Group Scheme of Roots of Unity
The group scheme of -th roots of unity is by definition the kernel of the -power map on the multiplicative group, considered as a group scheme. That is, for any integer we can consider the morphism on the multiplicative group that takes -th powers, and take an appropriate fiber product in the sense of scheme theory of it, with the morphism that serves as the identity.
The resulting group scheme is written . It gives rise to a reduced scheme, when we take it over a field, if and only if the characteristic of does not divide . This makes it a source of some key examples of non-reduced schemes (schemes with nilpotent elements in their structure sheaves); for example over a finite field with elements for any prime number .
This phenomenon is not easily expressed in the classical language of algebraic geometry. It turns out to be of major importance, for example, in expressing the duality theory of abelian varieties in characteristic (theory of Pierre Cartier). The Galois cohomology of this group scheme is a way of expressing Kummer theory.
Read more about this topic: Multiplicative Group
Famous quotes containing the words group, scheme, roots and/or unity:
“The boys think they can all be athletes, and the girls think they can all be singers. That’s the way to fame and success. ...as a group blacks must give up their illusions.”
—Kristin Hunter (b. 1931)
“The real security of Christianity is to be found in its benevolent morality, in its exquisite adaptation to the human heart, in the facility with which its scheme accommodates itself to the capacity of every human intellect, in the consolation which it bears to the house of mourning, in the light with which it brightens the great mystery of the grave.”
—Thomas Babington Macaulay (1800–1859)
“Einstein is not ... merely an artist in his moments of leisure and play, as a great statesman may play golf or a great soldier grow orchids. He retains the same attitude in the whole of his work. He traces science to its roots in emotion, which is exactly where art is also rooted.”
—Havelock Ellis (1859–1939)
“Hearing, seeing and understanding each other, humanity from one end of the earth to the other now lives simultaneously, omnipresent like a god thanks to its own creative ability. And, thanks to its victory over space and time, it would now be splendidly united for all time, if it were not confused again and again by that fatal delusion which causes humankind to keep on destroying this grandiose unity and to destroy itself with the same resources which gave it power over the elements.”
—Stefan Zweig (18811942)