Maths Olympiad Prep

Library / /7 of 15

Number theory Difficulty 5.1 AIME, harder Prove it Philippines

Problem:
Let N=201921N = 2019^{2} - 1. How many positive factors of N2N^{2} do not divide NN?

Solution

Solution:
Note that N=2351011009N = 2^{3} \cdot 5 \cdot 101 \cdot 1009 and so NN has 4222=324 \cdot 2 \cdot 2 \cdot 2 = 32 factors. On the other hand, N2=2652101210092N^{2} = 2^{6} \cdot 5^{2} \cdot 101^{2} \cdot 1009^{2} and so N2N^{2} has 7333=1897 \cdot 3 \cdot 3 \cdot 3 = 189 factors. Hence, 18932=157189 - 32 = 157 of these factors of N2N^{2} do not divide NN.

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.