Fixed Points Of Isometry Groups In Euclidean Space
A fixed point of an isometry group is a point that is a fixed point for every isometry in the group. For any isometry group in Euclidean space the set of fixed points is either empty or an affine space.
For an object, any unique center and, more generally, any point with unique properties with respect to the object is a fixed point of its symmetry group.
In particular this applies for the centroid of a figure, if it exists. In the case of a physical body, if for the symmetry not only the shape but also the density is taken into account, it applies to the center of mass.
If the set of fixed points of the symmetry group of an object is a singleton then the object has a specific center of symmetry. The centroid and center of mass, if defined, are this point. Another meaning of "center of symmetry" is a point with respect to which inversion symmetry applies. Such a point needs not be unique; if it is not, there is translational symmetry, hence there are infinitely many of such points. On the other hand, in the cases of e.g. C3h and D2 symmetry there is a center of symmetry in the first sense, but no inversion.
If the symmetry group of an object has no fixed points then the object is infinite and its centroid and center of mass are undefined.
If the set of fixed points of the symmetry group of an object is a line or plane then the centroid and center of mass of the object, if defined, and any other point that has unique properties with respect to the object, are on this line or plane.
Read more about Fixed Points Of Isometry Groups In Euclidean Space: 1D, 2D, 3D, Arbitrary Dimension
Famous quotes containing the words fixed, points, groups and/or space:
“Genius detects through the fly, through the caterpillar, through the grub, through the egg, the constant individual; through countless individuals the fixed species; through many species the genus; through all genera the steadfast type; through all the kingdoms of organized life the eternal unity. Nature is a mutable cloud which is always and never the same.”
—Ralph Waldo Emerson (1803–1882)
“It’s my feeling that God lends you your children until they’re about eighteen years old. If you haven’t made your points with them by then, it’s too late.”
—Betty Ford (b. 1918)
“Under weak government, in a wide, thinly populated country, in the struggle against the raw natural environment and with the free play of economic forces, unified social groups become the transmitters of culture.”
—Johan Huizinga (1872–1945)
“Mere human beings can’t afford to be fanatical about anything.... Not even about justice or loyalty. The fanatic for justice ends by murdering a million helpless people to clear a space for his law-courts. If we are to survive on this planet, there must be compromises.”
—Storm Jameson (1891–1986)