I found some more background here[1]. It's not clear (to me, at least) whether Euler would have employed parentheses in that case if they hadn't been necessary

I found some more background here[1]. It's not clear (to me, at least) whether Euler would have employed parentheses in that case if they hadn't been necessary to avoid ambiguity, i.e. if the argument had been x, he might have just written fx. [1] http://math.stackexchange.com/questions/636332/the-origin-of-the-function-fx-notation http://math.stackexchange.com/questions/636332/the-origin-of...