Contraction morphism

In algebraic geometry, a contraction morphism is a surjective projective morphism $f:X\to Y$ between normal projective varieties (or projective schemes) such that $f_{*}{\mathcal {O}}_{X}={\mathcal {O}}_{Y}$ or, equivalently, the geometric fibers are all connected (Zariski's connectedness theorem). It is also commonly called an algebraic fiber space, as it is an analog of a fiber space in algebraic topology.

By the Stein factorization, any surjective projective morphism is a contraction morphism followed by a finite morphism.

Examples include ruled surfaces and Mori fiber spaces.

Birational perspective

The following perspective is crucial in birational geometry (in particular in Mori's minimal model program).

Let X be a projective variety and ${\overline {NS}}(X)$ the closure of the span of irreducible curves on X in $N_{1}(X)$ = the real vector space of numerical equivalence classes of real 1-cycles on X. Given a face F of ${\overline {NS}}(X)$ , the contraction morphism associated to F, if it exists, is a contraction morphism $f:X\to Y$ to some projective variety Y such that for each irreducible curve $C\subset X$ , $f(C)$ is a point if and only if $[C]\in F$ .^[1] The basic question is which face F gives rise to such a contraction morphism (cf. cone theorem).

References

^ Kollár & Mori 1998, Definition 1.25.

Kollár, János; Mori, Shigefumi (1998), Birational geometry of algebraic varieties, Cambridge Tracts in Mathematics, vol. 134, Cambridge University Press, ISBN 978-0-521-63277-5, MR 1658959
Robert Lazarsfeld, Positivity in Algebraic Geometry I: Classical Setting (2004)