The invariants are numerical quantities derived from the molecular structure and used to characterize local properties of a molecule; these numbers are calculated in such a way as to be independent of any arbitrary atom/bond numbering. Local invariants can be distinguished into LOcal Vertex Invariants (LOVIs) and LOcal Edge Invariants (LOEIs), depending on whether they refer to atoms or bonds.
LOVIs of a molecule are usually collected into an N-dimensional vector (N = number of atoms). LOVIs and LOEIs (also known as LOBIs) are used to calculate several molecular (namely, topological indices) by applying different operators. L is here adopted as the general symbol for local invariants.
Over the years, it has been generally accepted that the definition of global (or local) indices from LOVIs (Li) implies the summation of the contributions of the elements that constitute a given molecular structure. However, summation (Minskowski’s first norm (N1) in our specific case) is just one of the many invariants capable of globally characterizing given LOVIs.