Combinatorics › Permutations and arrangements › Arrangements of letters
Seven-letter good words
High school · ●●○○○ 20 pts
Define a good word as a sequence of letters that consists only of the letters , , and (some of these letters may not appear in the sequence), and in which is never immediately followed by , is never immediately followed by , and is never immediately followed by . How many seven-letter good words are there?