1 Introduction
Throughout this paper, we work over the field of complex numbers
$\mathbb {C}$
.
A normal projective variety X is a Fano variety if
$-K_X$
is ample. According to the minimal model program, Fano varieties form a fundamental class in the birational classification of algebraic varieties. One recent breakthrough in birational geometry is the proof of the Borisov–Alexeev–Borisov conjecture by Birkar [Reference Birkar3, Reference Birkar4], which states that for a fixed positive integer d and a positive real number
$\epsilon $
, the set of d-dimensional Fano varieties with
$\epsilon $
-klt singularities forms a bounded family. During the proof, one important step is to establish the upper bound for the anti-canonical volume
$(-K_X)^d$
for an
$\epsilon $
-klt Fano variety X of dimension d ([Reference Birkar3, Theorem 1.6]).
Motivated by the classification theory of
$3$
-folds, we mainly focus on the anti-canonical volume
$(-K_X)^3$
for an
$\epsilon $
-klt Fano
$3$
-fold X. In this direction, Lai [Reference Lai13] gave an upper bound for those X which are
$\mathbb {Q}$
-factorial and of Picard rank
$1$
, which is over
$O((\frac {4}{\epsilon })^{384/\epsilon ^5})$
; later, the first author [Reference Jiang10] showed the existence of a nonexplicit upper bound; recently, Birkar [Reference Birkar5] gave the first explicit upper bound, which is about
$O(\frac {2^{1536/\epsilon ^3}}{\epsilon ^9})$
.
The main goal of this paper is to provide a reasonably small explicit upper bound with a sharp order, for the anti-canonical volume of an
$\epsilon $
-klt Fano
$3$
-fold. Here, we state the result for a larger class of varieties containing
$\epsilon $
-klt Fano
$3$
-folds. Recall that a normal projective variety X is said to be of
$\epsilon $
-Fano type if there exists an effective
$\mathbb {R}$
-divisor B, such that
$(X, B)$
is
$\epsilon $
-klt and
$-(K_X+B)$
is ample.
Theorem 1.1. Fix a real number
$0<\epsilon <\frac {1}{3}$
. Let X be a
$3$
-fold of
$\epsilon $
-Fano type. Then
$$ \begin{align*}\operatorname{Vol}(X, -K_{X})< \frac{3200}{\epsilon^4}.\end{align*} $$
The following example shows that the order
$O(\frac {1}{\epsilon ^4})$
in Theorem 1.1 is sharp.
Example 1.2. Ambro [Reference Ambro2, Example 6.3] showed that for each positive integer q, there exists a projective toric
$3$
-fold pair
$(X, B)$
, such that
-
•
$(X, B)$
is
$\frac {1}{q}$
-lc, -
•
$-(K_X+B)$
is ample (in fact,
$-K_X$
is ample as
$\rho (X)=1$
), and -
•
$(-K_X)^3> (-(K_X+B))^3=\frac {u_{4, q}}{q^4}=O(q^4)$
, where
$u_{4, q}=(q^2+q+1)^2(q^2+q)^2+ (q^2+q+1)(q^2+q)$
.
Remark 1.3. The ideas of this paper originate from [Reference Jiang9, Reference Jiang10], which we briefly explain in the following. Given a
$3$
-fold X of
$\epsilon $
-Fano type, in order to give an upper bound of the anti-canonical volume of X, we may reduce to the case that X admits a Mori fiber structure
$X\to T$
. Then we can split the discussion according to
$\dim T\in \{0,1,2\}$
. The case
$\dim T=0$
was solved in [Reference Prokhorov17] (or [Reference Lai13]) and other two cases were solved in [Reference Jiang10]. The main obstructions of getting a reasonably small explicit upper bound in [Reference Jiang10] are the following two issues:
-
(1) when
$\dim T=1$
, we reduce the upper bound problem to finding a lower bound of certain log canonical thresholds on surfaces, called
$\mu (2, \epsilon )$
, but the lower bound in [Reference Jiang10] is extremely small so that the resulting upper bound is extremely large; -
(2) when
$\dim T=2$
, we reduce the upper bound problem to the boundedness of such surfaces T (more precisely, the existence of very ample divisors on T with bounded self-intersection numbers), but the geometry of those T is quite complicated, which makes the upper bound nonexplicit; in fact, even if we can classify those surfaces T, then the resulting upper bound will be explicitly computable but still extremely large.
This paper is devoted to solving these two issues, and we put two main ingredients into the recipe.
The first one is a new reduction originated from [Reference Jiang and Zou11] (Proposition 4.1), which shows that we may further assume that X admits a better fibration structure
$X\to S$
so that if
$\dim S=2$
, then there exists a free divisor on S with small self-intersection number. This solves the second issue.
The second one is a more detailed estimate on the lower bound
$\mu (2, \epsilon )$
(Theorem 3.1), which solves the first issue. We significantly improve the lower bound from about
$O(2^{-64/\epsilon ^3})$
in [Reference Jiang10] to
$O(\epsilon ^3)$
(Theorem 3.1).
Remark 1.4. We shall also compare our result with [Reference Birkar5]. The method used in [Reference Birkar5, Theorem 1.2] is a slight modification of [Reference Birkar3, Theorem 1.6], which is different from our method (but it is a more general strategy that works in any dimension). One thing we share in common is that, [Reference Birkar5, Theorem 1.2] also reduces the problem to finding some kind of lower bound of log canonical thresholds on surfaces ([Reference Birkar5, Lemma 2.2]). Here, note that the constant
$\mu (2, \epsilon )$
in [Reference Birkar5, Lemma 2.2] is
$\frac {1}{\mu (2, \epsilon )}$
in our terminology. If we replace [Reference Birkar5, Lemma 2.2] by Theorem 3.1 in the proof of [Reference Birkar5, Theorem 1.2], then we get an explicit upper bound about
$O(\frac {1}{\epsilon ^{12}})$
.
2 Preliminaries
We adopt standard notation and definitions in [Reference Kollár and Mori12] and will freely use them. We use
$\sim _{\mathbb {Q}}, \sim _{\mathbb {R}}, \equiv $
to denote
${\mathbb {Q}}$
-linear equivalence,
$\mathbb {R}$
-linear equivalence, and numerical equivalence respectively.
2.1 Singularities of pairs
Definition 2.1. A pair
$(X, B)$
consists of a normal variety X and an effective
$\mathbb {R}$
-divisor B on X, such that
$K_X+B$
is
$\mathbb {R}$
-Cartier.
Definition 2.2. Let
$(X, B)$
be a pair. Let
$f: Y\to X$
be a log resolution of
$(X, B)$
, write
$$ \begin{align*}K_Y =f^*(K_X+B)+\sum a_iE_i, \end{align*} $$
where
$E_i$
are distinct prime divisors on Y satisfying
$f_*(\sum a_iE_i)=-B$
. The number
$a_i+1$
is called the log discrepancy of
$E_i$
with respect to
$(X, B)$
, and is denoted by
$a(E_i, X, B)$
. The pair
$(X,B)$
is called
-
(1) Kawamata log terminal (klt, for short) if
$a_i+1>0$
for all i; -
(2) log canonical (lc, for short) if
$a_i+1\geq 0$
for all i; -
(3)
$\epsilon $
-klt if
$a_i+1> \epsilon $
for all i and for some
$0<\epsilon <1$
; -
(4)
$\epsilon $
-lc if
$a_i+1\geq \epsilon $
for all i and for some
$0<\epsilon <1$
; -
(5) terminal if
$a_i> 0$
for all f-exceptional divisors
$E_i$
and for all f.
Usually, we write X instead of
$(X,0)$
in the case when
$B=0$
.
2.2 Varieties of Fano type
Definition 2.3. A variety X is said to be of
$\epsilon $
-Fano type if X is projective and there exists an effective
$\mathbb {R}$
-divisor B, such that
$-(K_X+B)$
is ample and
$(X, B)$
is
$\epsilon $
-klt for some
$0<\epsilon <1$
.
2.3 Volumes
Definition 2.4. Let X be a normal projective variety of dimension n, and let D be a Cartier divisor on X. The volume of D is defined by
$$ \begin{align*}\operatorname{Vol}(X, D)=\limsup_{m\to \infty}\frac{h^0(X, {\mathcal{O}}_X(mD))}{m^n/n!}. \end{align*} $$
Moreover, by the homogeneous property of volumes, the definition can be extended to
${\mathbb {Q}}$
-Cartier
${\mathbb {Q}}$
-divisors. Note that if D is a nef
${\mathbb {Q}}$
-Cartier
${\mathbb {Q}}$
-divisor, then
$\operatorname {Vol}(X, D)=D^n$
. We refer to [Reference Lazarsfeld14, Section 2.2.C] for more details and properties on volumes of divisors.
3 A lower bound of log canonical thresholds on surfaces
The main goal of this section is to prove the following theorem on certain log canonical thresholds on surfaces. This is the main ingredient of this paper and the most technical part. Note that in this paper, by a curve, we always mean an irreducible one.
Theorem 3.1. Fix
$0<\epsilon <\frac {1}{3}$
. Let S be a smooth projective surface. Suppose that there exists a real number
$0<t<1$
and effective
$\mathbb {R}$
-divisors
$B, D$
on S, such that
-
•
$(S, B)$
is
$\epsilon $
-lc; -
•
$(S, (1-t)B+tD)$
is not klt; -
•
$B\sim _{\mathbb {R}} D \sim _{\mathbb {R}} -K_S$
and
$-K_S$
is big.
Then
$t>\frac {3\epsilon ^3}{400}.$
As an immediate corollary, we confirm the generalized Ambro’s conjecture ([Reference Jiang10, Conjecture 2.7]) in dimension
$2$
with a greatly improved lower bound.
Corollary 3.2 (cf. [Reference Jiang10, Theorem 2.8]).
The generalized Ambro’s conjecture ([Reference Jiang10, Conjecture 2.7]) holds in dimension
$2$
with
$\mu (2, \epsilon )>\frac {3\epsilon ^3}{400}$
.
The reduction from Corollary 3.2 to Theorem 3.1 is standard (see [Reference Jiang10, Section 5, Page 1583]).
Remark 3.3. While the constant term might be improved slightly, the order
$\epsilon ^3$
in Theorem 3.1 is sharp. In fact, Ambro [Reference Ambro2, Theorem 1.1, Example 6.3] showed that for each positive integer q, there exists a projective toric surface pair
$(X, \Delta )$
, such that
-
•
$(X, \Delta )$
is
$\frac {1}{q}$
-lc, -
•
$-(K_X+\Delta )$
is ample, and -
• there exists an effective
$\mathbb {Q}$
-divisor
$H\sim _{\mathbb {Q}} -(K_X+\Delta )$
, such that
$(X, \Delta +tH)$
is not klt for
$t=\frac {1}{(q+1)(q^2+q+1)}=O(\frac {1}{q^3})$
.
We can modify this example to satisfy assumptions in Theorem 3.1. Take
$A\sim _{\mathbb {Q}} -(K_X+\Delta )$
to be a sufficiently general ample effective
$\mathbb {Q}$
-divisor, such that
$(X, \Delta +A)$
is still
$\frac {1}{q}$
-lc. Take
$\pi : S\to X$
to be the minimal resolution of X, then
$-K_S$
is big as
$-K_X$
is big. We may write
$$ \begin{align*}K_S+B=\pi^*(K_X+\Delta+A)\sim_{\mathbb{Q}}0 \end{align*} $$
for some effective
$\mathbb {Q}$
-divisor
$B\geq \pi ^*A$
. Then
$(S, B)$
is
$\frac {1}{q}$
-lc. In this case,
$D:=B-\pi ^*A+\pi ^*H$
is an effective
$\mathbb {Q}$
-divisor, such that
$D\sim _{\mathbb {Q}}B\sim _{\mathbb {Q}}-K_S$
and the pair
$(S, (1-t)B+tD)$
is not klt as
$$ \begin{align*}K_S+(1-t)B+tD=\pi^*(K_X+\Delta+tH+ (1-t)A).\end{align*} $$
3.1 Weighted dual graphs
In this subsection, we recall basic knowledge of weighted dual graphs of resolutions of surface singularities from [Reference Alexeev1] or [Reference Kollár and Mori12, 4.1].
Let Y be a normal surface, and let
$\pi : Y^{\prime }\to Y$
be a resolution with
$\pi $
-exceptional curves
$\{E_i\}_i$
. The weighted dual graph
$\Gamma $
of
$\pi $
is defined as the following: each vertex
$v_i$
of
$\Gamma $
corresponds to a
$\pi $
-exceptional curve
$E_i$
, and it has a positive weight
$-E_i^2$
; two vertices
$v_i$
and
$v_j$
are connected by an edge of weight
$m=(E_i\cdot E_j)$
if
$(E_i\cdot E_j)\neq 0$
. If Y is klt and has a unique singular point, then
$\Gamma $
is a tree with simple edges and all
$\pi $
-exceptional curves are smooth rational curves by [Reference Alexeev1, Lemma 3.2.7] or [Reference Kollár and Mori12, Theorem 4.7]. In this case, denote by
$\overline {v_iv_j}$
the path from
$v_i$
to
$v_j$
, that is, the unique shortest chain in
$\Gamma $
joining
$v_i$
and
$v_j$
.
For any subgraph
$\Gamma '\subset \Gamma $
, define
$\Delta (\Gamma ')$
to be the absolute value of the determinant of the matrix
$[(E_{i}\cdot E_{j})]$
, made up by vertices in
$\Gamma '$
. Here,
$\Delta (\emptyset )=1$
by default.
We will often use the following lemma to compute log discrepancies and multiplicities of exceptional divisors.
Lemma 3.4 (cf. [Reference Alexeev1, (3.1.10)]).
Let
$\pi : Y'\to Y$
be a resolution of a klt surface singularity
$P\in Y$
. Suppose that the set of
$\pi $
-exceptional curves is
$\{E_1, E_2, \dots , E_n\}$
. Denote by
$\Gamma $
the weighted dual graph of
$\pi $
.
-
(1) Then for each
$1\leq k\leq n$
,
$$ \begin{align*}a(E_k, Y, 0)=\frac{\sum_{j=1}^n (2-(\sum_{i\neq j}E_{i} \cdot E_j) )\cdot\Delta(\Gamma\setminus \overline{v_kv_j})}{\Delta(\Gamma)}.\end{align*} $$
-
(2) If C is an irreducible curve on Y, then for each
$1\leq k\leq n$
,
$$ \begin{align*}\operatorname{mult}_{E_k} \pi^*C=\frac{\sum_{j=1}^n(\pi^{-1}_*C\cdot E_j)\cdot\Delta(\Gamma\setminus \overline{v_kv_j})}{\Delta(\Gamma)}.\end{align*} $$
Proof. (1) is just [Reference Alexeev1, (3.1.10)] and (2) can be deduced in the same way by applying [Reference Alexeev1, Lemma 3.1.9] and the Cramer’s rule.
The following lemma will be used to deal with the weighted dual graph of the minimal resolution of a cyclic quotient singularity.
Lemma 3.5. Let
$\Gamma $
be a chain with vertices
$v_1, \dots , v_n$
ordering in the natural sense that
$v_i$
is connected to
$v_{i+1}$
by an edge for
$1\leq i\leq n-1$
. Suppose that for each
$1\leq i\leq n$
, the weight of
$v_i$
is
$m_i$
with
$m_i\geq 2$
. Then the following assertions hold:
-
(1)
$\Delta (\Gamma )=m_1\cdot \Delta (\Gamma \setminus \{v_1\})-\Delta (\Gamma \setminus \overline {v_1v_2})$
; -
(2)
$\Delta (\Gamma \setminus \overline {v_1v_k})=m_{k+1}\cdot \Delta (\Gamma \setminus \overline {v_1v_{k+1}})-\Delta (\Gamma \setminus \overline {v_1v_{k+2}})$
for
$1\leq k\leq n-2$
; -
(3)
$\Delta (\Gamma )>\Delta (\Gamma \setminus \{v_1\})>\Delta (\Gamma \setminus \overline {v_1v_2})>\dots > \Delta (\Gamma \setminus \overline {v_1v_n})=1$
; -
(4)
$\Delta (\Gamma )\geq n+1$
and
$\Delta (\Gamma \setminus \overline {v_1v_k})\geq n-k+1$
for
$1\leq k\leq n$
; moreover, the equalities hold if all
$m_i=2$
; -
(5) if
$\Delta (\Gamma )=\Delta (\Gamma \setminus \{v_1\})+1$
, then
$\Delta (\Gamma )=n+1$
; -
(6) if
$m_{i_0}\geq 3$
for some
$1\leq i_0\leq n$
, then
$\Delta (\Gamma )>(i_{0}+1)\Delta (\Gamma \setminus \overline {v_{1}v_{i_0}})$
.
Proof. Assertions (1) and (2) can be calculated easily from computing determinants. By Assertions (1)(2) and the fact that
$m_i\geq 2$
, we have
$$ \begin{align} \Delta(\Gamma)\geq 2 \Delta(\Gamma\setminus\{v_1\})-\Delta(\Gamma\setminus\overline{v_1v_2}) \end{align} $$
and
$$ \begin{align} \Delta(\Gamma\setminus\overline{v_1v_k})\geq 2 \Delta(\Gamma\setminus\overline{v_1v_{k+1}})-\Delta(\Gamma\setminus\overline{v_1v_{k+2}})\end{align} $$
for
$1\leq k\leq n-2$
. So Assertion (3) follows inductively from the fact that
$$ \begin{align*}\Delta(\Gamma\setminus\overline{v_1v_{n-1}})-\Delta(\Gamma\setminus\overline{v_1v_{n}})=m_n-1>0. \end{align*} $$
Assertion (4) follows from Assertion (3) and a direct computation.
For Assertion (5), the assumption combining with Eqs. (3.1) and (3.2) implies that
$$ \begin{align*}\Delta(\Gamma\setminus\overline{v_1v_{k+1}})-\Delta(\Gamma\setminus\overline{v_1v_{k+2}})=1 \end{align*} $$
for
$1\leq k\leq n-2$
. Moreover, Eqs. (3.1) and (3.2) become equalities, and hence, all
$m_i=2$
. So
$\Delta (\Gamma )=n+1$
by Assertion (4).
By applying Eqs. (3.1) and (3.2) inductively, one can see that
$$ \begin{align*} \Delta(\Gamma)\geq j \Delta(\Gamma\setminus\overline{v_1v_{j-1}})-(j-1)\Delta(\Gamma\setminus\overline{v_1v_j}) \end{align*} $$
for
$1\leq j\leq n$
. Here, we set
$\Gamma \setminus \overline {v_1v_{0}}=\Gamma $
. Then, by Assertions (1)(2)(3), for
$1\leq j\leq n$
, we have
$$ \begin{align*} \Delta(\Gamma){}&\geq j(m_j \Delta(\Gamma\setminus\overline{v_1v_{j}})-\Delta(\Gamma\setminus\overline{v_1v_{j+1}}))-(j-1)\Delta(\Gamma\setminus\overline{v_1v_j})\\ {}&=(j m_j-j+1) \Delta(\Gamma\setminus\overline{v_1v_{j}})-j \Delta(\Gamma\setminus\overline{v_1v_{j+1}})\\ {}&>(j m_j-2j+1) \Delta(\Gamma\setminus\overline{v_1v_{j}}). \end{align*} $$
Here, we set
$\Delta (\Gamma \setminus \overline {v_1v_{n+1}})=0$
. In particular, if
$m_{i_0}\geq 3$
, then
$\Delta (\Gamma )>(i_{0}+1) \Delta (\Gamma \setminus \overline {v_{1}v_{i_0}})$
.
3.2 Geometric structure of
$\delta $
-lc surface pairs
Lemma 3.6. Fix a real number
$0<\delta <\frac {1}{6}$
and a positive integer N. Let Y be a normal surface, and let C be an irreducible curve on Y, such that
$(Y, (1-\delta )C)$
is
$\delta $
-lc. Let
$\pi : Y'\to Y$
be the minimal resolution of Y. Suppose that
$\rho (Y'/Y)\leq N$
. Then
$\operatorname {mult}_{E}\pi ^*C> \frac {\delta }{N+1}$
for any prime divisor E on
$Y'$
, such that E is
$\pi $
-exceptional and
$\pi (E)\in C$
.
Proof. By shrinking Y if necessary, we may assume that
$P\in Y$
is the only singular point on Y and
$P\in C$
. Denote by
$\Gamma $
the weighted dual graph of
$\pi $
. For any
$\pi $
-exceptional prime divisor E, clearly
$\operatorname {mult}_{E}\pi ^*C$
is a positive rational number and its denominator divides
$\Delta (\Gamma )$
by Lemma 3.4. So it suffices to show that
$\Delta (\Gamma )<\frac {N+1}{\delta }$
.
By [Reference Prokhorov16, Corollary 6.0.9],
$(Y, C)$
is lc. Then the weighted dual graph
$\Gamma $
of the minimal resolution
$\pi : Y'\to Y$
are classified into three cases, as in [Reference Kollár and Mori12, Theorem 4.15]. We split the discussion into these three cases. Denote by
$m=\rho (Y'/Y)$
the number of
$\pi $
-exceptional curves on
$Y'$
.
As
$(Y, (1-\delta )C)$
is
$\delta $
-lc,
$a(E, Y, (1-\delta )C)\geq \delta $
for any prime divisor E over Y. We will apply this fact to some specially chosen E.
Case (1): For the case [Reference Kollár and Mori12, Theorem 4.15(1)],
$\Gamma $
is a chain with vertices
$v_1, \cdots , v_m$
corresponding to
$\pi $
-exceptional curves
$E_1, \cdots , E_m$
, such that
$\pi ^{-1}_*C$
intersects
$E_1$
and
$E_m$
.
If
$m\geq 2$
, by Lemma 3.4, we have
$$\begin{align*}a(E_1, Y, 0)= \operatorname{mult}_{E_1}\pi^*C=\frac{\Delta(\Gamma\setminus\{v_1\})+\Delta(\emptyset)}{\Delta(\Gamma)}. \end{align*}$$
Then
$$ \begin{align*} a(E_1, Y, (1-\delta)C)={}&a(E_1, Y, 0)-(1-\delta)\operatorname{mult}_{E_1}\pi^*C\\ ={}&\delta\cdot\frac{\Delta(\Gamma\setminus\{v_1\})+1}{\Delta(\Gamma)}. \end{align*} $$
Therefore,
$a(E_1, Y, (1-\delta )C)\geq \delta $
implies that
$\Delta (\Gamma \setminus \{v_1\})+1\geq \Delta (\Gamma )$
. So, by Lemma 3.5(5),
$\Delta (\Gamma )=m+1\leq N+1$
.
If
$m=1$
, then a similar computation by Lemma 3.4 shows that
$$\begin{align*}a(E_1, Y, (1-\delta)C)=\delta\cdot\frac{2\Delta(\Gamma\setminus\{v_1\})}{\Delta(\Gamma)}=\frac{2\delta}{\Delta(\Gamma)}. \end{align*}$$
Therefore,
$a(E_1, Y, (1-\delta )C)\geq \delta $
implies that
$\Delta (\Gamma )\leq 2$
.
Case (2): For the case [Reference Kollár and Mori12, Theorem 4.15(2)], we have
$m\geq 3$
.
If
$m\geq 4$
, then
$\Gamma $
is a tree with only one fork. Let
$E_{\text {f}}$
be the
$\pi $
-exceptional curve corresponding to the fork vertex
$v_{\text {f}}$
. Then
$\Gamma \setminus \{v_{\text {f}}\}=\Gamma '\cup \{v_1\}\cup \{v_2\}$
, where
$v_1, v_2$
correspond to
$(-2)$
-curves intersecting
$E_{\text {f}}$
and
$\Gamma '$
is the chain corresponding to curves connecting
$E_{\text {f}}$
and
$\pi ^{-1}_*C$
.
Then, by Lemma 3.4,
$$ \begin{align*} {}&a(E_{\text{f}}, Y, 0)\\ ={}&\frac{\Delta(v_1)\cdot \Delta(v_2)-\Delta(\Gamma')\cdot \Delta(v_1)\cdot \Delta(v_2)+\Delta(\Gamma')\cdot \Delta(v_2)+\Delta(\Gamma')\cdot \Delta(v_1)}{\Delta(\Gamma)}\\ ={}&\frac{4}{\Delta(\Gamma)} \end{align*} $$
and
$$\begin{align*}\operatorname{mult}_{E_{\text{f}}}\pi^*C=\frac{\Delta(v_1)\cdot \Delta(v_2)}{\Delta(\Gamma)}=\frac{4}{\Delta(\Gamma)}. \end{align*}$$
Then
$a(E_{\text {f}}, Y, (1-\delta )C)=\frac {4\delta }{\Delta (\Gamma )} $
. Therefore,
$a(E_{\text {f}}, Y, (1-\delta )C)\geq \delta $
implies that
$\Delta (\Gamma )\leq 4$
.
If
$m=3$
, then
$\Gamma $
is a chain consisting of three vertices
$v_1, v_2, v_3$
corresponding to
$E_1, E_2, E_3$
, such that
$E_1$
and
$E_3$
are
$(-2)$
-curves and
$\pi ^{-1}_*C$
intersects
$E_2$
. Then a similar computation by Lemma 3.4 shows that
$a(E_{2}, Y, (1-\delta )C)=\frac {4\delta }{\Delta (\Gamma )}$
. Therefore,
$a(E_{2}, Y, (1-\delta )C)\geq \delta $
implies that
$\Delta (\Gamma )\leq 4$
.
Case (3): For the case [Reference Kollár and Mori12, Theorem 4.15(3)],
$\Gamma $
is a chain with vertices
$v_1, \cdots , v_m$
corresponding to
$\pi $
-exceptional curves
$E_1, \cdots , E_m$
, such that
$\pi ^{-1}_*C$
intersects
$E_1$
. If all
$E_i$
are
$(-2)$
-curves for
$1\leq i\leq m$
, then by Lemma 3.5(4),
$\Delta (\Gamma )=m+1\leq N+1$
. If
$-E_{i_0}^2\geq 3$
for some
$1\leq i_0\leq m$
, take
$i_0$
to be the minimal one, then by Lemmas 3.4 and 3.5(4),
$$ \begin{align*} a(E_{i_0}, Y, (1-\delta)C) ={}&\frac{\Delta(\Gamma\setminus\overline{v_{i_0}v_m})}{\Delta(\Gamma)}+\delta\cdot\frac{\Delta(\Gamma\setminus\overline{v_{1}v_{i_0}})}{\Delta(\Gamma)} \\ ={}&\frac{i_0}{\Delta(\Gamma)}+\delta\cdot\frac{\Delta(\Gamma\setminus\overline{v_{1}v_{i_0}})}{\Delta(\Gamma)} \\ <{}&\frac{i_0}{\Delta(\Gamma)}+\frac{\delta}{i_0+1}. \end{align*} $$
Here, the last inequality is by Lemma 3.5(6). Therefore,
$a(E_{i_0}, Y, (1-\delta )C)\geq \delta $
implies that
$\Delta (\Gamma )<\frac {i_0+1}{\delta }\leq \frac {N+1}{\delta }$
.
Lemma 3.7 (cf. [Reference Jiang10, Claim 2]).
Fix
$0<\delta <1$
. Let X be a smooth projective surface, and let
$(X, B)$
be a
$\delta $
-lc pair, such that
$K_X+B\equiv 0$
. Then
$C^2\geq -\frac {2}{\delta }$
for any irreducible curve C on X.
Proof. We may assume that
$C^2<0$
. Then, by the genus formula,
$$ \begin{align*} -2{}&\leq 2p_a(C)-2=(K_X+C)\cdot C\\ {}&= \delta C^2+(K_X+(1-\delta)C)\cdot C\\ {}&\leq \delta C^2+(K_X+B)\cdot C=\delta C^2.\\[-37pt] \end{align*} $$
Lemma 3.8. Let X be a normal projective surface, such that
$K_X$
is
$\mathbb {Q}$
-Cartier and not pseudo-effective. Then either
$X\simeq \mathbb {P}^2$
or X is covered by a family of rational curves C, such that
$(-K_X\cdot C)\leq 2$
.
Proof. Take
$\pi : X'\to X$
to be the minimal resolution of X, then
$K_{X'}+G=\pi ^*K_X$
, where G is an effective
$\mathbb {Q}$
-divisor. Then
$K_{X'}$
is not pseudo-effective as
$K_X$
is not pseudo-effective. Suppose that
$X\not \simeq \mathbb {P}^2$
, then clearly
$X'\not \simeq \mathbb {P}^2$
. By the standard minimal model program, there exists a morphism
$X'\to T$
whose general fibers are
$\mathbb {P}^1$
. Therefore,
$X'$
is covered by a family of rational curves
$C'$
, such that
$(-K_{X'}\cdot C')=2$
. So
$(-K_X\cdot \pi (C'))\leq (-K_{X'}\cdot C')= 2$
.
3.3 Proof of Theorem 3.1
In this subsection, we give the proof of Theorem 3.1.
By [Reference Jiang8, Lemma 3.1], there is a birational morphism
$g: S\to S'$
, where
$S'$
is
$\mathbb {P}^2$
or the n-th Hirzebruch surface
$\mathbb {F}_n$
with
$n\leq \frac {2}{\epsilon }$
. Since
$B\sim _{\mathbb {R}}D\sim _{\mathbb {R}}-K_S$
, we may write
$$ \begin{align*} K_S+B={}&g^*(K_{S'}+g_*B);\\ K_S+(1-t)B+tD={}&g^*(K_{S'}+(1-t)g_*B+tg_*D). \end{align*} $$
Hence,
$({S'}, g_*B)$
is
$\epsilon $
-lc and
$({S'}, (1-t)g_*B+tg_*D)$
is not klt. By replacing the triple
$(S, B, D)$
with
$(S', g_*B, g_*D)$
, we may assume that S is
$\mathbb {P}^2$
or
$\mathbb {F}_n$
for some
$n\leq \frac {2}{\epsilon }$
.
Fix a positive real number
$0<\delta < {\epsilon }$
, such that
$\delta <\frac {1}{6}$
. Take
$$ \begin{align*}t_0=\max\{s\in \mathbb{R}\mid (S, (1-s)B+s D) \text{ is } \delta\text{-lc}\}. \end{align*} $$
Then clearly
$0<t_0<t$
. In the following, we will show that
$$ \begin{align} t_0>\frac{\delta^2(\epsilon-\delta)}{16+4\delta+\delta^2(\epsilon-1)}. \end{align} $$
In particular, we can take
$\delta =\frac {\epsilon }{2}$
, then Eq. (3.3) implies that
$t>t_0>\frac {3\epsilon ^3}{400}.$
By the definition of
$t_0$
, there exists a prime divisor E over S, such that
$$ \begin{align*}a(E, S, (1-t_0)B+t_0 D)=\delta.\end{align*} $$
If E is a prime divisor on S, then as
$(S, B)$
is
$\epsilon $
-lc, we have
$$ \begin{align*} {}&\operatorname{mult}_E B\leq 1-\epsilon;\\ {}&\operatorname{mult}_E ((1-t_0)B+t_0 D)= 1-\delta. \end{align*} $$
So
$$\begin{align*}t_0\geq\frac{\epsilon-\delta}{\operatorname{mult}_{E}D-1+\epsilon}\geq\frac{\epsilon(\epsilon-\delta)}{2+3\epsilon+\epsilon^2},\end{align*}$$
which implies Eq. (3.3). Here, we used the fact that
$$\begin{align*}\operatorname{mult}_{E}D\leq \begin{cases} 3 & \text{if } S=\mathbb{P}^2;\\ n+4\leq \frac{2}{\epsilon}+4 & \text{if } S=\mathbb{F}_n, \end{cases} \end{align*}$$
by applying [Reference Jiang10, Lemma 3.3] to a general point on E.
So, from now on, we may assume that E is exceptional over S. By [Reference Birkar, Cascini, Hacon and McKernan6, Corollary 1.4.3], there exists a projective birational morphism
$f: Y\to S$
, such that E is the unique
$\pi $
-exceptional divisor on Y. We have
$$ \begin{align} K_Y+(1-t_0)B_Y+t_0D_Y+(1-\delta)E= f^*(K_{S}+(1-t_0)B+t_0D). \end{align} $$
Here,
$B_Y$
and
$D_Y$
are strict transforms of B and D on Y. Write
$$ \begin{align*} K_{Y}+B_Y+bE={}&f^*(K_S+B)\equiv 0, \\ K_{Y}+D_Y+dE={}&f^*(K_S+D)\equiv 0. \end{align*} $$
Then
$(1-t_0)b+t_{0}d=1-\delta $
. Since
$b\leq 1-\epsilon $
as
$(S, B)$
is
$\epsilon $
-lc, we have
$$ \begin{align}t_0\geq\frac{\epsilon-\delta}{d+\epsilon-1}. \end{align} $$
So, in order to bound
$t_0$
from below, we need to bound d from above.
By Eq. (3.4), we know that
$(Y, (1-t_0)B_Y+t_0D_Y+(1-\delta )E)$
is
$\delta $
-lc, and
$$ \begin{align*} K_Y+(1-t_0)B_Y+t_0D_Y+(1-\delta)E\equiv 0. \end{align*} $$
We can run a
$(K_Y+(1-t_0)B_Y+t_0 D_Y)$
-MMP (which is also a
$(-E)$
-MMP) on Y to get a Mori fiber space
$Y'\to Z$
, such that
$E'$
is ample over Z, where
$E'$
is the strict transform of E on
$Y'$
. Here,
$(Y', (1-\delta )E')$
is again
$\delta $
-lc by the negativity lemma. We have
$-K_{Y'}\equiv D_{Y'}+dE'$
, where
$D_{Y'}$
is the strict transform of
$D_Y$
on
$Y'$
.
If
$\dim Z=1$
, then a general fiber of
$Y'\to Z$
is a smooth rational curve. By restricting
$-K_{Y'}\equiv D_{Y'}+dE'$
on a general fiber, we get
$d\leq 2$
.
If
$\dim Z=0$
, then
$Y'$
is of Picard rank
$1$
. In this case,
$-K_{Y'}\equiv e E'$
for some
$e\geq d$
. If
$Y'\simeq \mathbb {P}^2$
, then clearly
$d\leq e\leq 3$
. So, we may assume that
$Y'\not \simeq \mathbb {P}^2$
. By Lemma 3.8, there is a general rational curve C, such that
$(-K_{Y'}\cdot C)\leq 2$
. Take
$\pi ^{\prime }: Y^{\prime }_{\min }\to Y'$
to be the minimal resolution of
$Y'$
, and take
$Y_{\min }$
to be the minimal resolution of Y. Then the morphism
$Y_{\min }\to Y'$
factors through
$Y^{\prime }_{\min }$
.
Claim 3.9. We have
$\rho (Y_{\min }/Y)\leq \frac {8}{\delta }-1$
.
We grant Claim 3.9 for this moment and continue the proof of Theorem 3.1. The proof of Claim 3.9 will be provided later. By Claim 3.9,
$$ \begin{align*} \rho(Y^{\prime}_{\min}/Y'){}&\leq \rho(Y_{\min}/Y')= \rho(Y_{\min}/Y)+\rho(Y)-1\\ {}&\leq \rho(Y_{\min}/Y)+2\leq \frac{8}{\delta}+1. \end{align*} $$
Here, we used the fact that
$\rho (Y)=\rho (S)+1\leq 3$
. Recall that
$(Y', (1-\delta )E')$
is
$\delta $
-lc and
$\delta <\frac {1}{6}$
, then, by Lemma 3.6,
$\pi^{\prime*}E^{\prime }$
is an effective
$\mathbb {Q}$
-divisor with all coefficients larger than
$\frac {\delta ^2}{8+2\delta }$
. As C is general, by the projection formula,
$(E'\cdot C)> \frac {\delta ^2}{8+2\delta }$
, which implies that
$$ \begin{align*}\frac{e\delta^2}{8+2\delta}< (eE'\cdot C)=(-K_{Y'}\cdot C)\leq 2.\end{align*} $$
Hence,
$d\leq e\leq \frac {16+4\delta }{\delta ^2}$
.
In summary, we always have
$d \leq \frac {16+4\delta }{\delta ^2}$
. Therefore, by Eq. (3.5),
$$ \begin{align*}t_0\geq\frac{\epsilon-\delta}{d+\epsilon-1}\geq\frac{\delta^2(\epsilon-\delta)}{16+4\delta+\delta^2(\epsilon-1)}. \end{align*} $$
Proof of Claim 3.9.
Denote by
$\pi : Y_{\min }\to Y$
the minimal resolution of Y. Denote by
$E_0$
the strict transform of E on
$Y_{\min }$
. Denote by
$f_0=f\circ \pi : Y_{\min }\to S$
the induced morphism.
Denote
$P=f(E)\in S$
. Then
$Y\setminus E\simeq S\setminus \{P\}$
is smooth. Therefore,
$Y_{\min }$
and Y are isomorphic over
$Y\setminus E$
and
$$ \begin{align*}\text{Exc}(f_0)=f_0^{-1}(P)=\text{Exc}(\pi)\cup E_0.\end{align*} $$
Note that
$f_0: Y_{\min }\to S$
can be decomposed into successive blow-ups along smooth points and
$\text {Exc}(\pi )$
does not contain any
$(-1)$
-curves, so
$E_0$
is the unique
$(-1)$
-curve in
$\text {Exc}(f_0)$
. In other words, if we denote the last blow-up by
$Y_{\min }\to S_1$
, then
$E_0$
is the exceptional divisor over
$S_1$
.
Denote by
$\Gamma $
the weighted dual graph of
$f_0$
, and denote by
$v_0$
the vertex corresponding to
$E_0$
. Then
$\Gamma $
is a tree and
$\Gamma \setminus \{v_0\}$
is the weighted dual graph of
$\pi $
. Since the weighted dual graph of
$S_{1}\to S$
is also a tree,
$\Gamma \setminus \{v_0\}$
has at most two connected components (which implies that Y has at most two singular points).
Claim 3.10.
$\Gamma $
is a chain.
Proof of Claim 3.10.
Take
$\Gamma '$
to be a connected component of
$\Gamma \setminus \{v_0\}$
, then it corresponds to exceptional curves over a singular point on Y. Recall that
$(Y, (1-\delta )E)$
is
$\delta $
-lc by Eq. (3.4). As
$\delta <\frac {1}{6}$
,
$(Y, E)$
is lc by [Reference Prokhorov16, Corollary 6.0.9]. Then the weighted dual graph
$\Gamma '$
and its relation with
$v_0$
are classified into three cases, as in [Reference Kollár and Mori12, Theorem 4.15]. We shall rule out [Reference Kollár and Mori12, Theorem 4.15(1)(2)]. In the case of [Reference Kollár and Mori12, Theorem 4.15(1)],
$\Gamma $
contains a loop, which is absurd; in the case of [Reference Kollár and Mori12, Theorem 4.15(2)],
$\Gamma $
contains a fork with two
$(-2)$
-curves on two tails of the fork, so, by contracting
$(-1)$
-curves in the graph successively, we will reach some model with two
$(-1)$
-curves over
$P\in S$
, which is also absurd. Hence, we conclude that
$\Gamma '$
is a chain connecting to
$v_0$
by one edge at one end. Therefore,
$\Gamma $
is a chain.
If
$\Gamma \setminus \{v_0\}$
is empty, then clearly Y is smooth and
$\rho (Y_{\min }/Y)=0$
. So, in the following, we split the discussion into two cases, depending on the number of connected components of
$\Gamma \setminus \{v_0\}$
.
Case (a).
$\Gamma \setminus \{v_0\}$
has one connected component.
In this case, denote the
$\pi $
-exceptional curves by
$E_1,\dots , E_N$
, such that
$E_0$
intersects
$E_1$
, where
$N=\rho (Y_{\min }/Y)$
. Then all
$E_{i}$
are
$(-2)$
-curves for
$1\leq i\leq N$
.
Suppose that
$Y_{\min }\to S$
is decomposed into successive blow-ups at smooth points as
$$ \begin{align*}Y_{\min}=Y_0\to Y_1\to \dots \to Y_N\to Y_{N+1}=S, \end{align*} $$
then
$E_i$
is the strict transform of the exceptional divisor of
$Y_{i}\to Y_{i+1}$
. For each i, denote by
$G_i$
the strict transform of
$(1-t_0)B+t_0 D$
on
$Y_i$
and denote by
$P_i\in Y_i$
the blow-up center on
$Y_i$
.
Write
$$ \begin{align} {}&K_{Y_{\min}}+G_0+(1-\delta)E_0+\sum_{i=1}^N b_i E_i=f_0^* (K_S+(1-t_0)B+t_0 D). \end{align} $$
Then, the coefficient of
$E_0$
is computed from
$G_1$
and
$E_1$
by the formula
$$ \begin{align*}1-\delta=\operatorname{mult}_{P_1}G_1+b_1-1.\end{align*} $$
Since
$b_1\leq 1-\delta $
as
$( S, (1-t_0)B+t_0 D)$
is
$\delta $
-lc, we have
$\operatorname {mult}_{P_1}G_1\geq 1$
, hence,
$\operatorname {mult}_{P_i}G_i\geq \operatorname {mult}_{P_1}G_1\geq 1$
for
$1\leq i\leq N+1$
. Hence, the intersection number
$G_i^2 $
decreases at least by
$1$
after each blow-up for
$1\leq i\leq N+1$
. Therefore,
$$ \begin{align} N+1\leq G_{N+1}^2-G_0^2=K_S^2-G_0^2. \end{align} $$
Since
$Y_{\min }$
is the minimal resolution of Y, we have
$b_i\geq 0$
for
$1\leq i\leq N$
in Eq. (3.6). In particular,
$({Y_{\min }}, G_0+(1-\delta )E_0+\sum _{i=1}^N b_i E_i)$
is a
$\delta $
-lc pair, such that
$$ \begin{align*}K_{Y_{\min}}+G_0+(1-\delta)E_0+\sum_{i=1}^N b_i E_i\equiv 0. \end{align*} $$
Write
$G_0=\sum _{k}c_k C_k$
, where
$C_k$
are distinct prime divisors, then
$c_k\leq 1-\delta $
. By Lemma 3.7,
$C_k^2\geq -\frac {2}{\delta }$
.
If
$S=\mathbb {F}_n$
, then
$\sum _{k}c_k\leq 4$
by [Reference Jiang10, Lemma 3.3]. Hence
$$ \begin{align*} G_0^2=(\sum_k c_kC_k)^2\geq{}&(\sum_{k}c_{k}^2)\cdot(-\frac{2}{\delta}) \geq(\sum_{k}c_{k})\cdot(1-\delta)\cdot(-\frac{2}{\delta})\\ \geq{}&4(1-\delta)\cdot(-\frac{2}{\delta}) =8-\frac{8}{\delta}. \end{align*} $$
Combining with Eq. (3.7), we have
$$ \begin{align*} N+1\leq{}&K_S^2-8+\frac{8}{\delta}= \frac{8}{\delta}. \end{align*} $$
If
$S=\mathbb {P}^2$
, then we have
$\sum _{k}c_k\leq 3$
, and by the same argument, we get
$$ \begin{align*} N+1\leq{}&K_S^2-G_0^2\leq 9-6+\frac{6}{\delta}\leq \frac{8}{\delta}. \end{align*} $$
Case (b).
$\Gamma \setminus \{v_0\}$
has two connected components.
In this case, suppose that the two connected components are two chains
$\Gamma _1, \Gamma _2$
consisting of vertices
$v_1, \dots , v_p$
and
$u_1, \dots , u_q$
, corresponding to exceptional divisors
$E_1, \dots , E_p$
and
$F_1, \dots , F_q$
, respectively, where
$E_1$
and
$F_1$
intersect
$E_0$
. Here,
$p+q=\rho (Y_{\min }/Y)$
.
Set
$m_i=-E_{i}^2$
and
$l_j=-F_{j}^2$
for
$1\leq i \leq p$
and
$1\leq j \leq q$
. Recall that
$\Gamma $
is the weighted dual graph of a resolution over the smooth point
$P\in S$
, so after blowing down
$E_0$
, there is exactly one
$(-1)$
-curve among the strict transforms of
$E_{1}$
and
$F_{1}$
. So without loss of generality, we may assume that
$m_1\geq 3$
and
$l_1=2$
. Again, by the fact that a contraction of a
$(-1)$
-curve in the graph induces another unique
$(-1)$
-curve, we know that
$l_j=2$
for
$1\leq j\leq m_1-2\leq q$
, and
$l_{m_1-1}\geq 3$
if
$m_1-1\leq q$
.
Since
$Y_{\min }$
is the minimal resolution of Y, we may write
$$ \begin{align*}K_{Y_{\min}}+G'=f_0^*(K_S+(1-t_0)B+t_0 D)\equiv 0,\end{align*} $$
where
$(Y_{\min }, G')$
is a
$\delta $
-lc pair. By Lemma 3.7, we conclude that
$m_1=-E_1^2\leq \frac {2}{\delta }$
.
By Lemma 3.4, we have
$$ \begin{align} \kern-1pt & a(E_{i}, Y, (1-\delta)E)=\frac{\Delta(\Gamma_1\setminus \overline{v_{i}v_{p}})}{\Delta(\Gamma_1)}+\delta\cdot \frac{\Delta(\Gamma_1\setminus \overline{v_{1}v_{i}})}{\Delta(\Gamma_1)}; \end{align} $$
$$ \begin{align} &a(F_{j}, Y, (1-\delta)E)=\frac{\Delta(\Gamma_2\setminus \overline{u_{j}u_{q}})}{\Delta(\Gamma_2)}+\delta\cdot \frac{\Delta(\Gamma_2\setminus \overline{u_{1}u_{j}})}{\Delta(\Gamma_2)} \end{align} $$
for
$1\leq i \leq p$
and
$1\leq j \leq q$
.
To finish the proof, we need to show the following claim.
Claim 3.11. We have
$p\leq \frac {1}{\delta }$
and
$q\leq \frac {3}{\delta }-2$
.
Proof of Claim 3.11.
Recall that
$(Y, (1-\delta )E)$
is
$\delta $
-lc.
First, we show that
$p\leq \frac {1}{\delta }$
. We may assume that
$p\geq 2$
. By Eq. (3.8),
$$ \begin{align*}a(E_{1}, Y, (1-\delta)E)=\frac{1}{\Delta(\Gamma_1)}+\delta\cdot \frac{\Delta(\Gamma_1\setminus\{v_{1}\})}{\Delta(\Gamma_1)}.\end{align*} $$
Therefore,
$a(E_{1}, Y, (1-\delta )E)\geq \delta $
implies that
$$ \begin{align*} \frac{1}{\delta}\geq {}& \Delta(\Gamma_1)- \Delta(\Gamma_1\setminus\{v_{1}\})> \Delta(\Gamma_1\setminus\{v_{1}\}) \geq p. \end{align*} $$
Here, the second inequality is from Lemma 3.5(6) with
$i_0=1$
and the third is from Lemma 3.5(4).
Next, we show that
$q\leq \frac {3}{\delta }-2$
. If
$q=m_1-2$
, then clearly
$q\leq \frac {2}{\delta }-2$
. If
$q>m_1-2$
, then take
$j_0=m_1-1$
, we have
$l_{j_0}\geq 3$
. By Eq. (3.9),
$$ \begin{align*} a(F_{j_0}, Y, (1-\delta)E)={}&\frac{\Delta(\Gamma_2\setminus \overline{u_{j_0}u_{q}})}{\Delta(\Gamma_2)}+\delta\cdot \frac{\Delta(\Gamma_2\setminus \overline{u_{1}u_{j_0}})}{\Delta(\Gamma_2)}\\ ={}& \frac{j_0}{\Delta(\Gamma_2)}+ \delta\cdot \frac{\Delta(\Gamma_2\setminus \overline{u_{1}u_{j_0}})}{\Delta(\Gamma_2)}. \end{align*} $$
Therefore,
$a(F_{j_0}, Y, (1-\delta )E)\geq \delta $
implies that
$$ \begin{align*} \frac{j_0}{\delta}\geq {}& \Delta(\Gamma_2)- \Delta(\Gamma_2\setminus \overline{u_{1}u_{j_0}})> j_0\Delta(\Gamma_2\setminus \overline{u_{1}u_{j_0}}) \geq j_0(q-j_0+1). \end{align*} $$
Here, the second inequality is from Lemma 3.5(6) with
$i_0=j_0$
and the third is from Lemma 3.5(4). Recall that
$m_1=j_0+1$
, so
$q\leq \frac {1}{\delta }+m_1-2\leq \frac {3}{\delta }-2$
.
In summary,
$\rho (Y_{\min }/Y)=p+q\leq \frac {4}{\delta }-2$
.
4 Upper bound of anti-canonical volumes
In this section, we prove the main theorem.
4.1 A reduction step
The following proposition is a refinement of [Reference Jiang10, Theorem 4.1] by the idea of [Reference Jiang and Zou11, Proposition 4.1].
Proposition 4.1. Fix
$0<\epsilon <1$
. Let X be a
$3$
-fold of
$\epsilon $
-Fano type. Then X is birational to a normal projective
$3$
-fold W satisfying the following:
-
(1) W is
$\mathbb {Q}$
-factorial terminal; -
(2)
$\operatorname {Vol}(X, -K_{X})\leq \operatorname {Vol}(W, -K_{W})$
; -
(3) W is of
$\epsilon $
-Fano type; -
(4) there exists a projective morphism
$f: W\to Z$
with connected fibers, such that one of the following conditions holds:-
(a) Z is a point and W is a
$\mathbb {Q}$
-factorial terminal Fano
$3$
-fold with
$\rho (W)=1$
; -
(b)
$Z= \mathbb {P}^1$
; -
(c) Z is a del Pezzo surface with at worst Du Val singularities and
$\rho (Z)=1$
, and general fibers of f are
$\mathbb {P}^1$
.
-
Proof. By [Reference Jiang10, Theorem 4.1], X is birational to W with a Mori fiber structure (see [Reference Jiang10, Definition 2.1]), in particular, W satisfies Properties (1)(2)(3). So here, we only need to explain how to get Property (4) by the proof of [Reference Jiang and Zou11, Proposition 4.1(5)].
Denote by
$W\to T$
the Mori fiber structure on W. Note that
$\dim T\in \{0,1,2\}$
. By [Reference Zhang18, Theorem 1], W is rationally connected, which implies that T is also rationally connected.
If
$\dim T=0$
, then take
$Z=T$
and W is a
$\mathbb {Q}$
-factorial terminal Fano
$3$
-fold with
$\rho (W)=1$
. In this case, we get (a).
If
$\dim T=1$
, then
$T\simeq \mathbb {P}^1$
. In this case, we get (b).
If
$\dim T=2$
, then T is a rational surface as it is rationally connected, and T has at worst Du Val singularities by [Reference Mori and Prokhorov15, Theorem 1.2.7]. We can run a
$K_{T}$
-MMP on T which ends up with a surface
$T'$
, such that either
-
•
$T'$
is a del Pezzo surface with at worst Du Val singularities and
$\rho (T')=1$
, or -
• there is a morphism
$T'\to \mathbb {P}^1$
with connected fibers.
In the former case, take
$Z=T'$
and take
$f: W\to Z$
to be the induced morphism
$W\to T\to T'$
, then general fibers of f are smooth rational curves as
$-K_W$
is ample over T, then we get (c). In the latter case, take
$Z=\mathbb {P}^1$
and take
$f: W\to Z$
to be the induced morphism
$W\to T\to T'\to \mathbb {P}^1$
, then we get (b).
4.2 Proof of Theorem 1.1
According to Proposition 4.1, we can split the discussion into three cases. We essentially follow the proof in [Reference Jiang10].
Proposition 4.2 (cf. [Reference Jiang10, Corollary 6.3]).
Suppose that
$0<\epsilon <\frac {1}{3}$
. Keep the setting as in Proposition 4.1. Assume that case (b) holds. Then
$$ \begin{align*}\operatorname{Vol}(W, -K_{W})< \frac{3200}{\epsilon^4}.\end{align*} $$
Proof. By [Reference Jiang10, Theorem 6.1],
$$ \begin{align*}\operatorname{Vol}(W, -K_{W})\leq \frac{6M(2, \epsilon)}{\mu(2, \epsilon)},\end{align*} $$
where
$M(2, \epsilon )\leq \frac {2}{\epsilon }+4+\frac {2}{3}< \frac {4}{\epsilon }$
by [Reference Jiang8] or [Reference Jiang10, Corollary 4.5] and
$\mu (2, \epsilon )>\frac {3\epsilon ^3}{400}$
by Corollary 3.2.
Proposition 4.3 (cf. [Reference Jiang10, Theorem 6.6]).
Keep the setting as in Proposition 4.1. Assume that case (c) holds. Then
$$ \begin{align*}\operatorname{Vol}(W, -K_{W})\leq\frac{1152}{\epsilon^2}.\end{align*} $$
Proof. By the classification of del Pezzo surfaces with at worst Du Val singularities and Picard rank
$1$
(see [Reference Dolgachev7, Theorem 8.3.2]), there exists a base point free linear system
$\mathcal {H}$
on Z which defines a generically finite map, such that
$\mathcal {H}^2\leq 6$
(see also [Reference Jiang and Zou11, Proposition 4.3]). Take a general element
$H\in \mathcal {H}$
and denote
$G=f^{-1}(H)$
. Consider the self-intersection number
$d=H^2$
.
By [Reference Jiang10, Lemma 6.5], we have
$\operatorname {Vol}(G, -K_{W}|_G)\leq \frac {8(d+2)}{\epsilon }$
. Then by [Reference Jiang10, Theorem 6.6],
$\operatorname {Vol}(W, -K_{W}) \leq \frac {144(d+2)}{\epsilon ^2}$
. This proves the proposition as
$d\leq 6.$
Here, we remark that in [Reference Jiang10, Lemma 6.5, Theorem 6.6], the assumptions are
-
•
$W\to Z$
is a Mori fiber space and
$\dim Z=2$
; -
•
$\mathcal {H}$
is very ample.
But those assumptions can be slightly weakened as in our setting without any other changes to the proofs:
-
• general fibers of
$W\to Z$
are
$\mathbb {P}^1$
; -
•
$\mathcal {H}$
is base point free and defines a generically finite map.
Proof of Theorem 1.1.
By Proposition 4.1, it suffices to bound
$\operatorname {Vol}(W, -K_{W})$
. If case (a) holds, then
$\operatorname {Vol}(W, -K_{W})=(-K_W)^3\leq 64$
by [Reference Prokhorov17]. If case (b) or (c) holds, then the conclusion follows from Propositions 4.2 and 4.3.
Acknowledgements
The authors are grateful to Caucher Birkar for discussions and suggestions. The second author would like to thank her mentor, Professor Caucher Birkar, for his support and encouragement. This work was supported by National Key Research and Development Program of China #2023YFA1010600, NSFC for Innovative Research Groups #12121001, National Key Research and Development Program of China #2020YFA0713200, BMSTC and ACZSP #Z221100002722017. The first author is a member of LMNS, Fudan University. We would like to thank the referee for useful comments and suggestions.
