Intersection index (in algebraic geometry)

The number of points in the intersection of divisors (cf. Divisor) in an -dimensional algebraic variety with allowance for the multiplicities of these points. More precisely, let be an -dimensional non-singular algebraic variety over a field , and let be effective divisors in that intersect in a finite number of points. The local index (or multiplicity) of intersection of these divisors at a point is the integer

where is the local equation for the divisor in the local ring . In the complex case, the local index coincides with the residue of the form , and also with the degree of the germ of the mapping (cf. Degree of a mapping)

The global intersection index is the sum of the local indices over all points of the intersection . If this intersection is not empty, then .

See also Intersection theory.

References

[a1]	I.R. Shafarevich, "Basic algebraic geometry" , Springer (1974) pp. Chapt. IV (Translated from Russian)