The $4n^{2}$Inequality for Complete Intersection Singularities
pukh@liverpool.ac.uk
Abstract
The famous $4n^{2}$inequality is extended to generic complete intersection singularities: it is shown that the multiplicity of the selfintersection of a mobile linear system with a maximal singularity is greater than $4n^{2}\mu$, where $\mu$ is the multiplicity of the singular point.
Keywords
1 Statement of the Result
Let $(X,o)$ be a germ of a complete intersection singularity of codimension $l$ and type $\underline{\mu}=(\mu_{1},\dots,\mu_{l})$, where
$\displaystyle\text{dim}X=M\geqslant l+\mu_{1}+\dots+\mu_{l}+3.$ 
We will assume the singularity to be generic in the sense of Sect. 2 below. The aim of this note is to prove the following claim.
Theorem.
Let $\Sigma$ be a mobile linear system on X. Assume that for some positive $n\in{\mathbb{Q}}$ the pair $(X,\frac{1}{n}\Sigma)$ is not canonical at the point $o$ but canonical outside this point. Then the selfintersection $Z=(D_{1}\circ D_{2})$ of the system $\Sigma$ satisfies the inequality
$\mathop{\text{mult}}\nolimits_{o}Z>4n^{2}\mathop{\text{mult}}\nolimits_{o}X.$  (1) 
Remark 1.

(i)
The assumption of the theorem means that the pair $(X,\frac{1}{n}\Sigma)$ has a noncanonical singularity with the centre at the point $o$. Explicitly, for some exceptional divisor $R$ over $X$, the centre of which is the point $o$, the Noether–Fano inequality
$\displaystyle\mathop{\text{ord}}\nolimits_{R}\Sigma>n\cdot a(R,X)$ holds, where $a(R,X)$ is the discrepancy of $R$ with respect to $X$.

(ii)
The selfintersection $Z=(D_{1}\circ D_{2})$ is the schemetheoretic intersection of any two general divisors in $\Sigma$ which is well defined as $\Sigma$ is free from fixed components.

(iii)
When $\mathop{\text{mult}_{o}}X=1$, we get the standard $4n^{2}$inequality, see Pukhlikov ([Pukhlikov2013], Chapter 2). For that reason, we call the inequality (1) the $4n^{2}$inequality as well. The standard $4n^{2}$inequality (for the nonsingular case) was first shown in [Pukhlikov2000] on the basis of the technique developed in [Iskovskikh and Manin1971]. Later a different proof was found by [Corti2000] and various generalizations of the $4n^{2}$inequality were investigated ([Cheltsov2005b]; [Pukhlikov2010]), see Pukhlikov ([Pukhlikov2013], Chapter 2) for more details.
Note that in the smooth case (when $\mathop{\text{mult}}\nolimits_{o}X=1$) the $4n^{2}$inequality holds for $\mathop{\text{dim}}X\geqslant 3$ without any additional assumptions. This is because the exceptional divisor of the blow up of the point $o$ on $X$ is just the projective space, and in the projective space it is very easy to bound multiplicities in terms of degrees. Unfortunately, it is not so easy to do so (in the way we need) for hypersurfaces and complete intersections, which generate the need for additional assumptions.
The author thanks the referees for a number of useful suggestions, especially for spotting the insufficient lower bound for $\mathop{\text{dim}}X$ in the first version of the paper.
2 Generic Complete Intersection Singularities
The germ $(X,o)$ is given by a system of $l$ analytic equations
$\displaystyle\begin{array}[]{ccccccc}0&=&q_{1,\mu_{1}}&+&q_{1,\mu_{1}+1}&+& \dots\\ &\dots&&&&&\\ 0&=&q_{l,\mu_{l}}&+&q_{l,\mu_{l}+1}&+&\dots\\ \end{array}$ 
in ${\mathbb{C}}^{M+l}$, where $2\leqslant\mu_{1}\leqslant\dots\leqslant\mu_{l}$, $l\geqslant 1$ and the polynomials $q_{j,i}$ are homogeneous of degree $i$ in the coordinates $z_{1},\dots,z_{M+l}$; the point $o=(0,\dots,0)$ is the origin. We denote by
$\displaystyle\underline{\mu}=(\mu_{1},\dots,\mu_{l})$ 
the type of the singularity $o\in X$ and set
$\displaystyle\mu=\mu_{1}\cdots\mu_{l}=\mathop{\text{mult}}\nolimits_{o}X$ 
to be the multiplicity of the point $o$ (assuming the conditions of general position for the first polynomials $q_{1,\mu_{1}},q_{2,\mu_{2}},\dots,q_{l,\mu_{l}}$, stated below). Set also
$\displaystyle\underline{\mu}=\mu_{1}+\dots+\mu_{l}.$ 
Recall that by assumption $M\geqslant l+\underline{\mu}+3$. Let $P\ni o$ be a linear subspace in ${\mathbb{C}}^{M+l}$ of dimension $2l+\underline{\mu}+3$. Denote by $X_{P}$ the intersection $X\cap P$.
Definition 1.
We say that the complete intersection singularity $(X,o)$ is generic, if for a general subspace $P$ of dimension $2l+\underline{\mu}+3$ the singularity $o\in X_{P}$ is an isolated singularity, $\mathop{\text{dim}}X_{P}=l+\underline{\mu}+3$ and for the blow up
$\displaystyle\varphi_{P}\colon X^{+}_{P}\to X_{P}$ 
of the point $o$, the variety $X^{+}_{P}$ is nonsingular in neighborhood of the exceptional divisor $Q_{P}=\varphi^{1}_{P}(o)$, which is a nonsingular complete intersection
$\displaystyle Q_{P}=\{q_{1,\mu_{1}}=q_{2,\mu_{2}}=\dots=q_{l,\mu_{l}}=0\} \subset{\mathbb{P}}^{2l+\underline{\mu}+2}$ 
of codimension $l$ and type $\underline{\mu}=(\mu_{1},\dots,\mu_{l})$.
From now on, we assume that the singularity $o\in X$ is generic. In particular, by Grothendieck’s theorem on factoriality ([Call and Lyubeznik1994]), $X$ is a factorial variety near the point $o$.
3 Start of the Proof
The idea of the proof is as follows. We use as a model the proof of the standard $4n^{2}$inequality by means of the technique of counting multiplicities as it is given in Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2). First, we observe that by inversion of adjunction, the existence of a noncanonical singularity $R$ implies the existence of another singularity $E$ of the same pair $(X,\frac{1}{n}\Sigma)$ which satisfies a Noether–Fano type inequality. The latter is somewhat weaker (but sufficient for our purposes). However, the new singularity $E$ has the crucial advantage that its centre on the blow up $X^{+}$ of the point $o$ has a high dimension. This is done in the present section.
After that, in Sect. 4 we resolve the singularity $E$ and use the assumptions on the singular point $o\in X$ to relate the multiplicities of the system $\Sigma$ and its selfintersection at the point $o$ with the multiplicities of the strict transforms of $\Sigma$ and the selfintersection at the “higher storeys” of the resolution, at the centres of the singularity $E$ on those “higher storeys”.
This done, we apply the technique of counting multiplicities in word for word the same way as in Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2) and complete the proof.
Let us realize this programme.
For a general $(2l+\underline{\mu}+3)$subspace $P$ set $\Sigma_{P}=\Sigma_{P}$ to be the restriction of $\Sigma$ onto $P$. By inversion of adjunction ([Shokurov1993]; [Kollár et al.1993]), the pair $(X_{P},\frac{1}{n}\Sigma_{P})$ is not canonical (for $M>l+\underline{\mu}+3$, even nonlog canonical, but we do not need that.) Obviously,
$\displaystyle Z_{P}=Z_{P}=(Z\circ X_{P})$ 
is the selfintersection of the system $\Sigma_{P}$ and $\mathop{\text{ mult}_{o}}Z=\mathop{\text{mult}_{o}}Z_{P}$. Therefore, we may (and will) assume from the beginning that $M=l+\underline{\mu}+3$ and so $P={\mathbb{C}}^{M+l}$, so that already the original singularity $o\in X$ is isolated. Now we omit the index $P$ and write
$\displaystyle\varphi\,\,\colon X^{+}\to X$ 
for the blow up of the point $o$ and $Q=\varphi^{1}(o)$ for the exceptional divisor, which is a nonsingular complete intersection of type $\underline{\mu}$ in ${\mathbb{P}}^{2l+\underline{\mu}+2}$.
Now let $\Pi\ni o$ be a general linear subspace of dimension $\underline{\mu}+3$. By the symbol $X_{\Pi}$ we denote the intersection $X\cap\Pi$. Clearly, $o\in X_{\Pi}\subset\Pi={\mathbb{C}}^{\underline{\mu}+3}$ is an isolated complete intersection singularity of codimension $l$. Let $\varphi_{\Pi}\,\,\colon X^{+}_{\Pi}\to X_{\Pi}$ be the blow up of the point $o$ and $Q_{\Pi}=\varphi^{1}_{\Pi}(o)$ the exceptional divisor. Clearly $Q_{\Pi}\subset{\mathbb{P}}^{\underline{\mu}+2}$ is a nonsingular complete intersection of type $\underline{\mu}$ (and codimension $l$).
Note that by the adjunction formula for the discrepancy we have the equality $a(Q_{\Pi},X_{\Pi})=2$.
For a general divisor $D\in\Sigma$ and its strict transform $D^{+}\in\Sigma^{+}$ on $X^{+}$ we have
$\displaystyle D^{+}\sim\nu\,Q$ 
for some positive integer $\nu$ (recall that we consider a local situation: $o\in X$ is a germ). Obviously, if $\nu>2n$, then
$\displaystyle\mathop{\text{mult}}\nolimits_{o}Z\geqslant\nu^{2}\mu>4n^{2}\mu$ 
and the $4n^{2}$inequality holds. For that reason, from now on we assume that
$\displaystyle\nu\leqslant 2n.$ 
Setting $D_{\Pi}=D_{X_{\Pi}}$, we get $D^{+}_{\Pi}\sim\nu\,Q_{\Pi}$. By the inversion of adjunction the pair $\left(X_{\Pi},\frac{1}{n}D_{\Pi}\right)$ is not log canonical at the point $o$, the more so not canonical, so for some exceptional divisor $E_{\Pi}$ over $X_{\Pi}$ the Noether–Fano inequality
$\displaystyle\mathop{\text{ord}}\nolimits_{E_{\Pi}}\Sigma_{\Pi}>na(E_{\Pi},X_{ \Pi})$ 
is satisfied. As $\nu\leqslant 2n$ and $a(Q_{\Pi},X_{\Pi})=2$, we see that $E_{\Pi}\neq Q_{\Pi}$ and $E_{\Pi}$ is a non log canonical (and so not canonical) singularity of the pair
$\displaystyle\left(X^{+}_{\Pi},\frac{1}{n}D^{+}_{\Pi}+\frac{(\nu2n)}{n}Q_{\Pi }\right)$ 
(the more so, of the pair $\left(X^{+}_{\Pi},\frac{1}{n}D^{+}_{\Pi}\right))$. Denote by $\Delta_{\Pi}\subset Q_{\Pi}$ the centre of $E_{\Pi}$ on $X^{+}_{\Pi}$, an irreducible subvariety in $Q_{\Pi}$.
Proposition 1.
If $\mathop{\text{codim}}\,(\Delta_{\Pi}\subset Q_{\Pi})=1$, then the estimate
$\displaystyle\mathop{\text{mult}}\nolimits_{o}Z\geqslant 8n^{2}\mu$ 
holds.
Proof.
We note that $\mathop{\text{mult}_{o}}Z=\mathop{\text{mult}}_{o}Z_{\Pi}$. Arguing as in the proof of Proposition 4.1 in Pukhlikov ([Pukhlikov2013], Chapter 2) [see also Cheltsov ([Cheltsov2005b], Section 1.7)], we get the following estimate:
$\displaystyle\mathop{\text{mult}}\nolimits_{o}Z_{\Pi}\geqslant\nu^{2}\mu+4 \left(3\frac{\nu}{n}\right)n^{2}\mu,$ 
and easy calculations complete the proof. $\square$
Therefore we may assume that $\mathop{\text{codim}}(\Delta_{\Pi}\subset Q_{\Pi})\geqslant 2$.
Coming back to the variety $X$, we conclude that for some exceptional divisor $E$ over $X$ with the centre at $o$ the Noether–Fano type inequality
$\displaystyle\mathop{\text{ord}}\nolimits_{E}\Sigma>n(2\mathop{\text{ ord}}\nolimits_{E}Q+a(E,X^{+}))$ 
is satisfied. Moreover, the centre $\Delta\subset Q$ of $E$ on $X$ has codimension at least 2 and dimension at least $2l$.
4 Resolution of the Singularity $E$
Consider the sequence of blow ups
$\displaystyle X_{0}=X\leftarrow X_{1}=X^{+}\leftarrow X_{2}\leftarrow\dots \leftarrow X_{K},$ 
where $\varphi_{i,i1}\colon X_{i}\to X_{i1}$ is the blow up of the centre $B_{i1}\subset X_{i1}$ of the exceptional divisor $E$ on $X_{i1}$. In particular, $B_{0}=o$ and $B_{1}=\Delta$. Using the notations, identical to those in Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2), we set
$\displaystyle E_{i}=\varphi^{1}_{i,i1}(B_{i1})\subset X_{i}$ 
to be the exceptional divisor, so that $E_{1}=Q$. As $X_{1}=X^{+}$ is nonsingular in a neighborhood of $E_{1}$, all subsequent varieties $X_{i}$ are nonsingular at the generic point of $B_{i}$ and all constructions of Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2) work automatically for the blow ups $\varphi_{i,i1}$ with $i\geqslant 2$.
The last exceptional divisor $E_{K}$ defines the discrete valuation $\mathop{\text{ord}}_{E}$.
We divide the sequence $\varphi_{i,i1}$, $i=1,\dots,K$, of blow ups into the lower part, $i=1,\dots,L\leqslant K$, corresponding to the centres $B_{i1}$ of codimensions at least 3, and the upper part, $i=L+1,\dots,K$, corresponding to the centres $B_{i1}$ of codimension two. As usual, we denote the strict transform of any geometric object on $X_{i}$ by adding the upper index $i$ and set:
$\displaystyle\nu_{i}=\mathop{\text{mult}}\nolimits_{B_{i1}}\Sigma^{i}$ 
for any $i=2,\dots,K$ to be the elementary multiplicities. Let $\Gamma$ be the oriented graph of the resolution of the singularity $E$ and $p_{ij}$ the number of paths from the vertex $i$ to the vertex $j$, $p_{ii}=1$ by definition [see Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2) for the standard details]. We also set $p_{i}=p_{Ki}$, $i=1,\dots,K$. Now the Noether–Fano type inequality takes the form
$\sum^{K}_{i=1}p_{i}\nu_{i}>\left(2p_{1}+\sum^{K}_{i=2}p_{i}\delta_{i}\right),$  (2) 
where $\nu_{1}=\nu$ and $\delta_{i}=\mathop{\text{codim}}(B_{i1}\subset X_{i1})$ are the elementary discrepancies. By the linearity of the Noether–Fano type inequality (2) and the standard properties of the numbers $p_{ij}$ we may assume that $\nu_{K}>n$ (replacing, if necessary, $E_{K}$ by a lower singularity $E_{j}$ for some $j<K$). In order to proceed, we need the following known fact.
Proposition 2.
Let $Y\subset{\mathbb{P}}^{N}$ be a nonsingular complete intersection of codimension $l\geqslant 1$, $S\subset Y$ an irreducible subvariety of codimension $a\geqslant 1$ and $B\subset Y$ an irreducible subvariety of dimension $al$, where the estimate $N\geqslant(l+1)(a+1)$ is satisfied. Then the inequality
$\displaystyle\mathop{\text{mult}}\nolimits_{B}S\leqslant m$ 
holds, where $m\geqslant 1$ is defined by the condition $S\sim mH^{a}_{Y}$ and $H_{Y}\in A^{1}Y$ is the class of a hyperplane section of $Y$.
Proof.
for the case $l=1$ was given in [Pukhlikov2002b]. The argument extends directly to the general case of an arbitrary $l$, see [Suzuki2015] (also [Pukhlikov2006]; [Cheltsov2005a]). $\square$
Applying Proposition 2 to a divisor in the linear system $\Sigma^{1}_{Q}$, we conclude that $\nu_{1}\geqslant\nu_{2}$, since $\mathop{\text{dim}}B_{1}=\mathop{\text{dim}}\Delta\geqslant 2l$. The inequalities
$\displaystyle\nu_{2}\geqslant\nu_{3}\geqslant\dots\geqslant\nu_{K}$ 
are standard. We deduce that the upper part of the resolution of $E$ is nonempty (that is to say, $L<K$) and the upper part of the graph $\Gamma$ is a chain:
$\displaystyle L\leftarrow(L+1)\leftarrow\dots\leftarrow K;$ 
moreover, there are no arrows connecting either of the vertices $L+1,\dots,K$ with any of vertices $1,2,\dots,L1$. [These are the standard consequences of inequalities $\nu_{K}>n$ and $\nu_{1}\leqslant 2n$, see Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2).] We do not need this additional information for the proof of our theorem, but in particular geometric problems it might be useful.
5 The Technique of Counting Multiplicities
Now everything is ready for the proof of the desired inequality (1). Take a general pair of divisors $D_{1},D_{2}\in\Sigma$ and set
$\displaystyle Z=Z_{0}=(D_{1}\circ D_{2})$ 
to be their schemetheoretic intersection, the selfintersection of the mobile linear system $\Sigma$. Recall that the strict transform of an irreducible subvariety or an effective cycle, or a linear system on some $X_{i}$ is denoted by adding the upper index $i$. (This notation silently implies that the irreducible subvariety or the effective cycle etc. is sitting on a lower storey $X_{j}$, $j\leqslant i$, of the resolution and that the operation of taking the strict transform is well defined for that particular subvariety etc.) For $i\geqslant 1$ write
$\displaystyle(D^{i}_{1}\circ D^{i}_{2})=(D^{i1}_{1}\circ D^{i1}_{2})^{i}+Z_{ i},$ 
where the effective cycle $Z_{i}$ of codimension 2 is supported on $E_{i}$ and so may be viewed as an effective divisor on $E_{i}$. Thus for any $i\leqslant L$ we obtain the presentation
$\displaystyle(D^{i}_{1}\circ D^{i}_{2})=Z^{i}_{0}+Z^{i}_{1}+\dots+Z^{i}_{i1}+ Z_{i}.$ 
For any $j>i$, where $j\leqslant L$, set
$\displaystyle m_{i,j}=\mathop{\text{mult}}\nolimits_{B_{j1}}Z^{j1}_{i}$ 
and for $i=2,\dots,L$ set $d_{i}=\mathop{\text{deg}}Z_{i}$ in the same sense as in Pukhlikov ([Pukhlikov2013], Chapter 2, Section 2.2). For the effective divisor $Z_{1}$ on $E_{1}=Q$ we have the relation
$\displaystyle Z_{1}\sim d_{1}H_{Q}$ 
for some $d_{1}\in{\mathbb{Z}}_{+}$, where $H_{Q}$ is the class of a hyperplane section of the complete intersection $Q\subset{\mathbb{P}}^{4l+2}$. Following the procedure of Pukhlikov ([Pukhlikov2013], Chapter 2), we obtain the system of equalities
$\displaystyle\begin{array}[]{rccccccc}\mu&(\nu_{1}^{2}+d_{1})&=&m_{0,1},&&&&\\ &\nu_{2}^{2}+d_{2}&=&m_{0,2}&+&m_{1,2},&&\\ &&\dots&&&&&\\ &\nu_{i}^{2}+d_{i}&=&m_{0,i}&+&\dots&+&m_{i1,i},\\ &&\dots&&&&&\end{array}$ 
$i=2,\dots,L$, where the estimate
$\displaystyle d_{L}\geqslant\sum^{K}_{i=L+1}\nu^{2}_{i}$ 
holds as usual, see Pukhlikov ([Pukhlikov2013], p. 53).
Proposition 3.

(i)
The inequality
$\displaystyle d_{1}\geqslant m_{1,2}$ holds.

(ii)
The inequality
$\displaystyle m_{0,1}\geqslant\mu m_{0,2}$ holds.
Proof.
Part (i) follows from Proposition 2 as $Z_{1}\sim d_{1}H_{Q}$ and $\mathop{\text{dim}}B_{1}\geqslant 2l$. In order to show part (ii), we note that (numerically)
$\displaystyle(Z^{1}\circ E_{1})\sim\frac{1}{\mu}m_{0,1}H^{2}_{Q}$ 
as $m_{0,1}=\mathop{\text{deg}}(Z^{1}\circ E_{1})$, the cycle $(Z^{1}\circ E_{1})=(Z^{1}\circ Q)$ being of pure codimension 2 on $Q$. Applying Proposition 2 to the cycle $(Z^{1}\circ Q)$, we get the inequality
$\displaystyle m_{0,2}\leqslant\mathop{\text{mult}}\nolimits_{\Delta}(Z^{1} \circ Q)\leqslant\frac{1}{\mu}m_{0,1},$ 
which completes the proof of the proposition. $\square$
The more so, $m_{0,1}\geqslant\mu m_{0,i}$ for $i\geqslant 3$ as $m_{0,2}\geqslant m_{0,3}\geqslant\dots\geqslant m_{0,L}$.
Now set
$\displaystyle m^{*}_{i,j}=\mu m_{i,j}$ 
for $(i,j)\neq(0,1)$ and $m^{*}_{0,1}=m_{0,1}$. Also set
$\displaystyle d^{*}_{i}=\mu d_{i}$ 
for $i=1,\dots,L$. We obtain the following system of equalities:
$\displaystyle\begin{array}[]{ccccccc}\mu\nu_{1}^{2}+d^{*}_{1}&=&m^{*}_{0,1},&& &&\\ \mu\nu_{2}^{2}+d^{*}_{2}&=&m^{*}_{0,2}&+&m^{*}_{1,2},&&\\ &\dots&&&&&\\ \mu\nu_{i}^{2}+d^{*}_{i}&=&m^{*}_{0,i}&+&\dots&+&m^{*}_{i1,i},\\ &\dots&&&&&\end{array}$ 
where $i=1,\dots,L$, and
$\displaystyle d^{*}_{L}\geqslant\mu\sum^{K}_{i=L+1}\nu^{2}_{i},$ 
where the integers $m^{*}_{i,j}$ and $d^{*}_{i}$ satisfy precisely the same properties, as the integers $m_{i,j}$ and $d_{i}$ in the nonsingular case considered in Pukhlikov ([Pukhlikov2013], Chapter 2, p. 52–53). Now repeating the arguments of Pukhlikov ([Pukhlikov2013], Chapter 2, p. 52–53) word for word, we obtain the inequality
$\displaystyle\left(\sum^{L}_{i=1}p_{i}\right)\mathop{\text{ mult}}\nolimits_{o}Z\geqslant\mu\sum^{K}_{i=1}p_{i}\nu^{2}_{i},$ 
which in the standard way implies the desired estimate
$\displaystyle\mathop{\text{mult}}\nolimits_{o}Z>\mu\cdot 4n^{2}.$ 
Proof of the theorem is completed.
Remark 2.
The inequality (1) essentially simplifies the proof of birational superrigidity of Fano hypersurfaces with isolated singularities of general position given in [Pukhlikov2002a]. The cases of singular points of multiplicity $\mu=3$ and 4 in that paper are really hard. The inequality (1) gives for the multiplicity $\mathop{\text{mult}_{o}}Z$ at such points the lower bound $12n^{2}$ and $16n^{2}$, respectively, which is more than enough to exclude the maximal singularities over such points by the standard (in fact, relaxed) technique of hypertangent divisors. More applications of the inequality (1) in the spirit of [Eckl and Pukhlikov2014], [Evans and Pukhlikov2016] will be given separately.
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