## Asian Journal of Mathematics

- Asian J. Math.
- Volume 16, Number 3 (2012), 515-560.

### Fano Threefolds of Genus 6

#### Abstract

Ideas and methods of *Clemens C. H., Griffiths Ph. The intermediate Jacobian of
a cubic threefold* are applied to a Fano threefold $X$ of genus 6 — intersection of $G(2, 5) \subset P^9$ with
$P^7$ and a quadric. Main results:

1. The Fano surface $F(X)$ of $X$ is smooth and irreducible. Hodge numbers and some other invariants of $F(X)$ are calculated.

2. Tangent bundle theorem for $X$ is proved, and its geometric interpretation is given. It is shown that $F(X)$ defines $X$ uniquely.

3. The Abel-Jacobi map $\Phi : \operatorname{Alb} F(X) \to J^3(X)$ is an isogeny.

4. As a necessary step of calculation of $h^{1,0}(F(X))$ we describe a special intersection of 3 quadrics in $P^6$ (having 1 double point) whose Hesse curve is a smooth plane curve of degree 6.

5. $\operatorname{im} \Phi(F(X)) \subset J^3(X)$ is algebraically equivalent to $\frac{2\Theta^8}{8!}$ where $\Theta \subset J^3(X)$ is a Poincaré divisor (a sketch of the proof).

#### Article information

**Source**

Asian J. Math., Volume 16, Number 3 (2012), 515-560.

**Dates**

First available in Project Euclid: 23 November 2012

**Permanent link to this document**

https://projecteuclid.org/euclid.ajm/1353696020

**Mathematical Reviews number (MathSciNet)**

MR2989233

**Zentralblatt MATH identifier**

1263.14040

**Subjects**

Primary: 14J30: $3$-folds [See also 32Q25] 14J45: Fano varieties

Secondary: 14J25: Special surfaces {For Hilbert modular surfaces, see 14G35} 14C30: Transcendental methods, Hodge theory [See also 14D07, 32G20, 32J25, 32S35], Hodge conjecture

**Keywords**

Fano threefolds Fano surfaces middle Jacobian tangent bundle theorem global Torelli theorem

#### Citation

Logachev, Dmitry. Fano Threefolds of Genus 6. Asian J. Math. 16 (2012), no. 3, 515--560. https://projecteuclid.org/euclid.ajm/1353696020