# Hahn series

In mathematics, Hahn series (sometimes also known as Hahn-Mal'cev-Neumann series) are a type of formal infinite series. They are a generalization of Puiseux series (themselves a generalization of formal power series) and were first introduced by Hans Hahn in 1907[1] (and then further generalized by Anatoly Maltsev and Bernhard Neumann to a non-commutative setting). They allow for arbitrary exponents of the indeterminate so long as the set supporting them forms a well-ordered subset of the value group (typically $\mathbb{Q}$ or $\mathbb{R}$). Hahn series were first introduced, as groups, in the course of the proof of the Hahn embedding theorem and then studied by him as fields in his approach to Hilbert's seventeenth problem.

## Formulation

The field of Hahn series $K[[T^\Gamma]]$ (in the indeterminate T) over a field K and with value group Γ (an ordered group) is the set of formal expressions of the form $f = \sum_{e\in\Gamma} c_e T^e$ with $c_e \in K$ such that the support $\{ e\in \Gamma : c_e \neq 0 \}$ of f is well-ordered. The sum and product of $f = \sum_{e\in\Gamma} c_e T^e$ and $g = \sum_{e\in\Gamma} d_e T^e$ are given by $f+g = \sum_{e\in\Gamma} (c_e+d_e) T^e$ and $fg = \sum_{e\in\Gamma} \sum_{e'+e''=e} c_{e'} d_{e''} T^e$ (in the latter, the sum $\sum_{e'+e''=e} \cdot$ over values $(e',e'')$ such that $c_{e'} \neq 0$ and $d_{e''} \neq 0$ is finite because a well-ordered set cannot contain an infinite decreasing sequence).

For example, $T^{-1/p} + T^{-1/p^2} + T^{-1/p^3} + \cdots$ is a Hahn series (over any field) because the set of rationals $\{-\frac{1}{p}, -\frac{1}{p^2}, -\frac{1}{p^3}, \ldots\}$ is well-ordered; it is not a Puiseux series because the denominators in the exponents are unbounded. (And if the base field K has characteristic p, then this Hahn series satisfies the equation $X^p-X = T^{-1}$ so it is algebraic over $K(T)$.)

The valuation $v(f)$ of $f = \sum_{e\in\Gamma} c_e T^e$ is defined as the smallest e such that $c_e \neq 0$ (in other words, the smallest element of the support of f): this makes $K[[T^\Gamma]]$ into a spherically complete valued field with value group Γ (justifying a posteriori the terminology); in particular, v defines a topology on $K[[T^\Gamma]]$. If $\Gamma \subseteq \mathbb{R}$, then v corresponds to an ultrametric) absolute value $|f| = \exp(-v(f))$, with respect to which $K[[T^\Gamma]]$ is a complete metric space. However, unlike in the case of formal Laurent series or Puiseux series, the formal sums used in defining the elements of the field do not converge: in the case of $T^{-1/p} + T^{-1/p^2} + T^{-1/p^3} + \cdots$ for example, the absolute values of the terms tend to 1 (because their valuations tend to 0), so the series is not convergent (such series are sometimes known as "pseudo-convergent"[2]).

If K is algebraically closed (but not necessarily of characteristic zero) and Γ is divisible, then $K[[T^\Gamma]]$ is algebraically closed.[3] Thus, the algebraic closure of $K((T))$ is contained in $K[[T^{\mathbb{Q}}]]$ (when K is of characteristic zero, it is exactly the field of Puiseux series): in fact, it is possible to give a somewhat analogous description of the algebraic closure of $K((T))$ in positive characteristic as a subset of $K[[T^\Gamma]]$.[4]

If K is an ordered field then $K[[T^\Gamma]]$ is totally ordered by making the indeterminate T infinitesimal (greater than 0 but less than any positive element of K) or, equivalently, by using the lexicographic order on the coefficients of the series. If K is real-closed and Γ is divisible then $K[[T^\Gamma]]$ is itself real closed.[5] This fact can be used to analyse (or even construct) the field of surreal numbers (which is isomorphic, as an ordered field, to the field of Hahn series with real coefficients and value group the surreal numbers themselves[6]).

If κ is an infinite regular cardinal, one can consider the subset of $K[[T^\Gamma]]$ consisting of series whose support set $\{ e\in \Gamma : c_e \neq 0 \}$ has cardinality (strictly) less than κ: it turns out that this is also a field, with much the same algebraic closedness properties as the full $K[[T^\Gamma]]$: e.g., it is algebraically closed or real closed when K is so and Γ is divisible.[7]

## Hahn-Witt series

The construction of Hahn series can be combined with Witt vectors (at least over a perfect field) to form “twisted Hahn series” or “Hahn-Witt series”:[8] for example, over a finite field K of characteristic p (or their algebraic closure), the field of Hahn-Witt series with value group Γ (containing the integers) would be the set of formal sums $\sum_{e\in\Gamma} c_e p^e$ where now $c_e$ are Teichmüller representatives (of the elements of K) which are multiplied and added in the same way as in the case of ordinary Witt vectors (which is obtained when Γ is the group of integers). When Γ is the group of rationals or reals and K is the algebraic closure of the finite field with p elements, this construction gives a (ultra)metrically complete algebraically closed field containing the p-adics, hence a more or less explicit description of the field $\mathbb{C}_p$ or its spherical completion.[9]

## References

• Hahn, Hans (1907), "Über die nichtarchimedischen Größensysteme", Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften, Wien, Mathematisch - Naturwissenschaftliche Klasse (Wien. Ber.) 116: 601–655 (reprinted in: Hahn, Hans (1995), Gesammelte Abhandlungen I, Springer-Verlag)
• Alling, Norman L. (1987). Foundations of Analysis over Surreal Number Fields. Mathematics Studies 141. North-Holland. ISBN 0-444-70226-1.
• Poonen, Bjorn (1993), "Maximally complete fields", Enseign. Math. 39: 87–106
• Kedlaya, Kiran Sridhara (2001), "The algebraic closure of the power series field in positive characteristic", Proc. Amer. Math. Soc. 129: 3461–3470, doi:10.1090/S0002-9939-01-06001-4
• Kedlaya, Kiran Sridhara (2001), "Power Series and 𝑝-adic Algebraic Closures", J. Number Theory 89: 324–339, doi:10.1006/jnth.2000.2630

## Notes

1. ^ Hahn (1907)
2. ^ Kaplansky (1942, Duke Math. J., definition on p.303)
3. ^ MacLane (1939, Bull. Amer. Math. Soc., theorem 1 (p.889))
4. ^ Kedlaya (2001, Proc. Amer. Math. Soc.)
5. ^ Alling (1987, §6.23, (2) (p.218))
6. ^ Alling (1987, theorem of §6.55 (p. 246))
7. ^ Alling (1987, §6.23, (3) and (4) (p.218–219))
8. ^ Kedlaya (2001, J. Number Theory)
9. ^ Poonen (1993)