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 (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 or ). 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.
The field of Hahn series (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
with such that the support of f is well-ordered. The sum and product of
are given by
(in the latter, the sum over values such that and is finite because a well-ordered set cannot contain an infinite decreasing sequence).
For example, is a Hahn series (over any field) because the set of rationals
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 so it is algebraic over .)
The valuation of a non zero Hahn series
is defined as the smallest such that (in other words, the smallest element of the support of ): this makes into a spherically complete valued field with value group and residue field (justifying a posteriori the terminology). In fact, if has characteristic zero, then is up to (non unique) isomorphism the only spherically complete valued field with residue field and value group . The valuation defines a topology on . If , then v corresponds to an ultrametric absolute value , with respect to which 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 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").
If K is algebraically closed (but not necessarily of characteristic zero) and Γ is divisible, then is algebraically closed. Thus, the algebraic closure of is contained in , where is the algebraic closure of (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 in positive characteristic as a subset of .
If K is an ordered field then 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 is itself real closed. 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).
If κ is an infinite regular cardinal, one can consider the subset of consisting of series whose support set 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 : e.g., it is algebraically closed or real closed when K is so and Γ is divisible.
One can define a notion of summable families in . If is a set and is a family of Hahn series , then we say that is summable if the set is well-ordered, and each set for is finite.
We may then define the sum as the Hahn series
If are summable, then so are the families , and we have
This notion of summable family does not correspond to the notion of convergence in the valuation topology on . For instance, in , the family is summable but the sequence does not converge.
Let < and let denote the ring of real-valued functions which are analytic on a neighborhood of .
If contains , then we can evaluate every element of at every element of of the form , where the valuation of is strictly positive. Indeed, the family is always summable, so we can define . This defines a ring morphism .
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: 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 where now 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 or its spherical completion.