# Abhyankar's conjecture

In abstract algebra, Abhyankar's conjecture is a 1957 conjecture of Shreeram Abhyankar, on the Galois groups of algebraic function fields of characteristic p.[1] The soluble case was solved by Serre in 1990[2] and the full conjecture was proved in 1994 by work of Michel Raynaud and David Harbater.[3][4][5]

The problem involves a finite group G, a prime number p, and the function field K(C) of a nonsingular integral algebraic curve C defined over an algebraically closed field K of characteristic p.

The question addresses the existence of a Galois extension L of K(C), with G as Galois group, and with specified ramification. From a geometric point of view, L corresponds to another curve C′, together with a morphism

π : C′ → C.

Geometrically, the assertion that π is ramified at a finite set S of points on C means that π restricted to the complement of S in C is an étale morphism. This is in analogy with the case of Riemann surfaces. In Abhyankar's conjecture, S is fixed, and the question is what G can be. This is therefore a special type of inverse Galois problem.

The subgroup p(G) is defined to be the subgroup generated by all the Sylow subgroups of G for the prime number p. This is a normal subgroup, and the parameter n is defined as the minimum number of generators of

G/p(G).

Then for the case of C the projective line over K, the conjecture states that G can be realised as a Galois group of L, unramified outside S containing s + 1 points, if and only if

ns.

This was proved by Raynaud.

For the general case, proved by Harbater, let g be the genus of C. Then G can be realised if and only if

ns + 2 g.

## References

1. Abhyankar, Shreeram (1957), "Coverings of Algebraic Curves", American Journal of Mathematics 79 (4): 825–856, doi:10.2307/2372438 .
2. Serre, Jean-Pierre (1990), "Construction de revêtements étales de la droite affine en caractéristique p" (in French), Comptes Rendus de l'Académie des Sciences, Série I 311 (6): 341–346
3. Raynaud, Michel (1994), "Revêtements de la droite affine en caractéristique p > 0", Inventiones Mathematicae 116 (1): 425–462, doi:10.1007/BF01231568, Bibcode1994InMat.116..425R .
4. Harbater, David (1994), "Abhyankar's conjecture on Galois groups over curves", Inventiones Mathematicae 117 (1): 1–25, doi:10.1007/BF01232232, Bibcode1994InMat.117....1H .
5. Fried, Michael D.; Jarden, Moshe (2008), Field arithmetic, Ergebnisse der Mathematik und ihrer Grenzgebiete. 3. Folge, 11 (3rd ed.), Springer-Verlag, p. 70, ISBN 978-3-540-77269-9