Maths Olympiad Prep

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, 2022

Number theory Difficulty 5.1 AIME, harder Prove it Switzerland

Problem:
Determine all pairs (m,p)(m, p) of a positive integer mm and a prime number pp satisfying the equation
p2+pm=m3 p^{2} + p m = m^{3}

Solutions — 2

Solution 1

Solution:
Rewriting the equation as p2=m(m2p)p^{2} = m (m^{2} - p), we see that mm must divide p2p^{2}. However, since pp is prime, the only positive factors of p2p^{2} are 11, pp and p2p^{2}. We now check each case separately:

Case m=1m = 1:
The equation becomes p2+p=1p^{2} + p = 1. Since this would imply that pp divides 11, we do not get any solutions in this case.

Case m=pm = p:
The equation becomes 2p2=p32 p^{2} = p^{3} and cancelling a factor of p2p^{2} we find that p=2p = 2. The pair (2,2)(2, 2) is therefore the only solution in this case.

Case m=p2m = p^{2}:
The equation becomes p2+p3=p6p^{2} + p^{3} = p^{6} and after cancelling, we get 1+p=p41 + p = p^{4}. Again, this would imply that pp divides 11, which is not possible. No solutions in this case.

We conclude that (2,2)(2, 2) is the only pair satisfying the equation.

Solution 2

Solution:
We observe that pp divides the left-hand-side of the equation and therefore must divide the right-hand-side as well. Now if pp divides m3m^{3}, we must have that pp divides mm. Let us write m=pnm = p n for some positive integer nn. Substituting into our equation and cancelling a factor of p2p^{2} we are left with 1+n=pn31 + n = p n^{3}. This implies that nn must divide 11 and therefore n=1n = 1 and m=2m = 2. The equation now simplifies to p=2p = 2 and we conclude that the only solution is the pair (2,2)(2, 2).

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