Maths Olympiad Prep

Library / /71 of 129

, 2012

Number theory Difficulty 5.3 AIME, harder Prove it Slovenia

Find all natural numbers nn and prime numbers pp such that n+2pn\sqrt[n]{n+\frac{2}{p}} is a natural number.

Solution

Denote n+2pn=k\sqrt[n]{n + \frac{2}{p}} = k where kk is a natural number, hence n+2p=knn + \frac{2}{p} = k^n. Thus pp is a divisor of 22. Since pp is prime, we have p=2p = 2 or p=1p = 1 (but 11 is not prime). So p=2p = 2.

Now, n+22=kn    n+1=knn + \frac{2}{2} = k^n \implies n + 1 = k^n.

Try n=1n = 1: 1+1=2=k1    k=21 + 1 = 2 = k^1 \implies k = 2.
Try n=2n = 2: 2+1=3=k2    k=32 + 1 = 3 = k^2 \implies k = \sqrt{3} (not integer).
Try n=3n = 3: 3+1=4=k3    k=433 + 1 = 4 = k^3 \implies k = \sqrt[3]{4} (not integer).

So only n=1n = 1, p=2p = 2, k=2k = 2 is a solution.

Alternatively, if the original problem was n+2nn\sqrt[n]{n + \frac{2}{n}} (as in the solution context), then nn must be a divisor of 22, so n=1n = 1 or n=2n = 2.

For n=1n = 1: 1+21=3=k1    k=31 + \frac{2}{1} = 3 = k^1 \implies k = 3.
For n=2n = 2: 2+22=3=k2    k=32 + \frac{2}{2} = 3 = k^2 \implies k = \sqrt{3} (not integer).

Thus, the only solution is n=1n = 1, p=2p = 2, k=2k = 2.

Therefore, the solutions are n=1n = 1, p=2p = 2.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.