Degeneration (algebraic geometry)

In algebraic geometry, a degeneration (or specialization) is the act of taking a limit of a family of varieties. Precisely, given a morphism

$\pi :𝒳\to C,$

of a variety (or a scheme) to a curve C with origin 0 (e.g., affine or projective line), the fibers

${\pi }^{-1}(t)$

form a family of varieties over C. Then the fiber ${\pi }^{-1}(0)$ may be thought of as the limit of ${\pi }^{-1}(t)$ as $t\to 0$. One then says the family ${\pi }^{-1}(t),t\ne 0$ degenerates to the special fiber ${\pi }^{-1}(0)$. The limiting process behaves nicely when $\pi$ is a flat morphism and, in that case, the degeneration is called a flat degeneration. Many authors assume degenerations to be flat.

When the family ${\pi }^{-1}(t)$ is trivial away from a special fiber; i.e., ${\pi }^{-1}(t)$ is independent of $t\ne 0$ up to (coherent) isomorphisms, ${\pi }^{-1}(t),t\ne 0$ is called a general fiber.

Degenerations of curves

Template:Expand section In the study of moduli of curves, the important point is to understand the boundaries of the moduli, which amounts to understand degenerations of curves.

Stability of invariants

Ruled-ness specializes. Precisely, Matsusaka'a theorem says

Let X be a normal irreducible projective scheme over a discrete valuation ring. If the generic fiber is ruled, then each irreducible component of the special fiber is also ruled.

Infinitesimal deformations

Let D = k[ε] be the ring of dual numbers over a field k and Y a scheme of finite type over k. Given a closed subscheme X of Y, by definition, an embedded first-order infinitesimal deformation of X is a closed subscheme XTemplate:' of Y ×_Spec(k) Spec(D) such that the projection Template:Math is flat and has X as the special fiber.

If Y = Spec A and Template:Math are affine, then an embedded infinitesimal deformation amounts to an ideal Template:Mvar of A[ε] such that Template:Math is flat over D and the image of Template:Mvar in A = A[ε]/ε is Template:Mvar.

In general, given a pointed scheme (S, 0) and a scheme X, a morphism of schemes Template:Pi: XTemplate:' → S is called the deformation of a scheme X if it is flat and the fiber of it over the distinguished point 0 of S is X. Thus, the above notion is a special case when S = Spec D and there is some choice of embedding.

See also

• Deformation theory
• Differential graded Lie algebra
• Kodaira–Spencer map
• Frobenius splitting
• Relative effective Cartier divisor

References

• M. Artin, Lectures on Deformations of Singularities – Tata Institute of Fundamental Research, 1976
• Template:Hartshorne AG
• E. Sernesi: Deformations of algebraic schemes
• M. Gross, M. Siebert, An invitation to toric degenerations
• M. Kontsevich, Y. Soibelman: Affine structures and non-Archimedean analytic spaces, in: The unity of mathematics (P. Etingof, V. Retakh, I.M. Singer, eds.), 321–385, Progr. Math. 244, Birkhäuser 2006.
• Karen E Smith, Vanishing, Singularities And Effective Bounds Via Prime Characteristic Local Algebra.
• V. Alexeev, Ch. Birkenhake, and K. Hulek, Degenerations of Prym varieties, J. Reine Angew. Math. 553 (2002), 73–116.

External links

• http://mathoverflow.net/questions/88552/when-do-infinitesimal-deformations-lift-to-global-deformations