In mathematics, the Levi-Civita field, named after Tullio Levi-Civita, is a non-Archimedean ordered field; i.e., a system of numbers containing infinite and infinitesimal quantities. Each member can be constructed as a formal series of the form where are real numbers, is the set of rational numbers, and is to be interpreted as a positive infinitesimal. The support of, i.e., the set of indices of the nonvanishing coefficients must be a left-finite set: for any member of, there are only finitely many members of the set less than it; this restriction is necessary in order to make multiplication and division well defined and unique. The ordering is defined according to the dictionary ordering of the list of coefficients, which is equivalent to the assumption that is an infinitesimal. The real numbers are embedded in this field as series in which all of the coefficients vanish except.
the relation holds if and the least non-zero coefficient of is strictly positive.
Equipped with those operations and order, the Levi-Civita field is indeed an ordered field extension of where the series is a positive infinitesimal.
Properties and applications
The Levi-Civita field is real-closed, meaning that it can be algebraically closed by adjoining an imaginary unit, or by letting the coefficients be complex. It is rich enough to allow a significant amount of analysis to be done, but its elements can still be represented on a computer in the same sense that real numbers can be represented using floating point. It is the basis of automatic differentiation, a way to perform differentiation in cases that are intractable by symbolic differentiation or finite-difference methods. The Levi-Civita field is also Cauchy complete, meaning that relativizing the definitions of Cauchy sequence and convergent sequence to sequences of Levi-Civita series, each Cauchy sequence in the field converges. Equivalently, it has no proper dense ordered field extension. As an ordered field, it has a natural valuation given by the rational exponent corresponding to the first non zero coefficient of a Levi-Civita series. The valuation ring is that of series bounded by real numbers, the residue field is, and the value group is. The resulting valued field is Henselian but not spherically complete. Indeed, the field of Hahn series with real coefficients and value group is a proper immediate extension, containing series such as which are not in the Levi-Civita field.
Relations to other ordered fields
The Levi-Civita field is the Cauchy-completion of the field of Puiseux series over the field of real numbers, that is, it is a dense extension of without proper dense extension. Here is a list of some of its notable proper subfields and its proper ordered field extensions:
