nLab nodal curve




A nodal singularity of an algebraic curve is one of the form parameterized by the equation xy=0x y = 0. A nodal curve is a curve with a nodal singularity.

(e.g. Hain 08, p. 45)

The nodal cubic

The nodal cubic curve (over some base) is (see at elliptic curve – Nodal curves and cuspidal curves for notation and background) the solution to the Weierstrass equation for which the discriminant vanishes, but the modular invariant c 4c_4 does not.

This is equivalently the limit in which the j-invariant j=c 4 3Δj = \frac{c_4^3}{\Delta} goes to \infty.

Properties of the nodal cubic

Compactified moduli stack of elliptic curves and the Tate curve

The nodal cubic curve is not an elliptic curve, as it is singular, but adding it to the moduli stack of elliptic curves ell\mathcal{M}_{ell} produces the compactification ell¯\mathcal{M}_{\overline{ell}} which is often relevant.

The formal neighbourhood of the nodal curve in ell¯\mathcal{M}_{\overline{ell}} is the Tate curve.

Over the complex numbers

Over the complex numbers, the nodal cubic E 0E_0 is the Riemann sphere/complex projective space 1\mathbb{P}^1 with the pole points 0 and \infty identified (hence is a “complex torus with one cycle shrunk away”). Precisely: there is a holomorphic function

𝒫 1𝒫 2 \mathcal{P}^1 \to \mathcal{P}^2

which is onto E 0𝒫 2E_0 \subset \mathcal{P}_2, sends the unit of the multiplicative group 1 × 11 \in \mathbb{C}^\times \hookrightarrow \mathbb{P}^1 to the unit of E 0E_0, maps 0, 10,\infty \in \mathbb{P}^1 both to the nodal singular double point of E 0E_0 and is injective away from these points (e.g. Hain 08, exercise 47, p. 45)

× E 0 = 1{0,} 2. \array{ \mathbb{C}^\times &\hookrightarrow& E_0 \\ \downarrow^\mathrlap{=} && \downarrow \\ \mathbb{P}^1-\{0,\infty\} &\longrightarrow& \mathbb{P}^2 } \,.

ADE 2Cycle

Formal group and height

The formal group associated with a nodal cubic curve is of height 1. Indeed, passing to the point of the nodal curve in ell¯\mathcal{M}_{\overline{ell}} connects elliptic cohomology (of chromatic level 2) to topological K-theory (of chromatic level 1). For more on this see at moduli stack of tori and at tmf – Properties – Maps to K-theory and to Tate K-theory.

Relation to gauge enhancement in F-theory

In F-theory the nodal singularity locus of the given elliptic fibration is interpreted as the locus of D7-branes, see at F-brane scan.


Discussion over the complex numbers is in

Last revised on December 21, 2020 at 12:17:04. See the history of this page for a list of all contributions to it.