Contents

Ingredients

Concepts

Constructions

Examples

Theorems

# Contents

## Idea

Complex algebraic K3-surfaces admit the structure of elliptic fibrations over the Riemann sphere $\mathbb{C}P^1$. Equipped with such, they are also called elliptic K3-surfaces.

## Properties

### Singular points

Counted with multiplicity, these elliptic fibrations have 24 singular points (where the elliptic curve-fiber degenerates, to a nodal curve or cuspidal curve).

(review in Huybrechts 16, Section 2-2.4, Propp 18, p. 4).

This number 24 of singularities-with-multiplicities is identified with the Euler number of the topological space underlying the (complex analytic) K3-surface (e.g. Shimada 00, p. 432 (10 of 24), Schütt-Shioda 09, Section 6.7, Theorem 6.10, Huybrechts 16, Chapter 11, Remark 1.12, review includes Marquart 02, Section A.3.3, p. 59).

### Classification

Up to isomorphism, there are a finite number of possible such elliptic fibrations.

## In F-theory

Elliptically fibered Calabi-Yau manifolds play a central role in F-theory:

In passing from M-theory to type IIA string theory, the locus of any Kaluza-Klein monopole in 11d becomes the locus of D6-branes in 10d. The locus of the Kaluza-Klein monopole in turn (as discussed there) is the locus where the $S^1_A$-circle fibration degenerates. Hence in F-theory this is the locus where the fiber of the $S^1_A \times S^1_B$-elliptic fibration degenerates to the nodal curve. Since the T-dual of D6-branes are D7-branes, it follows that D7-branes in F-theory “are” the singular locus of the elliptic fibration.

Now an elliptically fibered complex K3-surface

$\array{ T &\longrightarrow& K3 \\ && \downarrow \\ && \mathbb{C}\mathbb{P}^1 }$

may be parameterized via the Weierstrass elliptic function as the solution locus of the equation

$y^2 = x^3 + f(z) x + g(z)$

for $x,y,z \in \mathbb{C}\mathbb{P}^1$, with $f$ a polynomial of degree 8 and $g$ of degree twelve. The j-invariant of the complex elliptic curve which this parameterizes for given $z$ is

$j(\tau(z)) = \frac{4 (24 f)^3}{27 g^2 + 4 f^3} \,.$

The poles $j\to \infty$ of the j-invariant correspond to the nodal curve, and hence it is at these poles that the D7-branes are located.

Since the order of the poles is 24 (the polynomial degree of the discriminant $\Delta = 27 g^2 + 4 f^3$) there are necessarily 24 D7-branes (Sen 96, page 5, Sen 97b, see also Morrison 04, sections 8 and 17, Denef 08, around (3.41), Douglas-Park-Schnell 14).

Under T-duality this translates to 24 D6-branes in type IIA string theory on K3 (Vafa 96, Footnote 2 on p. 6).

Notice that the net charge of these 24 D7-branes is supposed to vanish, due to S-duality effects (e.g. Denef 08, below (3.41)).

## References

Basics:

Further review:

Classification:

• Viacheslav Nikulin, Elliptic fibrations on K3 surfaces, Proceedings of the Edinburgh Mathematical Society, Volume 57 Issue 1 (arXiv:1010.3904, doi:10.1017/S0013091513000953)

• O. Lecacheux, Weierstrass Equations for the Elliptic Fibrations of a K3 Surface In: Balakrishnan J., Folsom A., Lalín M., Manes M. (eds.) Research Directions in Number Theory Association for Women in Mathematics Series, vol 19. Springer (2019) (doi:10.1007/978-3-030-19478-9_4)

• Marie Bertin, Elliptic Fibrations on K3 surfaces, 2013 (pdf)

Discussion in the context of K3-spectra:

Last revised on February 7, 2021 at 14:47:26. See the history of this page for a list of all contributions to it.