Olympiad Maths Prep

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Number theory Difficulty 5.4 AIME, harder Prove it Greece

Determine all pairs (x,y)(x, y) of positive integers satisfying the equation
yxy=y+1 yx^y = y + 1

Solution

Let y=pqy = \frac{p}{q}, with p,qNp, q \in \mathbb{N}^*, (p,q)=1(p, q) = 1. Then the equation is written in the form:
xq=y+1y=pq+1pq=p+qpx=(p+q)qpqq. x^q = \frac{y+1}{y} = \frac{\frac{p}{q}+1}{\frac{p}{q}} = \frac{p+q}{p} \Rightarrow x = \sqrt[q]{\frac{(p+q)^q}{p^q}}.
Therefore the right hand side must be rational. Since (p+q,p)=1(p+q, p)=1, it follows that the numerator and the denominator of the fraction must be powers of pp. Hence
p+q=apandp=bp, a,bN, a>1. p+q = a^p \quad \text{and} \quad p = b^p,\ a, b \in \mathbb{N}^*,\ a > 1.
If b2b \ge 2, then bp2p>pb^p \ge 2^p > p (absurd), and hence b=1b=1, which implies p=1p=1 and q=a1q = a - 1. Hence the solutions of the given equation are given parametrically in the form: (x,y)=(aa1,1a1), aN, a>1(x, y) = \left( a^{a-1}, \frac{1}{a-1} \right),\ a \in \mathbb{N}^*,\ a > 1.

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