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