A set of standard descriptive metadata elements used with web resources to aid in resource discovery. The elements are intended as a starting point for resource description. All elements are optional, and are intended to be extensible to richer descriptive elements when needed. The 15 elements are: title, author/creator, subject/keywords, description, publisher, other contributor, date, resource type, format, resource identifier, source, language, relation, coverage and rights management.
In mathematics, the core of a subgroup H of a group G with respect to a subset S of elements of G is defined to be the intersection of the subgroups conjugate to H under every element of the set S, i.e.