Stage: 3 Challenge Level: 
We define "strange" numbers in the following way. All strange numbers are prime. Every one digit prime number is strange and a number of two or more digits is strange if and only if so are the two numbers obtained from it by omitting either its first or its last digit. Find all strange numbers.
This problem comes from the Hungarian magazine KoMaL. You can find many other challenging problems in English on the KoMaL website.
Mathematical reasoning & proof. Properties of numbers. Prime numbers. Factorisation (algebraic). Modulus arithmetic. Place value. Number theory. Integers. Factors and multiples. Common factors.
Published July 2000.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk
