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.