Arithmetic surface

In more detail, an arithmetic surface ${\displaystyle S}$  (over the Dedekind domain ${\displaystyle R}$ ) is a scheme with a morphism ${\displaystyle p:S\rightarrow \mathrm {Spec} (R)}$  with the following properties: ${\displaystyle S}$  is integral, normal, excellent, flat and of finite type over ${\displaystyle R}$  and the generic fiber is a non-singular, connected projective curve over ${\displaystyle \mathrm {Frac} (R)}$  and for other ${\displaystyle t}$  in ${\displaystyle \mathrm {Spec} (R)}$ ,

${\displaystyle S{\underset {\mathrm {Spec} (R)}{\times }}\mathrm {Spec} (k_{t})}$ 

is a union of curves over ${\displaystyle R/t}$ .^[1]

In even more generality, arithmetic surfaces can be defined over Dedekind schemes, a typical example of which is the spectrum of the ring of integers of a number field (which is the case above). An arithmetic surface is then a regular fibered surface over a Dedekind scheme of dimension one.^[2] This generalisation is useful, for example, it allows for base curves which are smooth and projective over finite fields, which is important in positive characteristic.

Arithmetic surfaces over Dedekind domains are the arithmetic analogue of fibered surfaces over algebraic curves.^[1] Arithmetic surfaces arise primarily in the context of number theory.^[3] In fact, given a curve ${\displaystyle X}$  over a number field ${\displaystyle S}$ , there exists an arithmetic surface over the ring of integers ${\displaystyle O_{K}}$  whose generic fiber is isomorphic to ${\displaystyle X}$ . In higher dimensions one may also consider arithmetic schemes.^[3]

Arithmetic surfaces have dimension 2 and relative dimension 1 over their base.^[1]

We can develop a theory of Weil divisors on arithmetic surfaces since every local ring of dimension one is regular. This is briefly stated as "arithmetic surfaces are regular in codimension one."^[1] The theory is developed in Hartshorne's Algebraic Geometry, for example.^[4]

The projective line over Dedekind domain ${\displaystyle R}$  is a smooth, proper arithmetic surface over ${\displaystyle R}$ . The fiber over any maximal ideal ${\displaystyle {\mathfrak {m}}}$  is the projective line over the field ${\displaystyle R/{\mathfrak {m}}.}$ ^[5]

Néron models for elliptic curves, initially defined over a global field, are examples of this construction, and are much studied examples of arithmetic surfaces.^[6] There are strong analogies with elliptic fibrations.

Given two distinct irreducible divisors and a closed point on the special fiber of an arithmetic surface, we can define the local intersection index of the divisors at the point as you would for any algebraic surface, namely as the dimension of a certain quotient of the local ring at a point.^[7] The idea is then to add these local indices up to get a global intersection index. The theory starts to diverge from that of algebraic surfaces when we try to ensure linear equivalent divisors give the same intersection index, this would be used, for example in computing a divisors intersection index with itself. This fails when the base scheme of an arithmetic surface is not "compact". In fact, in this case, linear equivalence may move an intersection point out to infinity.^[8] A partial resolution to this is to restrict the set of divisors we want to intersect, in particular forcing at least one divisor to be "fibral" (every component is a component of a special fiber) allows us to define a unique intersection pairing having this property, amongst other desirable ones.^[9] A full resolution is given by Arakelov theory.

Arakelov theory offers a solution to the problem presented above. Intuitively, fibers are added at infinity by adding a fiber for each archimedean absolute value of K. A local intersection pairing that extends to the full divisor group can then be defined, with the desired invariance under linear equivalence.^[10]

• Glossary of arithmetic and Diophantine geometry
• Arakelov theory
• Néron model

• Hartshorne, Robin (1977). Algebraic Geometry. Graduate Texts in Mathematics. Vol. 52. Springer-Verlag. ISBN 0-387-90244-9. Zbl 0367.14001.
• Qing Liu (2002). Algebraic Geometry and Arithmetic Curves. Oxford University Press. ISBN 0-19-850284-2.
• Eisenbud, David; Harris, Joe (2000). The Geometry of Schemes. Graduate Texts in Mathematics. Vol. 197. Springer-Verlag. ISBN 0-387-98637-5. Zbl 0960.14002.
• Lang, Serge (1988). Introduction to Arakelov theory. New York: Springer-Verlag. ISBN 0-387-96793-1. MR 0969124. Zbl 0667.14001.
• Silverman, Joseph H. (1994). Advanced Topics in the Arithmetic of Elliptic Curves. Graduate Texts in Mathematics. Vol. 151. Springer-Verlag. ISBN 0-387-94328-5. Zbl 0911.14015.
• Soulé, C.; Abramovich, Dan; Burnol, J.-F.; Kramer, Jürg (1992). Lectures on Arakelov geometry. Cambridge Studies in Advanced Mathematics. Vol. 33. Joint work with H. Gillet. Cambridge: Cambridge University Press. ISBN 0-521-47709-3. Zbl 0812.14015.