TITLE: Is an empty parenthesis a valid mathematical expression?
QUESTION [11 upvotes]: Is using an empty parenthesis valid? 
For example, $15+()=15$. 
What is the meaning if it is valid? I need an academic reference to validate this.

REPLY [2 votes]: In mathematical logic, and computer programming languages like Haskell, F#, OCaml, and Scala, () represents the (single possible) value of the Unit type, which is used to denote a function that returns no value.
It is not valid to add a number with this representation of ().