Maths Olympiad Prep

Library / /3 of 22

Number theory Difficulty 4.7 AIME Prove it United States

Problem:
Show that there are infinitely many prime numbers whose last digit is not 11.

Solution

Solution:
Assume there are only finitely many such primes p1,,pkp_{1}, \ldots, p_{k}. Consider the number
N=10p1p2pk1 N = 10 p_{1} p_{2} \ldots p_{k} - 1
Since NN has last digit 99, there must be a prime pp dividing NN which does not have last digit 11 (otherwise NN must have last digit 11). But by construction, pp cannot divide NN, because it leaves a remainder of p1p-1 when divided by pp. This is a contradiction, so our assumption was wrong and there must be infinitely many such primes.

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.