16 words in a language of 2 letters
Source: Norwegian Mathematical Olympiad 2009 - Abel Competition p2
September 5, 2019
combinatorics
Problem Statement
There are two letters in a language.
Every word consists of seven letters, and two different words always have different letters on at least three places.
a. Show that such a language cannot have more than words.
b. Can there be words in the language?