The field of Hahn series (in the indeterminate ) over a field and with value group (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
and
are given by
and
(in the latter, the sum over values such that , and is finite because a well-ordered set cannot contain an infinite decreasing sequence).[2]
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 characteristicp, then this Hahn series satisfies the equation so it is algebraic over .)
Properties
edit
Properties of the valued field
edit
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 completevalued 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 .[3]
The valuation defines a topology on . If , then 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"[4]).
Algebraic properties
edit
If is algebraically closed (but not necessarily of characteristic zero) and is divisible, then is algebraically closed.[5] Thus, the algebraic closure of is contained in , where is the algebraic closure of (when 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 .[6]
If is an ordered field then is totally ordered by making the indeterminate infinitesimal (greater than 0 but less than any positive element of ) or, equivalently, by using the lexicographic order on the coefficients of the series. If is real-closed and is divisible then is itself real-closed.[7] 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[8]).
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 is so and is divisible.[9]
Summable families
edit
Summable families
edit
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[10]
and
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.
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,[11] so we can define . This defines a ring homomorphism .
Hahn–Witt series
edit
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:[12] for example, over a finite fieldK 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.[13]
The field of surreal numbers can be regarded as a field of Hahn series with real coefficients and value group the surreal numbers themselves.[14]
The Levi-Civita field can be regarded as a subfield of , with the additional imposition that the coefficients be a left-finite set: the set of coefficients less than a given coefficient is finite.
The field of transseries is a directed union of Hahn fields (and is an extension of the Levi-Civita field). The construction of resembles (but is not literally) , .
Hahn, Hans (1907), "Über die nichtarchimedischen Größensysteme", Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften, Wien, Mathematisch – Naturwissenschaftliche Klasse (Wien. Ber.), 116: 601–655, JFM 38.0501.01 (reprinted in: Hahn, Hans (1995), Gesammelte Abhandlungen I, Springer-Verlag)
Alling, Norman L. (1987). Foundations of Analysis over Surreal Number Fields. Mathematics Studies. Vol. 141. North-Holland. ISBN 0-444-70226-1. Zbl 0621.12001.