Example:Describing a function

A function need not have an algorithm for constructing the output; a description of the output is also fine. For example, let [math]dji : Days \href{/cs2800/wiki/index.php/%E2%86%92}{→} ℝ [/math] give, on input [math]d [/math], the closing Dow-Jones Industrial average on day [math]d [/math].