It is an accurate statement of the law in the US, but "purely functional expression" is not used in the sense of a mathematical function. It basically means exp

It is an accurate statement of the law in the US, but "purely functional expression" is not used in the sense of a mathematical function. It basically means expressions that solely reflect the way you need to do the thing to accomplish the function you are trying to achieve, without any significant creative element. For example, if I asked you to write Java code to print the sum of two integer variables named a and b, followed by a newline, you'd probably write pretty much exactly this: System.out.println(a + b); You might vary the spacing around the punctuation, or a few other unimportant details, but there's not much room for creativity there while still accomplishing the desired function. It isn't copyrightable. And this is a good example of how the word functional means something non-mathematical here: that isn't even a functional expression in the mathematical sense, since there's no return value and the printing is a side effect. It's purely imperative.