structural:summarization, information:extraction
In mathematics, a 3-sphere, glome or hypersphere is a higher-dimensional analogue of a sphere. In 4-dimensional Euclidean space, it is the set of points equidistant from a fixed central point. Analogous to how the boundary of a ball in three dimensions is an ordinary sphere (or 2-sphere, a two-dimensional surface), the boundary of a ball in four dimensions (a gongyl) is a 3-sphere (a 3- dimensional "surface"). Topologically, a 3-sphere is an example of a 3-manifold, and it is also an n -sphere.