A Hahn series is a generalization of a formal power series which allows non-integer exponents and transfinite sums, as long as the exponents are well-ordered.
Let be a linearly ordered abelian group, and a field.
The ring of Hahn series with value group? , denoted (or sometimes or ), is the ring of functions such that is well-ordered as as a subset of (or, sometimes, as a subset of the opposite poset ). Addition is defined pointwise, and multiplication is defined by the convolution product:
Notationally, we may write a Hahn series as .
The ring is a field. If is algebraically closed, then is algebraically closed provided that is divisible.
As a corollary, if is divisible, is real closed if is real closed. This is because the adjunction of a square root of would make algebraically closed, since this gives the same result as constructing the Hahn series over the algebraically closed field .
The multiplicative valuation of an element is the least for which . This yields a valuation ring, such a valuation ring determines and is determined by a valuation on a field. The field is a complete ultrametric space (and indeed a spherically complete field?) with respect to this valuation, but unlike for formal power series the formal series do not actually “converge” in this metric.
If , then we have Hahn series such as
Via Conway normal forms, the ring of Hahn series with and with value group the surreal numbers is isomorphic to the surreal numbers themselves. In other words, the surreals are a fixed point of the “Hahn series with real coefficients” functor on the category of abelian groups. However, the surreals are not the initial fixed point of this functor, since there are surreals that appear as exponents in their own Conway normal form — for instance, the ∞-numbers.
Well-based transseries can be constructed by iterating the Hahn series construction.
If , then a Hahn series is precisely a formal Laurent series?, i.e. .
If , then properly contains the Levi-Civita field, as the subclass of those Hahn series whose support is finite below any fixed rational.
More generally, any Hahn series field contains the Novikov field of with value group .
Last revised on July 22, 2019 at 21:44:00. See the history of this page for a list of all contributions to it.