Cycloaddition
In the (i+j+...) cycloaddition notation i and j refer to the number of atoms involved in the cycloaddition. In this notation a Diels-Alder reaction is a (4+2)cycloaddition and a 1,3-dipolar addition such as the first step in ozonolysis is a (3+2)cycloaddition. This notation uses parenthesis. The IUPAC preferred notation however,  with [i+j+...] counts electrons and not atoms. It uses square brackets. In this notation, the Diels–Alder reaction and the dipolar reaction both become a [4+2]cycloaddition. The reaction between norbornadiene and an activated alkyne is a [2+2+2]cycloaddition.