Absolute Galois group

The absolute Galois group of the real numbers is a cyclic group of order 2 generated by complex conjugation, since C is the separable closure of R and [C:R] = 2.

In mathematics, particularly in anabelian geometry and p-adic geometry, the absolute Galois groupG_K of a field K is the Galois group of K^sep over K, where K^sep is a separable closure of K. Alternatively it is the group of all automorphisms of the algebraic closure of K that fix K. The absolute Galois group is well-defined up to inner automorphism. It is a profinite group.

(When K is a perfect field, K^sep is the same as an algebraic closure K^alg of K. This holds e.g. for K of characteristic zero, or K a finite field.)

Examples

• The absolute Galois group of an algebraically closed field is trivial.
• The absolute Galois group of the real numbers is a cyclic group of two elements (complex conjugation and the identity map), since C is the separable closure of R, and its degree over R is [C:R] = 2.
• The absolute Galois group of a finite field K is isomorphic to the group of profinite integers

${\displaystyle {\hat {\mathbf {Z} }}=\varprojlim \mathbf {Z} /n\mathbf {Z} .}$ ^[1]

(For the notation, see Inverse limit.)

The Frobenius automorphism Fr is a canonical (topological) generator of G_K. (If K has q elements, Fr is given by Fr(x) = x^q for all x in K^alg.)

• The absolute Galois group of the field of rational functions with complex coefficients is free (as a profinite group). This result is due to Adrien Douady and has its origins in Riemann's existence theorem. ^[2]
• More generally, let C be an algebraically closed field and x an indeterminate. Then the absolute Galois group of K = C(x) is free of rank equal to the cardinality of C. This result is due to David Harbater and Florian Pop, and was also proved later by Dan Haran and Moshe Jarden using algebraic methods. ^{[3] [4] [5]}
• Let K be a finite extension of the p-adic numbers Q_p. For p ≠ 2, its absolute Galois group is generated by [K:Q_p] + 3 elements and has an explicit description by generators and relations. This is a result of Uwe Jannsen and Kay Wingberg. ^{[6] [7]} Some results are known in the case p = 2, but the structure for Q₂ is not known. ^[8]
• Another case in which the absolute Galois group has been determined is for the largest totally real subfield of the field of algebraic numbers. ^[9]

Problems

• No direct description is known for the absolute Galois group of the rational numbers. In this case, it follows from Belyi's theorem that the absolute Galois group has a faithful action on the dessins d'enfants of Grothendieck (maps on surfaces), enabling us to "see" the Galois theory of algebraic number fields. It is one of the goals of anabelian geometry to solve this problem
• Let K be the maximal abelian extension of the rational numbers. Then Shafarevich's conjecture asserts that the absolute Galois group of K is a free profinite group. ^[10]
• An interesting problem is to settle Ján Mináč and Nguyên Duy Tân's conjecture about vanishing of ${\displaystyle n}$- Massey products for ${\displaystyle n\geq 3}$. ^{[11] [12]}

Some general results

• The Neukirch–Uchida theorem asserts that every isomorphism of the absolute Galois groups of algebraic number fields arises from a field automorphism. In particular, two absolute Galois groups of number fields are isomorphic if and only if the base fields are isomorphic.
• Every profinite group occurs as a Galois group of some Galois extension; ^[13] however, not every profinite group occurs as an absolute Galois group. For example, the Artin–Schreier theorem asserts that the only finite absolute Galois groups are either trivial or of order 2, that is only two isomorphism classes.
• Every projective profinite group can be realized as an absolute Galois group of a pseudo algebraically closed field. This result is due to Alexander Lubotzky and Lou van den Dries. ^[14]

Uses in the geometrization of the local Langlands correspondence

In their 2022 paper on the geometrization of the local Langlands correspondence, Laurent Fargues and Peter Scholze looked to recover information about a local field E via its absolute Galois group, which is isomorphic to the étale fundamental group of Spec(E). This result was calculated while trying to evaluate the Weil group (which itself is a variant of the absolute Galois group) of E. This result arrives from the idea of the automorphism group G(E) of the trivial G-torsor over Spec(E); thus, G(E) relates to information over Spec(E), which is an anabelian question. ^{[15] [ non-primary source needed ]}

References

1. Szamuely 2009, p. 14.
2. Douady 1964
3. Harbater 1995
4. Pop 1995
5. Haran & Jarden 2000
6. Jannsen & Wingberg 1982
7. Neukirch, Schmidt & Wingberg 2000 , theorem 7.5.10
8. Neukirch, Schmidt & Wingberg 2000 , §VII.5
9. "qtr" (PDF). Retrieved 2019-09-04.
10. Neukirch, Schmidt & Wingberg 2000, p. 449.
11. Mináč & Tân (2016) pp.255,284
12. Harpaz & Wittenberg (2023) pp.1,41
13. Fried & Jarden (2008) p.12
14. Fried & Jarden (2008) pp.208,545
15. "Fargues, Scholze (2021)" (PDF).

Sources

• Douady, Adrien (1964), "Détermination d'un groupe de Galois", Comptes Rendus de l'Académie des Sciences de Paris, 258: 5305–5308, MR   0162796
• Fried, Michael D.; Jarden, Moshe (2008), Field arithmetic, Ergebnisse der Mathematik und ihrer Grenzgebiete. 3. Folge, vol. 11 (3rd ed.), Springer-Verlag, ISBN   978-3-540-77269-9, Zbl   1145.12001
• Haran, Dan; Jarden, Moshe (2000), "The absolute Galois group of C(x)", Pacific Journal of Mathematics, 196 (2): 445–459, doi: 10.2140/pjm.2000.196.445 , MR   1800587
• Harbater, David (1995), "Fundamental groups and embedding problems in characteristic p", Recent developments in the inverse Galois problem (Seattle, WA, 1993), Contemporary Mathematics, vol. 186, Providence, Rhode Island: American Mathematical Society, pp. 353–369, MR   1352282
• Jannsen, Uwe; Wingberg, Kay (1982), "Die Struktur der absoluten Galoisgruppe ${\displaystyle {\mathfrak {p}}}$-adischer Zahlkörper" (PDF), Inventiones Mathematicae , 70: 71–78, Bibcode:1982InMat..70...71J, doi:10.1007/bf01393199, S2CID   119378923
• Neukirch, Jürgen; Schmidt, Alexander; Wingberg, Kay (2000), Cohomology of Number Fields, Grundlehren der Mathematischen Wissenschaften, vol. 323, Berlin: Springer-Verlag, ISBN   978-3-540-66671-4, MR   1737196, Zbl   0948.11001
• Pop, Florian (1995), "Étale Galois covers of affine smooth curves. The geometric case of a conjecture of Shafarevich. On Abhyankar's conjecture", Inventiones Mathematicae , 120 (3): 555–578, Bibcode:1995InMat.120..555P, doi:10.1007/bf01241142, MR   1334484, S2CID   128157587
• Mináč, Ján; Tân, Nguyên Duy (2016), "Triple Massey products and Galois Theory", Journal of European Mathematical Society, 19 (1): 255–284
• Harpaz, Yonatan; Wittenberg, Olivier (2023), "The Massey vanishing conjecture for number fields", Duke Mathematical Journal, 172 (1): 1–41
• Szamuely, Tamás (2009), Galois Groups and Fundamental Groups, Cambridge studies in advanced mathematics, vol. 117, Cambridge: Cambridge University Press