Frobenius endomorphism

Fixed points of the Frobenius endomorphism

Consider the finite field F_p. By Fermat's little theorem, every element x of F_p satisfies x^p = x. Equivalently, it is a root of the polynomial X^p − X. The elements of F_p therefore determine p roots of this equation, and because this equation has degree p it has no more than p roots over any extension. In particular, if K is an algebraic extension of F_p (such as the algebraic closure or another finite field), then F_p is the fixed field of the Frobenius automorphism of K.

Let R be a ring of characteristic p > 0. If R is an integral domain, then by the same reasoning, the fixed points of Frobenius are the elements of the prime field. However, if R is not a domain, then X^p − X may have more than p roots; for example, this happens if R = F_p × F_p.

A similar property is enjoyed on the finite field ${\displaystyle {\mathbf{F}}_{p^n}}$ by the nth iterate of the Frobenius automorphism: Every element of ${\displaystyle {\mathbf{F}}_{p^n}}$ is a root of ${\displaystyle X^{p^n}-X}$, so if K is an algebraic extension of ${\displaystyle {\mathbf{F}}_{p^n}}$ and F is the Frobenius automorphism of K, then the fixed field of Fⁿ is ${\displaystyle {\mathbf{F}}_{p^n}}$. If R is a domain that is an ${\displaystyle {\mathbf{F}}_{p^n}}$-algebra, then the fixed points of the nth iterate of Frobenius are the elements of the image of ${\displaystyle {\mathbf{F}}_{p^n}}$.

Iterating the Frobenius map gives a sequence of elements in R:

${\displaystyle x,x^p,x^{p^2},x^{p^3},\dots .}$

This sequence of iterates is used in defining the Frobenius closure and the tight closure of an ideal.

As a generator of Galois groups

The Galois group of an extension of finite fields is generated by an iterate of the Frobenius automorphism. First, consider the case where the ground field is the prime field F_p. Let F_q be the finite field of q elements, where q = pⁿ. The Frobenius automorphism F of F_q fixes the prime field F_p, so it is an element of the Galois group Gal(F_q/F_p). In fact, since ${\displaystyle {\mathbf{F}}_q^{\times }}$ is cyclic with q − 1 elements, we know that the Galois group is cyclic and F is a generator. The order of F is n because F^j acts on an element x by sending it to x^pj, and ${\displaystyle x^{p^j}=x}$ can only have ${\displaystyle p^j}$ many roots, since we are in a field. Every automorphism of F_q is a power of F, and the generators are the powers Fⁱ with i coprime to n.

Now consider the finite field F_q^f as an extension of F_q, where q = pⁿ as above. If n > 1, then the Frobenius automorphism F of F_q^f does not fix the ground field F_q, but its nth iterate Fⁿ does. The Galois group Gal(F_q^f /F_q) is cyclic of order f and is generated by Fⁿ. It is the subgroup of Gal(F_q^f /F_p) generated by Fⁿ. The generators of Gal(F_q^f /F_q) are the powers Fⁿⁱ where i is coprime to f.

The Frobenius automorphism is not a generator of the absolute Galois group

${\displaystyle \mathrm{Gal}\left(\overline{{\mathbf{F}}_q}/{\mathbf{F}}_q\right),}$

because this Galois group is isomorphic to the profinite integers

${\displaystyle \hat{\mathbf{Z}}=\underset{n}{\underleftarrow{\lim }}\mathbf{Z}/n\mathbf{Z},}$

which are not cyclic. However, because the Frobenius automorphism is a generator of the Galois group of every finite extension of F_q, it is a generator of every finite quotient of the absolute Galois group. Consequently, it is a topological generator in the usual Krull topology on the absolute Galois group.

Frobenius for schemes

There are several different ways to define the Frobenius morphism for a scheme. The most fundamental is the absolute Frobenius morphism. However, the absolute Frobenius morphism behaves poorly in the relative situation because it pays no attention to the base scheme. There are several different ways of adapting the Frobenius morphism to the relative situation, each of which is useful in certain situations.

The absolute Frobenius morphism

Suppose that X is a scheme of characteristic p > 0. Choose an open affine subset U = Spec A of X. The ring A is an F_p-algebra, so it admits a Frobenius endomorphism. If V is an open affine subset of U, then by the naturality of Frobenius, the Frobenius morphism on U, when restricted to V, is the Frobenius morphism on V. Consequently, the Frobenius morphism glues to give an endomorphism of X. This endomorphism is called the absolute Frobenius morphism of X, denoted F_X. By definition, it is a homeomorphism of X with itself. The absolute Frobenius morphism is a natural transformation from the identity functor on the category of F_p-schemes to itself.

If X is an S-scheme and the Frobenius morphism of S is the identity, then the absolute Frobenius morphism is a morphism of S-schemes. In general, however, it is not. For example, consider the ring ${\displaystyle A={\mathbf{F}}_{p^2}}$. Let X and S both equal Spec A with the structure map X → S being the identity. The Frobenius morphism on A sends a to a^p. It is not a morphism of ${\displaystyle {\mathbf{F}}_{p^2}}$-algebras. If it were, then multiplying by an element b in ${\displaystyle {\mathbf{F}}_{p^2}}$ would commute with applying the Frobenius endomorphism. But this is not true because:

${\displaystyle b\cdot a=ba\ne F(b)\cdot a=b^{p}a.}$

The former is the action of b in the ${\displaystyle {\mathbf{F}}_{p^2}}$-algebra structure that A begins with, and the latter is the action of ${\displaystyle {\mathbf{F}}_{p^2}}$ induced by Frobenius. Consequently, the Frobenius morphism on Spec A is not a morphism of ${\displaystyle {\mathbf{F}}_{p^2}}$-schemes.

The absolute Frobenius morphism is a purely inseparable morphism of degree p. Its differential is zero. It preserves products, meaning that for any two schemes X and Y, F_X×Y = F_X × F_Y.

Restriction and extension of scalars by Frobenius

Suppose that φ : X → S is the structure morphism for an S-scheme X. The base scheme S has a Frobenius morphism F_S. Composing φ with F_S results in an S-scheme X_F called the restriction of scalars by Frobenius. The restriction of scalars is actually a functor, because an S-morphism X → Y induces an S-morphism X_F → Y_F.

For example, consider a ring A of characteristic p > 0 and a finitely presented algebra over A:

${\displaystyle R=A[X_1,\dots ,X_n]/(f_1,\dots ,f_m).}$

The action of A on R is given by:

${\displaystyle c\cdot \sum a_{\alpha }{X}^{\alpha }=\sum c{a}_{\alpha }{X}^{\alpha },}$

where α is a multi-index. Let X = Spec R. Then X_F is the affine scheme Spec R, but its structure morphism Spec R → Spec A, and hence the action of A on R, is different:

${\displaystyle c\cdot \sum a_{\alpha }{X}^{\alpha }=\sum F(c)a_{\alpha }{X}^{\alpha }=\sum c^p{a}_{\alpha }{X}^{\alpha }.}$

Because restriction of scalars by Frobenius is simply composition, many properties of X are inherited by X_F under appropriate hypotheses on the Frobenius morphism. For example, if X and S_F are both finite type, then so is X_F.

The extension of scalars by Frobenius is defined to be:

${\displaystyle X^{(p)}=X{\times }_S{S}_F.}$

The projection onto the S factor makes X^(p) an S-scheme. If S is not clear from the context, then X^(p) is denoted by X^(p/S). Like restriction of scalars, extension of scalars is a functor: An S-morphism X → Y determines an S-morphism X^(p) → Y^(p).

As before, consider a ring A and a finitely presented algebra R over A, and again let X = Spec R. Then:

${\displaystyle X^{(p)}=\mathrm{Spec}R{\otimes }_A{A}_F.}$

A global section of X^(p) is of the form:

${\displaystyle \sum _i\left(\sum _{\alpha }{a}_{i\alpha }{X}^{\alpha }\right)\otimes b_i=\sum _i\sum _{\alpha }{X}^{\alpha }\otimes a_{i\alpha }^p{b}_i,}$

where α is a multi-index and every a_iα and b_i is an element of A. The action of an element c of A on this section is:

${\displaystyle c\cdot \sum _i\left(\sum _{\alpha }{a}_{i\alpha }{X}^{\alpha }\right)\otimes b_i=\sum _i\left(\sum _{\alpha }{a}_{i\alpha }{X}^{\alpha }\right)\otimes b_{i}c.}$

Consequently, X^(p) is isomorphic to:

${\displaystyle \mathrm{Spec}A[X_1,\dots ,X_n]/\left(f_1^{(p)},\dots ,f_m^{(p)}\right),}$

where, if:

${\displaystyle f_j=\sum _{\beta }{f}_{j\beta }{X}^{\beta },}$

then:

${\displaystyle f_j^{(p)}=\sum _{\beta }{f}_{j\beta }^p{X}^{\beta }.}$

A similar description holds for arbitrary A-algebras R.

Because extension of scalars is base change, it preserves limits and coproducts. This implies in particular that if X has an algebraic structure defined in terms of finite limits (such as being a group scheme), then so does X^(p). Furthermore, being a base change means that extension of scalars preserves properties such as being of finite type, finite presentation, separated, affine, and so on.

Extension of scalars is well-behaved with respect to base change: Given a morphism S′ → S, there is a natural isomorphism:

${\displaystyle X^{(p/S)}{\times }_S{S}^{\prime }\cong (X{\times }_S{S}^{\prime }{)}^{(p/S^{\prime })}.}$

Relative Frobenius

Let X be an S-scheme with structure morphism φ. The relative Frobenius morphism of X is the morphism:

${\displaystyle F_{X/S}:X\to X^{(p)}}$

defined by the universal property of the pullback X^(p) (see the diagram above):

${\displaystyle F_{X/S}=(F_X,\phi ).}$

Because the absolute Frobenius morphism is natural, the relative Frobenius morphism is a morphism of S-schemes.

Consider, for example, the A-algebra:

${\displaystyle R=A[X_1,\dots ,X_n]/(f_1,\dots ,f_m).}$

We have:

${\displaystyle R^{(p)}=A[X_1,\dots ,X_n]/(f_1^{(p)},\dots ,f_m^{(p)}).}$

The relative Frobenius morphism is the homomorphism R^(p) → R defined by:

${\displaystyle \sum _i\sum _{\alpha }{X}^{\alpha }\otimes a_{i\alpha }\mapsto \sum _i\sum _{\alpha }{a}_{i\alpha }{X}^{p\alpha }.}$

Relative Frobenius is compatible with base change in the sense that, under the natural isomorphism of X^(p/S) ×_S S′ and (X ×_S S′)^(p/S′), we have:

${\displaystyle F_{X/S}\times 1_{S^{\prime }}=F_{X{\times }_S{S}^{\prime }/S^{\prime }}.}$

Relative Frobenius is a universal homeomorphism. If X → S is an open immersion, then it is the identity. If X → S is a closed immersion determined by an ideal sheaf I of O_S, then X^(p) is determined by the ideal sheaf I^p and relative Frobenius is the augmentation map O_S/I^p → O_S/I.

X is unramified over S if and only if F_X/S is unramified and if and only if F_X/S is a monomorphism. X is étale over S if and only if F_X/S is étale and if and only if F_X/S is an isomorphism.

Arithmetic Frobenius

The arithmetic Frobenius morphism of an S-scheme X is a morphism:

${\displaystyle F_{X/S}^a:X^{(p)}\to X{\times }_{S}S\cong X}$

defined by:

${\displaystyle F_{X/S}^a=1_X\times F_S.}$

That is, it is the base change of F_S by 1_X.

Again, if:

${\displaystyle R=A[X_1,\dots ,X_n]/(f_1,\dots ,f_m),}$

${\displaystyle R^{(p)}=A[X_1,\dots ,X_n]/(f_1,\dots ,f_m){\otimes }_A{A}_F,}$

then the arithmetic Frobenius is the homomorphism:

${\displaystyle \sum _i\left(\sum _{\alpha }{a}_{i\alpha }{X}^{\alpha }\right)\otimes b_i\mapsto \sum _i\sum _{\alpha }{a}_{i\alpha }{b}_i^p{X}^{\alpha }.}$

If we rewrite R^(p) as:

${\displaystyle R^{(p)}=A[X_1,\dots ,X_n]/\left(f_1^{(p)},\dots ,f_m^{(p)}\right),}$

then this homomorphism is:

${\displaystyle \sum a_{\alpha }{X}^{\alpha }\mapsto \sum a_{\alpha }^p{X}^{\alpha }.}$

Geometric Frobenius

Assume that the absolute Frobenius morphism of S is invertible with inverse ${\displaystyle F_S^{-1}}$. Let ${\displaystyle S_{F^{-1}}}$ denote the S-scheme ${\displaystyle F_S^{-1}:S\to S}$. Then there is an extension of scalars of X by ${\displaystyle F_S^{-1}}$:

${\displaystyle X^{(1/p)}=X{\times }_S{S}_{F^{-1}}.}$

If:

${\displaystyle R=A[X_1,\dots ,X_n]/(f_1,\dots ,f_m),}$

then extending scalars by ${\displaystyle F_S^{-1}}$ gives:

${\displaystyle R^{(1/p)}=A[X_1,\dots ,X_n]/(f_1,\dots ,f_m){\otimes }_A{A}_{F^{-1}}.}$

If:

${\displaystyle f_j=\sum _{\beta }{f}_{j\beta }{X}^{\beta },}$

then we write:

${\displaystyle f_j^{(1/p)}=\sum _{\beta }{f}_{j\beta }^{1/p}{X}^{\beta },}$

and then there is an isomorphism:

${\displaystyle R^{(1/p)}\cong A[X_1,\dots ,X_n]/(f_1^{(1/p)},\dots ,f_m^{(1/p)}).}$

The geometric Frobenius morphism of an S-scheme X is a morphism:

${\displaystyle F_{X/S}^g:X^{(1/p)}\to X{\times }_{S}S\cong X}$

defined by:

${\displaystyle F_{X/S}^g=1_X\times F_S^{-1}.}$

It is the base change of ${\displaystyle F_S^{-1}}$ by 1_X.

Continuing our example of A and R above, geometric Frobenius is defined to be:

${\displaystyle \sum _i\left(\sum _{\alpha }{a}_{i\alpha }{X}^{\alpha }\right)\otimes b_i\mapsto \sum _i\sum _{\alpha }{a}_{i\alpha }{b}_i^{1/p}{X}^{\alpha }.}$

After rewriting R^(1/p) in terms of ${\displaystyle \{f_j^{(1/p)}\}}$, geometric Frobenius is:

${\displaystyle \sum a_{\alpha }{X}^{\alpha }\mapsto \sum a_{\alpha }^{1/p}{X}^{\alpha }.}$

Arithmetic and geometric Frobenius as Galois actions

Suppose that the Frobenius morphism of S is an isomorphism. Then it generates a subgroup of the automorphism group of S. If S = Spec k is the spectrum of a finite field, then its automorphism group is the Galois group of the field over the prime field, and the Frobenius morphism and its inverse are both generators of the automorphism group. In addition, X^(p) and X^(1/p) may be identified with X. The arithmetic and geometric Frobenius morphisms are then endomorphisms of X, and so they lead to an action of the Galois group of k on X.

Consider the set of K-points X(K). This set comes with a Galois action: Each such point x corresponds to a homomorphism O_X → K from the structure sheaf to K, which factors via k(x), the residue field at x, and the action of Frobenius on x is the application of the Frobenius morphism to the residue field. This Galois action agrees with the action of arithmetic Frobenius: The composite morphism

${\displaystyle {\mathcal{O}}_X\to k(x)\stackrel{\stackrel{}{F}}{\to }k(x)}$

is the same as the composite morphism:

${\displaystyle {\mathcal{O}}_X\stackrel{{\stackrel{}{F}}_{X/S}^a}{\to }{\mathcal{O}}_X\to k(x)}$

by the definition of the arithmetic Frobenius. Consequently, arithmetic Frobenius explicitly exhibits the action of the Galois group on points as an endomorphism of X.

Frobenius for local fields

Given an unramified finite extension L/K of local fields, there is a concept of Frobenius endomorphism that induces the Frobenius endomorphism in the corresponding extension of residue fields.

Suppose L/K is an unramified extension of local fields, with ring of integers O_K of K such that the residue field, the integers of K modulo their unique maximal ideal φ, is a finite field of order q, where q is a power of a prime. If Φ is a prime of L lying over φ, that L/K is unramified means by definition that the integers of L modulo Φ, the residue field of L, will be a finite field of order q^f extending the residue field of K where f is the degree of L/K. We may define the Frobenius map for elements of the ring of integers O_L of L as an automorphism s_Φ of L such that

${\displaystyle s_{\mathrm{\Phi }}(x)\equiv x^q(\mathrm{mod}\mathrm{\Phi }).}$

Frobenius for global fields

In algebraic number theory, Frobenius elements are defined for extensions L/K of global fields that are finite Galois extensions for prime ideals Φ of L that are unramified in L/K. Since the extension is unramified the decomposition group of Φ is the Galois group of the extension of residue fields. The Frobenius element then can be defined for elements of the ring of integers of L as in the local case, by

${\displaystyle s_{\mathrm{\Phi }}(x)\equiv x^q(\mathrm{mod}\mathrm{\Phi }),}$

where q is the order of the residue field O_K/(Φ ∩ O_K).

Lifts of the Frobenius are in correspondence with p-derivations.

Examples

The polynomial

x⁵ − x − 1

has discriminant

19 × 151,

and so is unramified at the prime 3; it is also irreducible mod 3. Hence adjoining a root ρ of it to the field of 3-adic numbers Q₃ gives an unramified extension Q₃(ρ) of Q₃. We may find the image of ρ under the Frobenius map by locating the root nearest to ρ³, which we may do by Newton's method. We obtain an element of the ring of integers Z₃[ρ] in this way; this is a polynomial of degree four in ρ with coefficients in the 3-adic integers Z₃. Modulo 3⁸ this polynomial is

${\displaystyle {\rho }^3+3(460+183\rho -354{\rho }^2-979{\rho }^3-575{\rho }^4)}$.

This is algebraic over Q and is the correct global Frobenius image in terms of the embedding of Q into Q₃; moreover, the coefficients are algebraic and the result can be expressed algebraically. However, they are of degree 120, the order of the Galois group, illustrating the fact that explicit computations are much more easily accomplished if p-adic results will suffice.

If L/K is an abelian extension of global fields, we get a much stronger congruence since it depends only on the prime φ in the base field K. For an example, consider the extension Q(β) of Q obtained by adjoining a root β satisfying

${\displaystyle {\beta }^5+{\beta }^4-4{\beta }^3-3{\beta }^2+3\beta +1=0}$

to Q. This extension is cyclic of order five, with roots

${\displaystyle 2\cos \frac{2\pi n}{11}}$

for integer n. It has roots that are Chebyshev polynomials of β:

β² − 2, β³ − 3β, β⁵ − 5β³ + 5β

give the result of the Frobenius map for the primes 2, 3 and 5, and so on for larger primes not equal to 11 or of the form 22n + 1 (which split). It is immediately apparent how the Frobenius map gives a result equal mod p to the pth power of the root β.