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Register a new userA classification of terminal quartic 3-folds and applications to rationality questions
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Anne-Sophie Kaloghiros
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This paper studies the birational geometry of terminal Gorenstein Fano 3-folds. If Y is not Q-factorial, in most cases, it is possible to describe explicitly the divisor class group Cl Y by running a Minimal Model Program (MMP) on X, a small Q-factorialisation of Y. In this case, Weil non-Cartier divisors are generated by topological traces of K-negative extremal contractions on X. One can show, as an application of these methods, that a number of families of non-factorial terminal Gorenstein Fano 3-folds are rational. In particular, I give some examples of rational quartic hypersurfaces with Cl Y of rank 2, and show that when Cl Y has rank greater than 6, Y is always rational.
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In this thesis, I determine a bound on the defect of terminal Gorenstein quartic 3-folds. More generally, I study the defect of terminal Gorenstein Fano 3-folds of Picard rank 1 and genus at least 3. I state a geometric motivation of non Q-factoriality in the case of quartics.
Let $Xsubset mathbb{P}^4$ be a terminal factorial quartic $3$-fold. If $X$ is non-singular, $X$ is emph{birationally rigid}, i.e. the classical MMP on any terminal $mathbb{Q}$-factorial projective variety $Z$ birational to $X$ always terminates with $X$. This no longer holds when $X$ is singular, but very few examples of non-rigid factorial quartics are known. In this article, we first bound the local analytic type of singularities that may occur on a terminal factorial quartic hypersurface $Xsubset mathbb{P}^4$. A singular point on such a hypersurface is either of type $cA_n$ ($ngeq 1$), or of type $cD_m$ ($mgeq 4$), or of type $cE_6, cE_7$ or $cE_8$. We first show that if $(P in X)$ is of type $cA_n$, $n$ is at most $7$, and if $(P in X)$ is of type $cD_m$, $m$ is at most $8$. We then construct examples of non-rigid factorial quartic hypersurfaces whose singular loci consist (a) of a single point of type $cA_n$ for $2leq nleq 7$ (b) of a single point of type $cD_m$ for $m= 4$ or $5$ and (c) of a single point of type $cE_k$ for $k=6,7$ or $8$.
In this note we speculate about the structure of maximal product subvarieties in moduli stacks of Calabi-Yau manifolds. We discuss examples for quintic hypersurfaces in the four dimensional projective space.
Nonsingular projective 3-folds $V$ of general type can be naturally classified into 18 families according to the {it pluricanonical section index} $delta(V):=text{min}{m|P_mgeq 2}$ since $1leq delta(V)leq 18$ due to our previous series (I, II). Based on our further classification to 3-folds with $delta(V)geq 13$ and an intensive geometrical investigation to those with $delta(V)leq 12$, we prove that $text{Vol}(V) geq frac{1}{1680}$ and that the pluricanonical map $Phi_{m}$ is birational for all $m geq 61$, which greatly improves known results. An optimal birationality of $Phi_m$ for the case $delta(V)=2$ is obtained. As an effective application, we study projective 4-folds of general type with $p_ggeq 2$ in the last section.
This paper studies the defect of terminal Gorenstein Fano 3 folds. I determine a bound on the defect of terminal Gorenstein Fano 3-folds of Picard rank 1 that do not contain a plane. I give a general bound for quartic 3-folds and indicate how to study the defect of terminal Gorenstein Fano 3-folds with Picard rank 1 that contain a plane.
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