Maths Olympiad Prep

Library / /19 of 43

Number theory Difficulty 5.6 AIME, harder Prove it JBMO

Problem:
Solve in positive integers the equation 1005x+2011y=1006z1005^{x} + 2011^{y} = 1006^{z}.

Solution

Solution:
We have 1006z>2011y>20111006^{z} > 2011^{y} > 2011, hence z2z \geq 2. Then 1005x+2011y0(mod4)1005^{x} + 2011^{y} \equiv 0 \pmod{4}.
But 1005x1(mod4)1005^{x} \equiv 1 \pmod{4}, so 2011y1(mod4)y2011^{y} \equiv -1 \pmod{4} \Rightarrow y is odd, i.e. 2011y1(mod1006)2011^{y} \equiv -1 \pmod{1006}.
Since 1005x+2011y0(mod1006)1005^{x} + 2011^{y} \equiv 0 \pmod{1006}, we get 1005x1(mod1006)x1005^{x} \equiv 1 \pmod{1006} \Rightarrow x is even.
Now 1005x1(mod8)1005^{x} \equiv 1 \pmod{8} and 2011y3(mod8)2011^{y} \equiv 3 \pmod{8}, hence 1006z4(mod8)z=21006^{z} \equiv 4 \pmod{8} \Rightarrow z = 2.
It follows that y<2y=1y < 2 \Rightarrow y = 1 and x=2x = 2. The solution is (x,y,z)=(2,1,2)(x, y, z) = (2, 1, 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.