Hasse–Arf theorem

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][4]

The theorem deals with the upper numbered higher ramification groups of a finite abelian extension ${\displaystyle L/K}$. So assume ${\displaystyle L/K}$ is a finite Galois extension, and that ${\displaystyle v_{K}}$ 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 ${\displaystyle v_{L}}$ the associated normalised valuation ew of L and let ${\displaystyle \scriptstyle {\mathcal {O}}}$ be the valuation ring of L under ${\displaystyle v_{L}}$. Let ${\displaystyle L/K}$ have Galois group G and define the s-th ramification group of ${\displaystyle L/K}$ for any real s ≥ −1 by

${\displaystyle G_{s}(L/K)=\{\sigma \in G\,:\,v_{L}(\sigma a-a)\geq s+1{\text{ for all }}a\in {\mathcal {O}}\}.}$

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

${\displaystyle \eta _{L/K}(s)=\int _{0}^{s}{\frac {dx}{|G_{0}:G_{x}|}}.}$

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 {G^t(L/K) : t ≥ −1} 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.

With the above set up of an abelian extension L/K, the theorem states that the jumps of the filtration {G^t(L/K) : t ≥ −1} are all rational integers.^[4][5]

Suppose G is cyclic of order ${\displaystyle p^{n}}$, ${\displaystyle p}$ residue characteristic and ${\displaystyle G(i)}$ be the subgroup of ${\displaystyle G}$ of order ${\displaystyle p^{n-i}}$. The theorem says that there exist positive integers ${\displaystyle i_{0},i_{1},...,i_{n-1}}$ such that

${\displaystyle G_{0}=\cdots =G_{i_{0}}=G=G^{0}=\cdots =G^{i_{0}}}$

${\displaystyle G_{i_{0}+1}=\cdots =G_{i_{0}+pi_{1}}=G(1)=G^{i_{0}+1}=\cdots =G^{i_{0}+i_{1}}}$

${\displaystyle G_{i_{0}+pi_{1}+1}=\cdots =G_{i_{0}+pi_{1}+p^{2}i_{2}}=G(2)=G^{i_{0}+i_{1}+1}}$

...

${\displaystyle G_{i_{0}+pi_{1}+\cdots +p^{n-1}i_{n-1}+1}=1=G^{i_{0}+\cdots +i_{n-1}+1}.}$^[4]

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 ${\displaystyle Q_{8}}$ of order 8 with

• ${\displaystyle G_{0}=Q_{8}}$
• ${\displaystyle G_{1}=Q_{8}}$
• ${\displaystyle G_{2}=\mathbb {Z} /2\mathbb {Z} }$
• ${\displaystyle G_{3}=\mathbb {Z} /2\mathbb {Z} }$
• ${\displaystyle G_{4}=1}$

The upper numbering then satisfies

• ${\displaystyle G^{n}=Q_{8}}$   for ${\displaystyle n\leq 1}$
• ${\displaystyle G^{n}=\mathbb {Z} /2\mathbb {Z} }$   for ${\displaystyle 1<n\leq 3/2}$
• ${\displaystyle G^{n}=1}$   for ${\displaystyle 3/2<n}$

so has a jump at the non-integral value ${\displaystyle n=3/2}$.

In 2025, mathematician Faruk Alpay proposed an integration of the Hasse–Arf theorem into a broader recursive framework known as Alpay Algebra, redefining the theorem's integral jumps as fixed points within recursive identity structures. This reinterpretation views the integral jumps of ramification filtrations as invariants in recursive processes, providing a modern algorithmic perspective to classical local class field theory.

• Alpay, Faruk (2025). "Arf Teorilerinin Alpay Cebiri ile Bütünleşik Matematiksel Çerçevesi". Zenodo. doi:10.5281/zenodo.15337176.

• Neukirch, Jürgen (1999). Algebraische Zahlentheorie. Grundlehren der mathematischen Wissenschaften. Vol. 322. Berlin: Springer-Verlag. ISBN 978-3-540-65399-8. MR 1697859. Zbl 0956.11021.
• Serre, Jean-Pierre (1979), Local Fields, Graduate Texts in Mathematics, vol. 67, translated by Greenberg, Marvin Jay, Springer-Verlag, ISBN 0-387-90424-7, MR 0554237, Zbl 0423.12016