Maths Olympiad Prep

Library / /15 of 39

Number theory Difficulty 5.3 AIME, harder Prove it Ukraine

Find all pairs of positive integers (x,y)(x, y) satisfying the equation:
xy+yx=2008.x^y + y^x = 2008.

Solution

Let xyx \le y. x=1x=1 gives us an obvious solution y=2007y=2007.

In what follows, suppose that 2xy2 \le x \le y. We can check for small values of xx:

210=1024<2008<211=20482^{10} = 1024 < 2008 < 2^{11} = 2048, 36=729<2008<37=21873^6 = 729 < 2008 < 3^7 = 2187,
45=1024<2008<46=40964^5 = 1024 < 2008 < 4^6 = 4096, 54=625<2008<55=31255^4 = 625 < 2008 < 5^5 = 3125, 64=1296<2008<65=77766^4 = 1296 < 2008 < 6^5 = 7776, 73=343<2008<74=24017^3 = 343 < 2008 < 7^4 = 2401, 83=512<2008<84=40968^3 = 512 < 2008 < 8^4 = 4096, 93=729<2008<94=65619^3 = 729 < 2008 < 9^4 = 6561, 103=1000<2008<104=1000010^3 = 1000 < 2008 < 10^4 = 10000.

It is clear, that there is no need to continue. That is, it follows from the relations 210+102=1124<20082^{10} + 10^2 = 1124 < 2008, 36+63=945<20083^6 + 6^3 = 945 < 2008, 45+54=1649<20084^5 + 5^4 = 1649 < 2008 that there are no more new solutions (for further values of xx, the condition 2xy2 \le x \le y breaks).

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 and solution reproduced as published; topic and difficulty added by this site.