Maths Olympiad Prep

Library / /9 of 299

Number theory Difficulty 5.1 AIME, harder Prove it Iran

Prove that for every natural number mm, there exists a natural number NN such that for every natural number bb that 2b13892 \le b \le 1389, the sum of digits of NN in base bb, is more than mm.

Solution

We claim that N=(1389!)m+11N = (1389!)^{m+1} - 1 has the desired properties. Let 2b13892 \le b \le 1389, we know that there are at least m+1m+1 zeros in the rightmost of (1389!)m+1(1389!)^{m+1} in base bb, because (1389!)m+1(1389!)^{m+1} is divisible by bm+1b^{m+1}. Hence, the m+1m+1 rightmost digits of (1389!)m+11(1389!)^{m+1} - 1 is b1b-1, then sum of the digits of NN in base bb is at least (m+1)(b1)(m+1)(b-1) which is greater than or equal to m+1m+1.

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.