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Number theory Difficulty 4.4 AIME Prove it North Macedonia

Find all prime numbers pp and qq which satisfy the equation
(p+q)p=(qp)2q1. (p+q)^p = (q-p)^{2q-1}.

Solution

It cannot be that qp=1q-p=1, since (q+p)p>1(q+p)^p > 1, for all prime numbers pp and qq.
Let rr be a prime divisor of qpq-p. Then rr is also a divisor of q+pq+p, so it is a divisor of 2q=(q+p)+(qp)2q = (q+p)+(q-p) and of 2p=(q+p)(qp)2p = (q+p)-(q-p). It follows that p=q=rp=q=r or r=2r=2. The case p=qp=q is impossible, since the right-hand side will be zero, but not the left-hand side. According to this it has to be that qp=2kq-p=2^k and q+p=2lq+p=2^l for some positive integers kk and ll. From q=(q+p)+(qp)2=2l1+2k1q = \frac{(q+p)+(q-p)}{2} = 2^{l-1} + 2^{k-1}, it follows that for k>1k>1, 2q2|q, which is possible only for q=2q=2, but then (qp)2q1<0<(p+q)p(q-p)^{2q-1} < 0 < (p+q)^p, so it has to be that k=1k=1. This means that qp=2q-p=2. On the other hand we get lp=2q1=2p+3lp = 2q-1 = 2p+3, hence 3p3|p, i.e. p=3p=3 and q=5q=5. By checking we obtain that these numbers satisfy the equation.

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