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Number theory Difficulty 5.2 AIME, harder Prove it Mongolia

Find all triples of positive integers (m,n,p)(m, n, p) with m,nm, n positive integers and pp a prime number such that m2025+n2024=pmnm^{2025} + n^{2024} = pmn.

Solution

Answer: (m,n,p)=(1,1,2)(m, n, p) = (1, 1, 2).

Setting k=2024k = 2024, m1=m/dm_1 = m/d and n1=n/dn_1 = n/d, the given identity becomes dk2(dm1k+1+n1k)=pm1n1d^{k-2}(dm_1^{k+1} + n_1^k) = pm_1n_1, where dd is the greatest common divisor (m,n)(m, n) of nn and mm. Put a=n1/ba = n_1/b and c=d/bc = d/b, where bb denotes (d,n1)(d, n_1). Then we get (cb)k2(cm1k+1+akbk1)=pm1a(cb)^{k-2}(cm_1^{k+1} + a^k b^{k-1}) = pm_1a. Observe that cm1k+1+akbk1cm_1^{k+1} + a^k b^{k-1} is greater than 1 and is relatively prime to m1am_1a. Then (cb)k2(cb)^{k-2} is divisible by m1am_1a and
(cb)k2m1a(cm1k+1+akbk1)=p(0.3) \frac{(cb)^{k-2}}{m_1a}(cm_1^{k+1} + a^k b^{k-1}) = p \quad (0.3)
which implies (cb)k2=m1a(cb)^{k-2} = m_1a. Since (m1,b)=1(m_1, b) = 1 and (a,c)=1(a, c) = 1 we must have a=bk2a = b^{k-2} and m1=ck2m_1 = c^{k-2}. Thus, it follows from (0.3) that ck2k+1+bk2k+1=pc^{k^2-k+1} + b^{k^2-k+1} = p, hence pp must be divisible by b+cb+c since k2k+1k^2-k+1 is odd. Therefore p=c+bp = c+b which gives us c+b=ck2k+1+bk2k+1c+b = c^{k^2-k+1} + b^{k^2-k+1}. Hence we conclude that c=b=1c = b = 1 and so m=n=1,p=2m = n = 1, p = 2.

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