# Arithmetic genus

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A numerical invariant of algebraic varieties (cf. Algebraic variety). For an arbitrary projective variety (over a field ) all irreducible components of which have dimension , and which is defined by a homogeneous ideal in the ring , the arithmetic genus is expressed using the constant term of the Hilbert polynomial of by the formula This classical definition is due to F. Severi . In the general case it is equivalent to the following definition: where is the Euler characteristic of the variety with coefficients in the structure sheaf . In this form the definition of the arithmetic genus can be applied to any complete algebraic variety, and this definition also shows the invariance of relative to biregular mappings. If is a non-singular connected variety, and is the field of complex numbers, then where is the dimension of the space of regular differential -forms on . Such a definition for was given by the school of Italian geometers. For example, if , then is the genus of the curve ; if , where is the irregularity of the surface , while is the geometric genus of .

For any divisor on a normal variety , O. Zariski (see ) defined the virtual arithmetic genus as the constant term of the Hilbert polynomial of the coherent sheaf corresponding to . If the divisors and are algebraically equivalent, one has The arithmetic genus is a birational invariant in the case of a field of characteristic zero; in the general case this has so far (1977) been proved for dimensions only.

How to Cite This Entry:
Arithmetic genus. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Arithmetic_genus&oldid=14873
This article was adapted from an original article by I.V. Dolgachev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article