| − | Homology groups, while lacking the simple definition of the homotopy groups, tend to be more studied and more useful for classification of topological spaces. There are two approaches to the study of homology: simplicial, which forms groups out of chain complexes, and de Rham, which forms groups from differential forms. | + | Homology groups, while lacking the simple definition of the homotopy groups, tend to be more studied and more useful for classification of topological spaces. There are several approaches to the study of homology: simplicial, which forms groups out of chain complexes, and de Rham, which forms groups from differential forms. |