IMO Shortlist 1974 problem 12
Dodao/la: arhiva2. travnja 2012.
In a certain language words are formed using an alphabet of three letters. Some words of two or more letters are not allowed, and any two such distinct words are of different lengths. Prove that one can form a word of arbitrary length that does not contain any non-allowed word.
Izvor: Međunarodna matematička olimpijada, shortlist 1974