The rule applied by the function appears to be limited to a specific set of words that have a known association or can be categorized in some way. For example, the input "The Grapes of Wrath" returns "John Steinbeck", indicating a book-author association. However, for most inputs (such as "Rhine", "Ankara", "Marco Rubio", etc.), the function returns "undefined", suggesting that the rule does not apply to these words or there is no known association for them.

It is possible that the function f(x) is designed to recognize and produce an output for a specific set of words that have been pre-programmed into it, while returning "undefined" for other inputs that do not fall into this category.