Bundeswettbewerb Mathematik 1978 Problem 2.4
Source: Bundeswettbewerb Mathematik 1978 Round 2
September 23, 2022
number theoryprimedecimal representationpermutation
Problem Statement
A prime number has the property that however its decimal digits are permuted, the obtained number is also prime. Prove that this number has at most three different digits. Also prove a stronger statement.