Birkhoff–Grothendieck theorem

Short description: Classifies holomorphic vector bundles over the complex projective line

In mathematics, the Birkhoff–Grothendieck theorem classifies holomorphic vector bundles over the complex projective line. In particular every holomorphic vector bundle over ${\mathbb{ℂ}\mathbb{\mathbb{P}}}^1$ is a direct sum of holomorphic line bundles. The theorem was proved by Alexander Grothendieck (1957, Theorem 2.1),

${ℰ}\cong {𝒪}(a_1)\oplus \cdots \oplus {𝒪}(a_n).$

The notation implies each summand is a Serre twist some number of times of the trivial bundle. The representation is unique up to permuting factors.

Generalization

The same result holds in algebraic geometry for algebraic vector bundle over ${\mathbb{\mathbb{P}}}_k^1$ for any field $k$. It also holds for ${\mathbb{\mathbb{P}}}^1$ with one or two orbifold points, and for chains of projective lines meeting along nodes.

Applications

One application of this theorem is it gives a classification of all coherent sheaves on ${\mathbb{ℂ}\mathbb{\mathbb{P}}}^1$. We have two cases, vector bundles and coherent sheaves supported along a subvariety, so ${𝒪}(k),{{𝒪}}_{nx}$ where n is the degree of the fat point at $x\in {\mathbb{ℂ}\mathbb{\mathbb{P}}}^1$. Since the only subvarieties are points, we have a complete classification of coherent sheaves.

See also

• Algebraic geometry of projective spaces
• Euler sequence
• Splitting principle
• K-theory
• Jumping line

Further reading

• Huybrechts, Daniel (2004-11-18) (in en). Complex geometry: An introduction. Universitext. Springer Science+Business Media. ISBN 978-3540212904. https://link.springer.com/book/10.1007/b137952. 
• Okonek, Christian; Schneider, Michael; Spindler, Heinz (1980). Vector Bundles on Complex Projective Spaces. Modern Birkhäuser Classics. Birkhäuser Basel. doi:10.1007/978-3-0348-0151-5. ISBN 978-3-0348-0150-8. 
• Salamon, S. M.; Burstall, F. E. (1987). "Tournaments, Flags, and Harmonic Maps". Mathematische Annalen 277 (2): 249–266. doi:10.1007/BF01457363. http://eudml.org/doc/164249. 

External links

• Roman Bezrukavnikov. 18.725 Algebraic Geometry (LEC # 24 Birkhoff–Grothendieck, Riemann-Roch, Serre Duality) Fall 2015. Massachusetts Institute of Technology: MIT OpenCourseWare Creative Commons BY-NC-SA.