Hasse–Arf theorem explained

In mathematics, specifically in local class field theory, the Hasse–Arf theorem is a result concerning jumps of the upper numbering filtration of the Galois group of a finite Galois extension. A special case of it when the residue fields are finite was originally proved by Helmut Hasse,^{[1] [2]} and the general result was proved by Cahit Arf.^[3]

Statement

Higher ramification groupsL/K . So assume L/K is a finite Galois extension, and that v _{K} is a discrete normalised valuation of K, whose residue field has characteristic p > 0, and which admits a unique extension to L, say w. Denote by v _{L} the associated normalised valuation ew of L and let \scriptstyle{ l{O}} be the valuation ring of L under v _{L} . Let L/K have Galois group G and define the s-th ramification group of L/K for any real s ≥ -1 byG _{s(L/K)=\{\sigma\in} G:v _{L(\sigma} a - a)\geq s +1 forall a\in l{O}\}.

So, for example, G_-1 is the Galois group G. To pass to the upper numbering one has to define the function ψ_L/K which in turn is the inverse of the function η_L/K defined by

η _{L/K} (s) = \int ^{s} _{0} (dx)/(|G _{0:G} _{x|}) .

The upper numbering of the ramification groups is then defined by G^t(L/K) = G_s(L/K) where s = ψ_L/K(t).

These higher ramification groups G^t(L/K) are defined for any real t ≥ -1, but since v_L is a discrete valuation, the groups will change in discrete jumps and not continuously. Thus we say that t is a jump of the filtration if G^t(L/K) ≠ G^u(L/K) for any u > t. The Hasse–Arf theorem tells us the arithmetic nature of these jumps.

Statement of the theorem

With the above set up, the theorem states that the jumps of the filtration are all rational integers.^[4]

Example

Suppose G is cyclic of order

p ^{n} , p residue characteristic and G(i) be the subgroup of G of order p ^{n-i} . The theorem says that there exist positive integers i _{0,} i _{1,} ..., i _{n-1} such thatG _{0} = … = G _{i _{0}} = G = G ^{0} = … = G ^{i _{0}} G _{i _{0} +1} = … = G _{i _{0} + p i _{1}} = G( 1 ) = G ^{i _{0} +1} = … = G ^{i _{0} + i _{1}} G _{i _{0} + p i _{1} +1} = … = G _{i _{0} + p i _{1} + p ^{2} i _{2}} = G( 2 ) = G ^{i _{0} + i _{1} +1}

...

G _{i _{0} + p i _{1} + … + p ^{n-1} i _{n-1} +1} = 1 = G ^{i _{0} + … + i _{n-1} +1} . ^[5]

Non-abelian extensions

For non-abelian extensions the jumps in the upper filtration need not be at integers. Serre gave an example of a totally ramified extension with Galois group the quaternion group

Q _{8} of order 8 withG _{0} = Q _{8} G _{1} = Q _{8} G _{2} = \Z/ 2 \Z G _{3} = \Z/ 2 \Z G _{4} = 1 The upper numbering then satisfiesG ^{n} = Q _{8}   for n \leq 1 G ^{n} = \Z/ 2 \Z   for 1 < n\leq 3 / 2 G ^{n} = 1   for 3 / 2 < n so has a jump at the non-integral value n =3 / 2 .

Notes and References

1. Hasse . Helmut . Helmut Hasse. 1930 . Führer, Diskriminante und Verzweigungskörper relativ-Abelscher Zahlkörper. de. . 162 . 169–184 . 10.1515/crll.1930.162.169 . 1581221.
2. H. Hasse, Normenresttheorie galoisscher Zahlkörper mit Anwendungen auf Führer und Diskriminante abelscher Zahlkörper, J. Fac. Sci. Tokyo 2 (1934), pp.477 - 498.
3. Cahit . Arf . Cahit Arf . Untersuchungen über reinverzweigte Erweiterungen diskret bewerteter perfekter Körper . . 181 . 1939 . 1–44 . 0021.20201 . German. 10.1515/crll.1940.181.1 . 0000018.
4. Neukirch (1999) Theorem 8.9, p.68
5. Serre (1979) IV.3, p.76