words of max 3n letters in alphabet of n letters
Source: 7th QEDMO problem 3 (14. - 17. 1. 2010) https://artofproblemsolving.com/community/c1512515_qedmo_200507
May 9, 2021
combinatorics
Problem Statement
An alphabet has letters. A word is called differentiated if it has the following property fulfilled: No letter occurs more than once between two identical letters. For example with the alphabet the word abbdacbdd is not, the word bbacbadcdd is differentiated.
(a) Each differentiated word has a maximum of letters.
(b) How many differentiated words with exactly letters are ther