# Berkovich space

In mathematics, a **Berkovich space**, introduced by Berkovich (1990), is a version of an analytic space over a non-Archimedean field (e.g. p-adic field), refining Tate's notion of a rigid analytic space.

## Motivation

In the complex case we start by defining the complex affine space to be For each we define the ring of analytic functions on to be the ring of holomorphic function, i.e. functions on that can be written as a convergent power series in a neighborhood of each point.

We then define a local model space for to be

with A **complex analytic space** is a locally ringed -space which is locally isomorphic to a local model space.

When is a complete non-Archimedean field, we have that is totally disconnected. In such a case, if we continue with the same definition as in the complex case, we wouldn't get a good analytic theory. Berkovich gave a definition which gives nice analytic spaces over such , and also gives back the usual definition over

In addition to defining analytic functions over non-Archimedean fields, Berkovich spaces also have a nice underlying topological space.

## Berkovich spectrum

A **seminorm** on a ring is a non-constant function from to the non-negative reals such that

It is called **multiplicative** if and is called a **norm** if implies .

If is a normed ring with norm then the **Berkovich spectrum** of , is the set of multiplicative seminorms on that are bounded by the norm of .

The Berkovich spectrum is equipped with the weakest topology such that for any the map

is continuous.

The Berkovich spectrum of a normed ring is non-empty if is non-zero and is compact if is complete.

If is a point of the spectrum of then the elements with form a prime ideal of . The quotient field of the quotient by this prime ideal is a normed field, whose completion is a complete field with a multiplicative norm, this field is denoted by and the image of an element is denoted by . The field is generated by the image of .

Conversely a bounded map from to a complete normed field with a multiplicative norm that is generated by the image of gives a point in the spectrum of .

The spectral radius of

is equal to

### Examples

- The spectrum of a field complete with respect to a valuation is a single point corresponding to its valuation.

- If is a commutative C*-algebra then the Berkovich spectrum is the same as the Gelfand spectrum. A point of the Gelfand spectrum is essentially a homomorphism to , and its absolute value is the corresponding seminorm in the Berkovich spectrum.

- Ostrowski's theorem shows that the Berkovich spectrum of the integers (with the usual norm) consists of the powers of the usual valuation, for a prime or . If is a prime then and if then When these all coincide with the trivial valuation that is on all non-zero elements. For each (prime or infinity) we get a branch which is homeomorphic to a real interval, the branches meet at the point corresponding to the trivial valuation. The open neighborhoods of the trivial valuations are such that they contain all but finitely many branches, and their intersection with each branch is open.

## Berkovich affine space

If is a field with a valuation, then the ** n-dimensional Berkovich affine space** over , is the set of multiplicative seminorms on extending the norm on .

The Berkovich affine space is equipped with the weakest topology such that for any the map taking to is continuous. This is not a Berkovich spectrum, but is an increasing union of the Berkovich spectrums of rings of power series that converge in some ball (so it is locally compact).

We define an analytic function on an open subset is a map

with which is a local limit of rational functions, i.e., such that every point has an open neighborhood with the following property:

Continuing with the same definitions as in the complex case one can define the ring of analytic function, local model space, and analytic spaces over any field with a valuation (one can also define similar objects over normed rings). This gives reasonable objects for fields complete with respect to a nontrivial valuation and the ring of integers

In the case where this will give the same objects as described in the motivation section.

These analytic spaces are not all analytic spaces over non-Archimedean fields.

### Berkovich affine line

The **1-dimensional Berkovich affine space** is called the **Berkovich affine line**. When algebraically closed non-Archimedean field, complete with respects to its valuation, one can describe all the points of the affine line.

There is a canonical embedding .

The space is a locally compact, Hausdorff, and uniquely path-connected topological space which contains as a dense subspace.

One can also define the Berkovich projective line by adjoining to in a suitable manner a point at infinity. The resulting space is a compact, Hausdorff, and uniquely path-connected topological space which contains as a dense subspace.

## References

- Baker, Matthew; Conrad, Brian; Dasgupta, Samit; Kedlaya, Kiran S.; Teitelbaum, Jeremy (2008), Thakur, Dinesh S.; Savitt, David (eds.),
*p-adic geometry*, University Lecture Series,**45**, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-4468-7, MR 2482343 - Baker, Matthew; Rumely, Robert (2010),
*Potential theory and dynamics on the Berkovich projective line*, Mathematical Surveys and Monographs,**159**, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-4924-8, MR 2599526 - Berkovich, Vladimir G. (1990),
*Spectral theory and analytic geometry over non-Archimedean fields*, Mathematical Surveys and Monographs,**33**, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-1534-2, MR 1070709 - Berkovich, Vladimir G. (1993), "Étale cohomology for non-Archimedean analytic spaces",
*Publications Mathématiques de l'IHÉS*(78): 5–161, ISSN 1618-1913, MR 1259429