View source for Infinitely near point

{{Short description|Concept in algebraic geometry}}
In [[algebraic geometry]], an '''infinitely near point''' of an algebraic surface ''S'' is a point on a surface obtained from ''S'' by repeatedly blowing up points. Infinitely near points of [[algebraic surface]]s were introduced by {{harvs|txt|first=Max|last=Noether|authorlink=Max Noether|year=1876}}.<ref>''Infinitely Near Points on Algebraic Surfaces'', Gino Turrin, ''American Journal of Mathematics'', Vol. 74, No. 1 (Jan., 1952), pp. 100–106</ref>

There are some other meanings of "infinitely near point". Infinitely near points can also be defined for higher-dimensional varieties: there are several inequivalent ways to do this, depending on what one is allowed to blow up. Weil gave a definition of infinitely near points of smooth varieties,<ref>[4] Weil, A., ''Theorie des points proches sur les variétés differentielles'', Colloque de Topologie et Geometrie Diferentielle, Strasbourg, 1953, 111–117; in his ''Collected Papers'' II. The notes to the paper there indicate this was a rejected project for the [[Bourbaki group]]. Weil references [[Pierre de Fermat]]'s approach to calculus, as well as the jets of [[Charles Ehresmann]]. For an extended treatment, see O. O. Luciano, ''Categories of multiplicative functors and Weil's infinitely near points'',  Nagoya Math. J.  109 (1988), 69–89 (online [https://projecteuclid.org/euclid.nmj/1118780892 here]) for a full discussion.</ref> though these are not the same as infinitely near points in algebraic geometry.
In the line of [[hyperreal number]]s, an extension of the [[real number]] line, two points are called infinitely near if their difference is [[infinitesimal]].

== Definition==

When [[blowing up]] is applied to a point ''P'' on a surface ''S'', the new surface ''S''* contains a whole curve ''C'' where ''P'' used to be. The points of ''C'' have the geometric interpretation as the tangent directions at ''P'' to ''S''. They can be called infinitely near to ''P'' as way of visualizing them on ''S'', rather than ''S''*. More generally this construction can be iterated by blowing up a point on the new curve ''C'', and so on.

An '''infinitely near point''' (of order ''n'') ''P''<sub>''n''</sub> on a surface ''S''<sub>0</sub> is given by a sequence of points ''P''<sub>0</sub>, ''P''<sub>1</sub>,...,''P''<sub>''n''</sub> on surfaces ''S''<sub>0</sub>, ''S''<sub>1</sub>,...,''S''<sub>''n''</sub> such that  ''S''<sub>''i''</sub> is given by blowing up ''S''<sub>''i''–1</sub> at the point ''P''<sub>''i''–1</sub> and ''P''<sub>i</sub> is a point of the surface ''S''<sub>i</sub> with image ''P''<sub>''i''–1</sub>.

In particular the points of the surface ''S'' are the infinitely near points on ''S'' of order 0.

Infinitely near points correspond to 1-dimensional valuations of the function field of ''S'' with 0-dimensional center, and in particular correspond to some of the points of the [[Zariski–Riemann surface]]. (The 1-dimensional valuations with 1-dimensional center correspond to irreducible curves of ''S''.) It is also possible to iterate the construction infinitely often, producing an infinite sequence ''P''<sub>0</sub>, ''P''<sub>1</sub>,... of infinitely near points. These infinite sequences correspond to the 0-dimensional valuations of the function field of the surface, which correspond to the "0-dimensional" points of the [[Zariski–Riemann surface]].

==Applications==

If ''C'' and ''D'' are distinct irreducible curves on a smooth surface ''S'' intersecting at a point ''p'', then the multiplicity of their intersection at ''p'' is given by 
:<math>\sum_{x \text{ infinitely near }p} m_x(C)m_x(D)</math>
where ''m''<sub>''x''</sub>(''C'') is the multiplicity of ''C'' at ''x''. In general this is larger than ''m''<sub>''p''</sub>(''C'')''m''<sub>''p''</sub>(''D'') if ''C'' and ''D'' have a common tangent line at ''x'' so that they also intersect at infinitely near points of order greater than&nbsp;0, for example if ''C'' is the line ''y''&nbsp;=&nbsp;0 and ''D'' is the parabola ''y''&nbsp;=&nbsp;''x''<sup>2</sup> and ''p''&nbsp;=&nbsp;(0,0).

The genus of ''C'' is given by 
:<math> g(C)=g(N)+\sum_{\text{infinitely near points }x}m_x(m_x-1)/2</math>
where ''N'' is the normalization of ''C'' and ''m''<sub>''x''</sub> is the multiplicity of the infinitely near point ''x'' on ''C''.

==References==

<references/>
*{{citation|first=M. |last=Noether|title=Ueber die singularen Werthsysteme einer algebraischen Function und die singularen Punkte einer algebraischen Curve|journal= Mathematische Annalen
|volume= 9 |year=1876|issue=2 |pages=166–182|doi=10.1007/BF01443372|s2cid=120376948}}

[[Category:Geometry]]
[[Category:Differential calculus]]
[[Category:Nonstandard analysis]]
[[Category:Birational geometry]]