Question 4
An alphabet contains $29$ characters. A finite sequence of characters is a word, and $X_n$ is the set of all words of length $n$. Consider functions $f:X_n\to X_2$ that assign to each word a two-character subsequence obtained by deleting any $n-2$ characters, without changing the order of those retained. Write $V(f)$ for the image. Find the minimum possible $|V(f)|$ when (a) $n=3$; (b) $n=4$.