Maths Olympiad Prep

Library / /11 of 43

Number theory Difficulty 5.4 AIME, harder Prove it JBMO

Problem:
Determine all four digit numbers abcd\overline{a b c d} such that
a(a+b+c+d)(a2+b2+c2+d2)(a6+2b6+3c6+4d6)=abcd a(a+b+c+d)\left(a^{2}+b^{2}+c^{2}+d^{2}\right)\left(a^{6}+2 b^{6}+3 c^{6}+4 d^{6}\right)=\overline{a b c d}

Solution

Solution:
From abcd<10000\overline{a b c d}<10000 and
a10a(a+b+c+d)(a2+b2+c2+d2)(a6+2b6+3c6+4d6)=abcd a^{10} \leq a(a+b+c+d)\left(a^{2}+b^{2}+c^{2}+d^{2}\right)\left(a^{6}+2 b^{6}+3 c^{6}+4 d^{6}\right)=\overline{a b c d}
follows that a2a \leq 2. We thus have two cases:

Case I: a=1a=1.

Obviously 2000>1bcd=(1+b+c+d)(1+b2+c2+d2)(1+2b6+3c6+4d6)2000>\overline{1 b c d}=(1+b+c+d)\left(1+b^{2}+c^{2}+d^{2}\right)\left(1+2 b^{6}+3 c^{6}+4 d^{6}\right) \geq (b+1)(b2+1)(2b6+1)(b+1)\left(b^{2}+1\right)\left(2 b^{6}+1\right), so b2b \leq 2. Similarly one gets c<2c<2 and d<2d<2. By direct check there is no solution in this case.

Case II: a=2a=2.

We have 3000>2bcd=2(2+b+c+d)(4+b2+c2+d2)(64+2b6+3c6+4d6)3000>\overline{2 b c d}=2(2+b+c+d)\left(4+b^{2}+c^{2}+d^{2}\right)\left(64+2 b^{6}+3 c^{6}+4 d^{6}\right) \geq 2(b+2)(b2+4)(2b6+64)2(b+2)\left(b^{2}+4\right)\left(2 b^{6}+64\right), imposing b1b \leq 1. In the same way one proves c<2c<2 and d<2d<2. By direct check, we find out that 2010 is the only solution.

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.