In 3-dimensional space, the sphere with centre C and radius r is the locus of all points whose distance from C is equal to r. If C has Cartesian coordinates (a,b,c), this sphere has equation
The interior of a sphere of radius r has volume , and the surface area equals 4πr2.
In topology, the n-dimensional sphere is denoted as Sn and is often identified with the unit sphere in ℝn+1, that is,
The spheres described above are topologically S2.