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ABSTRACT
We collect a list of known four-dimensional Fano manifolds and compute their quantum periods. This list includes all four-dimensional Fano manifolds of index greater than one, all four-dimensional toric Fano manifolds, all four-dimensional products of lower-dimensional Fano manifolds, and certain complete intersections in projective bundles.
MATHEMATICS SUBJECT CLASSIFICATION:
1. Introduction
In this paper, we take the first step toward implementing a program, laid out in [CitationCoates et al. 12], to find and classify four-dimensional Fano manifolds using mirror symmetry. We compute quantum periods and quantum differential equations for many known four-dimensional Fano manifolds, using techniques described in [CitationCoates et al. 16]. Our basic reference for the theory of Fano manifolds is the book by Iskovskikh–Prokhorov [CitationIskovskikh and Prokhorov 99]. Recall that the index of a Fano manifold X is the largest integer r such that − KX = rH for some ample divisor H. A four-dimensional Fano manifold has index at most 5 [CitationShokurov 85]. Four-dimensional Fano manifolds with index r > 1 have been classified. In what follows we compute the quantum periods and quantum differential equations for all four-dimensional Fano manifolds of index r > 1, for all four-dimensional Fano toric manifolds, and for certain other four-dimensional Fano manifolds of index 1.
Highlights
We draw the reader’s attention to:
Section 6.2.4, where new tools for computing Gromov–Witten invariants (twisted I-functions for toric complete intersections [CitationCoates et al. 14] and an improved Quantum Lefschetz theorem [CitationCoates 14]) make a big practical difference to the computation of quantum periods. This should be contrasted with [CitationCoates et al. 16, Section 19], where the new techniques were not available.
Section 6.2.11, which relies on a new construction of Szurek–Wiśniewski’s null-correlation bundle [CitationSzurek and Wiśniewski 90] that may be of independent interest.
The tables of regularized quantum period sequences in Appendix A.
The numerical calculation of quantum differential operators in Section 9 and Appendix B. This suggests in particular that, for each four-dimensional Fano manifold X with Fano index r > 1, the regularized quantum differential equation of X is either extremal or of low ramification.
Section 6.2.17 and Section B.34, which together give an example of a product such that the regularized quantum differential equation for each factor is extremal, but the regularized quantum differential equation for the product itself is not.
This paper is accompanied by fully commented source code, written in the computational algebra system Magma [CitationBosma et al. 97]. This will allow the reader to verify the calculations presented here, or to perform similar computations.
2. Methodology
The quantum period GX of a Fano manifold X is a generating function
(2--1)
(2--1) for certain genus-zero Gromov–Witten invariants cd of X. A precise definition can be found in [CitationCoates et al. 16, Section B], but roughly speaking cd is the “virtual number” of degree-d rational curves C in X that pass through a given point and satisfy certain constraints on their complex structure. (The degree of a curve C here is the quantity ⟨ − KX, C⟩.) The quantum period is discussed in detail in [CitationCoates et al. 12, CitationCoates et al. 16]; one property that will be important in what follows is that the regularized quantum period
(2--2)
(2--2) satisfies a differential equation called the regularized quantum differential equation of X:
(2--3)
(2--3) where the pm are polynomials and
. It is expected that the regularized quantum differential equation for a Fano manifold X is extremal or of low ramification, as described in Section 9 below. This is a strong constraint on the Gromov–Witten invariants cd of X.
Quantum periods for a broad class of toric complete intersections can be computed using Givental’s mirror theorem [CitationGivental 96].
Theorem 2.1 ([CitationCoates et al. 16, Corollary C.2]).
Let X be a toric Fano manifold and let be the cohomology classes Poincaré-dual to the torus-invariant divisors on X. The quantum period of X is
Theorem 2.2 ([CitationCoates et al. 16, Corollary D.5]).
Let Y be a toric Fano manifold, and let be the cohomology classes Poincaré-dual to the torus-invariant divisors on Y. Let X be the complete intersection in Y defined by a regular section of E = L1⊕⋅⋅⋅⊕Ls where each Li is a nef line bundle, and let ρi = c1(Li), 1 ⩽ i ⩽ s. Suppose that the class c1(Y) − Λ is ample on Y, where Λ = c1(L1) + ⋅⋅⋅ + c1(Ls). Then X is Fano, and the quantum period of X is
where c is the unique rational number such that the right-hand side has the form 1 + O(t2).
An analogous mirror theorem holds for certain complete intersections in toric Deligne–Mumford stacks, but we will need only the case where the ambient stack is a weighted projective space.
Theorem 2.3 ([CitationCoates et al. 16, Proposition D.9]).
Let Y be the weighted projective space , let X be a smooth Fano manifold given as a complete intersection in Y defined by a section of
, and let − k = w0 + ⋅⋅⋅ + wn − d1 − ⋅⋅⋅ − dm. Suppose that each di is a positive integer, that − k > 0, and that wi divides dj for all i, j such that 0 ⩽ i ⩽ n and 1 ⩽ j ⩽ m. Then the quantum period of X is
where c is the unique rational number such that the right-hand side has the form 1 + O(t2).
The quantum period of a product is the product of the quantum periods.
Theorem 2.4 ([CitationCoates et al. 16, Corollary E.4]).
Let X and Y be smooth projective complex manifolds. Then:
As we will see below, another powerful tool for computing quantum periods is the Abelian/non-Abelian Correspondence of Ciocan-Fontanine–Kim–Sabbah [CitationCiocan-Fontanine et al. 08]. We now proceed to the calculation of quantum periods.
3. Four-dimensional Fano manifolds of index 5
The only example here is [CitationKobayashi and Ochiai 73, CitationKollár 81, CitationSerpico 80]. This is a toric variety. Theorem 2.1 yields
4. Four-dimensional Fano manifolds of index 4
The only example here is the quadric [CitationKollár 81, CitationSerpico 80]. This is a complete intersection in a toric variety. Theorem 2.2 yields
5. Four-dimensional Fano manifolds of index 3
There are six examples [CitationFujita 80, CitationFujita 81, CitationFujita 84, CitationFujita 90, CitationIskovskih 77, CitationIskovskih 79, CitationIskovskikh and Prokhorov 99], which are known as del Pezzo fourfolds:
a sextic hypersurface FI41 in the weighted projective space
;
a quartic hypersurface FI42 in the weighted projective space
;
a cubic hypersurface
;
a complete intersection
of type (2H)∩(2H), where
;
a complete intersection FI45⊂Gr(2, 5) of type H∩H, where H is the hyperplane bundle; and
.
The first four examples here are complete intersections in weighted projective spaces. Theorem 2.3 yields
For FI45⊂Gr(2, 5), we use the Abelian/non-Abelian Correspondence, applying Theorem F.1 in [CitationCoates et al. 16] with a = 2, b = c = d = e = 0. This yields
where Hm is the mth harmonic number. For
, combining Theorem 2.4 with [CitationCoates et al. 16, Example G.2] yields
Table
6. Four-dimensional Fano manifolds of index 2
Consider now a four-dimensional Fano manifold with index r = 2 and Picard rank ρ.
6.1. The case ρ = 1
Four-dimensional Fano manifolds with index r = 2 and Picard rank ρ = 1 have been classified [CitationMukai 89, CitationWilson 87], [CitationIskovskikh and Prokhorov 99, Chapter 5]. Up to deformation, there are nine examples: the “linear unsections” of smooth three-dimensional Fano manifolds with ρ = 1, r = 1, and degree at most 144. We compute the quantum periods of these examples using the constructions in [CitationCoates et al. 16, Sections 8–16], writing V4k for a four-dimensional Fano manifold with ρ = 1, r = 2, and degree 16k.
6.1.1. V42. [regularized quantum period p. 16, operator p. 28]
This is a sextic hypersurface in . Proposition D.9 in [CitationCoates et al. 16] yields
6.1.2. V44. [regularized quantum period p. 16, operator p. 28]
This is a quartic hypersurface in . Theorem 2.2 yields
6.1.3. V46. [regularized quantum period p. 16, operator p. 28]
This is a complete intersection of type (2H)∩(3H) in , where
. Theorem 2.2 yields
6.1.4. V48. [regularized quantum period p. 16, operator p. 29]
This is a complete intersection of type (2H)∩(2H)∩(2H) in , where
. Theorem 2.2 yields
6.1.5. V410. [regularized quantum period p. 16, operator p. 29]
This is a complete intersection in Gr(2, 5), cut out by a regular section of where
is the pullback of
on projective space under the Plücker embedding. We apply Theorem F.1 in [CitationCoates et al. 16] with a = b = 1 and c = d = e = 0. This yields
where Hm is the mth harmonic number.
6.1.6. V412. [regularized quantum period p. 16, operator p. 29]
This is the subvariety of Gr(2, 5) cut out by a regular section of , where S is the universal bundle of subspaces on Gr(2, 5). We apply Theorem F.1 in [CitationCoates et al. 16] with c = 1 and a = b = d = e = 0. This yields
6.1.7. V414. [regularized quantum period p. 16, operator p. 30]
This is a complete intersection in Gr(2, 6), cut out by a regular section of where
is the pullback of
on projective space under the Plücker embedding. We apply Theorem F.1 in [CitationCoates et al. 16] with a = 4 and b = c = d = e = 0. This yields
6.1.8. V416. [regularized quantum period p. 16, operator p. 30]
This is the subvariety of Gr(3, 6) cut out by a regular section of , where S is the universal bundle of subspaces on Gr(3, 6). We apply Theorem F.1 in [CitationCoates et al. 16] with a = 2, b = c = d = 0, and e = 1. This shows that the quantum period
is the coefficient of (p2 − p1)(p3 − p1)(p3 − p2) in the expression:
(Since this expression is totally antisymmetric in p1, p2, p3, it is divisible by (p2 − p1)(p3 − p1)(p3 − p2).)
6.1.9. V418. [regularized quantum period p. 16, operator p. 30]
This is the subvariety of Gr(5, 7) cut out by a regular section of , where S is the universal bundle of subspaces on Gr(5, 7). We apply Theorem F.1 in [CitationCoates et al. 16] with a = d = 1 and b = c = e = 0. This shows that the quantum period
is the coefficient of ∏1 ⩽ i < j ⩽ 5(pj − pi) in the expression:
where |l| = l1 + l2 + ⋅⋅⋅ + l5. (As above, antisymmetry implies that the long formula here is divisible by ∏1 ⩽ i < j ⩽ 5(pj − pi).)
6.2. The case ρ > 1
Four-dimensional Fano manifolds with ρ > 1 and r = 2 have been classified by Mukai [CitationMukai 89, CitationMukai 88] and Wiśniewski [CitationWiśniewski 90]. There are 18 deformation families, as follows. We denote the kth such deformation family, as given in [CitationIskovskikh and Prokhorov 99, Table 12.7], by MW4k.
6.2.1. MW41. [regularized quantum period p. 16, operator p. 31]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] and [CitationCoates et al. 16, Section 3] yields
6.2.2. MW42. [regularized quantum period p. 16, operator p. 32]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] and [CitationCoates et al. 16, Section 4] yields
6.2.3. MW43. [regularized quantum period p. 16, operator p. 33]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] and [CitationCoates et al. 16, Section 5] yields
6.2.4. MW44. [regularized quantum period p. 16, operator p. 34]
This is a double cover of , branched over a divisor of bidegree (2, 2). Consider the toric variety F with weight data:
and
. Let X be a member of the linear system |2L + 2M| defined by the equation w2 = f2, 2, where f2, 2 is a bihomogenous polynomial of degrees 2 in x0, x1, x2 and 2 in y0, y1, y2. Let
be the rational map which sends (contravariantly) the homogenous co-ordinate functions [x0, x1, x2, y0, y1, y2] on
to [x0, x1, x2, y0, y1, y2]. The restriction of p to X is a morphism, which exhibits X as a double cover of
branched over the locus
. Thus X = MW44.
Recall the definition of the J-function JX(t, z) from [CitationCoates and Givental 07, equation 11]. Recall from [CitationCoates 14] that there is a Lagrangian cone that encodes all genus-zero Gromov–Witten invariants of X, and a Lagrangian cone
that encodes all genus-zero
-twisted Gromov–Witten invariants of F. Here ΛX and ΛF are certain Novikov rings and
is the total Chern class with parameter λ (or, equivalently,
is the S1-equivariant Euler class with respect to an action of S1 described in [CitationCoates 14]; in this case one should regard λ as the standard generator for the S1-equivariant cohomology algebra of a point). The J-function JX is characterized by the fact that JX(t, − z) is the unique point on
of the form − z + t + O(z− 1).
Let p1, denote the first Chern class of L, L + M, respectively, and let P1,
denote the pullbacks of p1, p2 along the inclusion map i: X → F. Let Q1, Q2 denote the elements of the Novikov ring ΛX that are dual, respectively, to P1, P2, and note that ΛX and ΛF are canonically isomorphic (via i⋆). Theorem 22 in [CitationCoates et al. 14] implies that:
satisfies
. Theorem 1.1 in [CitationCoates 14] gives that
, and therefore that:
Since the hypersurface X misses the locus y1 = y2 = y3 = 0 in F, we have that i⋆(p2 − p1)3 = 0. Thus:
In particular, i⋆I(t1, t2, 0, − z) has the form − z + t1P1 + t2P2 + O(z− 1) and, from the characterization of JX discussed above, we conclude that JX(t1P1 + t2P2, − z) = i⋆I(t1, t2, 0, − z).
To extract the quantum period GX from the J-function JX(t1P1 + t2P2, z), we take the component along the unit class , set z = 1, set t1 = t2 = 0, and set Q1 = 1, Q2 = t2, obtaining:
6.2.5. MW45. [regularized quantum period p. 16, operator p. 34]
This is a divisor on of bidegree (1, 2). Theorem 2.2 yields
6.2.6. MW46. [regularized quantum period p. 16, operator p. 35]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] and [CitationCoates et al. 16, Section 6] yields
6.2.7. MW47. [regularized quantum period p. 16, operator p. 36]
This is a complete intersection of two divisors in , each of bidegree (1, 1). Theorem 2.2 yields
6.2.8. MW48. [regularized quantum period p. 16, operator p. 36]
This is a divisor on of bidegree (1, 1). Theorem 2.2 yields
6.2.9. MW49. [regularized quantum period p. 16, operator p. 37]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] and [CitationCoates et al. 16, Section 7] yields
6.2.10. MW410. [regularized quantum period p. 16, operator p. 38]
This is the blow-up of the quadric Q4 along a conic that is not contained in a plane lying in Q4. Consider the toric variety F with weight data:
and
. The morphism
that sends (contravariantly) the homogenous co-ordinate functions [x0, x1, …, x5] to [xs0, xs1, xs2, x3, x4, x5] blows up the plane Π = (x0 = x1 = x2 = 0) in
. Thus, a general member of |2M| on F is the blow-up of Q4 with center a conic on Π. In other words, a general member of |2M| on F is MW410. We have
− KF = 2L + 4M is ample, so that F is a Fano variety;
MW410 ∼ 2M is ample;
− (KF + 2M) ∼ 2L + 2M is ample.
Theorem 2.2 yields
6.2.11. MW411. [regularized quantum period p. 16, operator p. 38]
This is the projective bundle , where
is the null-correlation bundle of Szurek–Wiśniewski [CitationSzurek and Wiśniewski 90].
Remark 6.1.
For us denotes the projective bundle of lines in E, whereas in Szurek–Wiśniewski and Iskovskikh–Prokhorov,
denotes the projective bundle of one-dimensional quotients. With our conventions, if
is a projective bundle then
, and so a regular section
vanishes on
, where the vector bundle F → X is the cokernel of
.
Proposition 6.2.
Let , so that
. Consider the partial flag manifold Fl1, 2(V) and the natural projections
(6--4)
(6--4) Let |L| denote the linear system defined by
for the projective bundle p1. Then a general element of |L| is
, where
is the null-correlation bundle.
Proof.
The null-correlation bundle has rank 2, and so the perfect pairing gives canonical isomorphisms
and
. There is an exact sequence:
and the map
therein defines a section
. The construction in Remark 6.1 now exhibits
as the locus (s = 0) in
. We will identify
with the partial flag manifold Fl1, 2(V).
For a vector bundle of rank 3, the perfect pairing
gives a canonical isomorphism
. Applying this with
equal to
gives
where
. We thus need to identify
with Fl1, 2(V).
The Plücker embedding maps a subspace W ∈ Gr(2, V) to the antisymmetric linear map LW: V⋆ → V, well-defined up to scale, given by
where {w1, w2} is a basis for W. The kernel of LW is the annihilator W⊥⊂V⋆. If
then
; this implies in particular that rkLW = 2. Thus, the image of the Plücker embedding consists of (the lines spanned by) antisymmetric linear maps LW: V⋆ → V of rank 2, and one can recover W ∈ Gr(2, V) from its image ⟨LW⟩ by taking the annihilator of the kernel:
There is a canonical isomorphism Ann: Gr(2, V) → Gr(2, V⋆) which maps W ∈ Gr(2, V) to W⊥.
Recall that our goal is to identify with Fl1, 2(V). Let
denote the projection. The Euler sequence:
gives, via [CitationHartshorne 77, II, Exercise 5.16]:
and thus:
(6--5)
(6--5) This defines a map
. Consider the fiber of the sequence (Equation6–5
(6--5)
(6--5) ) over
. The map π⋆(∧2V⋆) → π⋆V⋆(1) here is given by contraction with v, and so non-zero elements of the kernel are antisymmetric linear maps V → V⋆ of rank 2. (They are antisymmetric, hence have rank 0, 2, or 4; they are non-zero, hence are not of rank 0; and they have the non-zero element v in their kernel, hence are not of rank 4.) In particular, we see that the image of f lies in
. Given
, write W[x]⊂V⋆ for the linear subspace defined by f([x]). Suppose that
lies over
. Then, applying the discussion in the previous paragraph but with V there replaced by V⋆, we see that v ∈ W⊥[x]. Thus, writing
for the composition
we have that q1([x])⊂q2([x]), i.e., that the diagram:
coincides with the diagram (Equation6–4
(6--4)
(6--4) ). This identifies
with the partial flag manifold Fl1, 2(V), and exhibits
as an element of the linear system |L| as claimed.
Abelianization:
To compute the quantum period, we use the Abelian/non-Abelian Correspondence of Ciocan-Fontanine–Kim–Sabbah, as in [CitationCoates et al. 16, Section 39]. Consider the situation as in Section 3.1 of [CitationCiocan-Fontanine et al. 08] with:
, regarded as the space of pairs:
, acting on X as:
, the diagonal subtorus in G;
the group that is denoted by S in [CitationCiocan-Fontanine et al. 08] set equal to the trivial group;
equal to the representation of G given by the determinant of the standard representation of the second factor
.
Then X//G is the partial flag manifold , whereas X//T is the toric variety with weight data:
and
, that is, X//T is the projective bundle
over
. The non-trivial element of the Weyl group
exchanges the two factors of
. The representation
induces the line bundle
over X//G = Fl, where L was defined in the statement of Proposition 6.2, whereas the representation
induces the line bundle
over X//T.
The Abelian/non-Abelian correspondence:
Let p1, p2, and denote the first Chern classes of the line bundles L1, L2, and H, respectively. We fix a lift of
to
in the sense of [CitationCiocan-Fontanine et al. 08, Section 3]; there are many possible choices for such a lift, and the precise choice made will be unimportant in what follows. The lift allows us to regard
as a subspace of
, which maps isomorphically to the Weyl-anti-invariant part
of
via
We compute the quantum period of MW411⊂X//G by computing the J-function of Fl = X//G twisted, in the sense of [CitationCoates and Givental 07], by the Euler class and the bundle
, using the Abelian/non-Abelian Correspondence.
Our first step is to compute the J-function of X//T twisted by the Euler class and the bundle . As in [CitationCoates et al. 16, Section D.1] and as in [CitationCiocan-Fontanine et al. 08], consider the bundles
and
equipped with the canonical
-action that rotates fibers and acts trivially on the base. Recall the definition of the twisted J-function
of X//T from [CitationCoates et al. 16, Section D.1]. We will compute
using the Quantum Lefschetz theorem;
is the restriction to the locus τ ∈ H0(X//T)⊕H2(X//T) of what was denoted by
in [CitationCiocan-Fontanine et al. 08]. The toric variety X//T is Fano, so Theorem C.1 in [CitationCoates et al. 16] gives
where τ = τ1p1 + τ2p2 + τ3p3 and we have identified the group ring
with
via the
-linear map that sends Qβ to Q⟨β, p1⟩1Q⟨β, p2⟩2Q⟨β, p3⟩3. The line bundles L1, L2, and H are nef, and
is ample, so Theorem D.3 in [CitationCoates et al. 16] gives
Consider now Fl = X//G and a point t ∈ H•(Fl). Recall that is the projectivization of the universal bundle S of subspaces on Gr ≔ Gr(2, 4). Let
be the pullback to Fl (under the projection map p2: Fl → Gr) of the ample generator of H2(Gr), and let
be the first Chern class of
. Identify the group ring
with
via the
-linear map which sends Qβ to q⟨β, ε1⟩1q⟨β, ε2⟩2. In [CitationCiocan-Fontanine et al. 08, Section 6.1], the authors consider the lift
of their twisted J-function
determined by a choice of lift
. We restrict to the locus
, considering the lift:
of our twisted J-function
determined by our choice of lift
. Theorems 4.1.1 and 6.1.2 in [CitationCiocan-Fontanine et al. 08] imply that
for some function
. Setting t = 0 gives
For symmetry reasons, the right-hand side here is divisible by p2 − p1; it takes the form:
whereas:
We conclude that ϕ(0) = 0. Thus,
(6--6)
(6--6)
To extract the quantum period G4MW11 from the twisted J-function , we proceed as in [CitationCoates et al. 16, Example D.8]: we take the non-equivariant limit, extract the component along the unit class
, set z = 1, and set Qβ = t⟨β, − K⟩ where K = K4MW11. Thus, we consider the right-hand side of (Equation6–6
(6--6)
(6--6) ), take the non-equivariant limit, extract the coefficient of p2 − p1, set z = 1, and set q1 = q2 = 2t, obtaining:
Remark 6.3.
The quantum period of can also be computed using Strangeway’s reconstruction theorem for the quantum cohomology of Fano bundles [CitationStrangeway 15, Theorem 1]. Thus, the quantum period of MW411 can be derived from this result together with the Quantum Lefschetz theorem. The Gromov–Witten invariants required as input to the reconstruction theorem can be computed via [CitationStrangeway 15, Lemma 1], using Schubert calculus on Gr(2, 4) and intersection numbers in
.
6.2.12. MW412. [regularized quantum period p. 16, operator p. 39]
This is the blow-up of the quadric Q4 along a line. Consider the toric variety F with weight data:
and
. The morphism
that sends (contravariantly) the homogenous co-ordinate functions [x0, x1, …, x5] to [xs0, xs1, xs2, xs3, x4, x5] blows up the line (x0 = x1 = x2 = x3 = 0) in
, and MW412 is the proper transform of a quadric containing this line. Thus MW412 is a member of |L + M| in the toric variety F. We have:
− KF = 3L + 3M is ample, so that F is a Fano variety;
MW412 ∼ L + M is ample;
− (KF + L + M) ∼ 2L + 2M is ample.
Theorem 2.2 yields
6.2.13. MW413. [regularized quantum period p. 16, operator p. 40]
This is the projective bundle or, equivalently, a member of |2L| in the toric variety F with weight data:
and
. We have
− KF = 4L + 2M is ample, that is, F is a Fano variety;
MW413 ∼ 2L is nef;
− (KF + 2L) ∼ 2L + 2M is ample.
The projection [x0: x1: x2: x3: x4: x5: u: v]↦[x0: x1: x2: x3: x4: x5] exhibits F as the scroll over
, and passing to a member of |2L| restricts this scroll to
. Theorem 2.2 yields
6.2.14. MW414. [regularized quantum period p. 16, operator p. 40]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] and [CitationCoates et al. 16, Section 1] yields
6.2.15. MW415. [regularized quantum period p. 16, operator p. 41]
This is the projective bundle , or in other words, the toric variety with weight data:
and
. Theorem 2.1 yields
6.2.16. MW416. [regularized quantum period p. 16, operator p. 42]
This is the product , where
is a divisor of bidegree (1, 1). Theorem 2.2 yields
6.2.17. MW417. [regularized quantum period p. 16, operator p. 43]
This is the product , where B37 is the blow-up of
at a point. Note that B37 is the projective bundle
. It follows that MW417 is the toric variety with weight data:
and
. Theorem 2.1 yields
6.2.18. MW418. [regularized quantum period p. 16, operator p. 44]
This is the product . Combining Theorem 2.4 with [CitationCoates et al. 16, Example G.1] yields
7. Four-dimensional Fano toric manifolds
Four-dimensional Fano toric manifolds were classified by Batyrev [CitationBatyrev 99] and Sato [CitationSato 00]. Øbro classified Fano toric manifolds in dimensions 2–8 [CitationØbro 07] and, to standardize notation, we will write for the kth four-dimensional Fano toric manifold in Øbro’s list.
is the (23 + k)th Fano toric manifold in the Graded Ring Database [CitationBrown and Kasprzyk], as the list there is the concatenation of Øbro’s lists in dimensions 2–8. We can compute the quantum periods of the
using Theorem 2.1; the first few Taylor coefficients of their regularized quantum periods can be found in the tables in the Appendix.
8. Product manifolds and other index 1 examples
Quantum periods for one-, two- and three-dimensional Fano manifolds were computed in [CitationCoates et al. 16]. Combining these results with Theorem 2.4 allows us to compute the quantum period of any four-dimensional Fano manifold that is a product of lower-dimensional manifolds. Many of these examples have Fano index r = 1.
In his thesis [CitationStrangeway 14], Strangeway determined the quantum periods of two four-dimensional Fano manifolds of index r = 1 that have not yet been discussed. These manifolds arise as complete intersections in the nine-dimensional projective bundle . Let
denote the canonical projection, let p ∈ H2(F) be the first Chern class of
, and let ξ ∈ H2(F) be the first Chern class of the tautological bundle
. The manifold F is Fano of Picard rank 2, with nef cone generated by {ξ, p} and − KF = 6ξ + 2p. Let:
We consider also:
which was unaccountably omitted from [CitationStrangeway 14].
The manifolds Strk, k ∈ {1, 2, 3}, each have Picard rank two. To see this, observe that the ambient manifold F is the blow-up of along Gr(2, 5), where
is the Plücker embedding [CitationStrangeway 15]; the blow-up
and the projection
are the extremal contractions corresponding to the two extremal rays in
. Thus, Str1 is the blow-up of
along an elliptic curve
of degree 5. Consider the five-dimensional Fano manifold F5 given by the complete intersection of four divisors of type ξ in F. Then F5 is the blow-up of
along a del Pezzo surface S5 of degree 5; in particular, F5 has Picard rank two. Str3 is an ample divisor (of type ξ + p) in F5, so the Picard rank of Str3 is also two. The manifold Str2 is a divisor in F5 of type p, and F5 arises as the closure of the graph of the map
given by the five-dimensional linear system of quadrics passing through S5. This exhibits Str2 as the blow-up of a smooth four-dimensional quadric Q4 along S5, which implies that the Picard rank of Str2 is two.
We can compute the quantum periods of Strk, k ∈ {1, 2, 3}, by observing that a complete intersection in F of five divisors of type ξ and one divisor of type p is a three-dimensional Fano manifold , “unsectioning” to compute the quantum period for F, and then applying the quantum Lefschetz theorem to compute the quantum periods for Str1, Str2, and Str3. Recall the definition of the J-function JX(t, z) from [CitationCoates and Givental 07, equation 11]. The identity component of the J-function of
is
(8--7)
(8--7) where q1, q2 are generators of the Novikov ring for
dual, respectively, to ξ and p; see [CitationCoates et al. 16, Section 34]. The identity component of the J-function of F takes the form:
for some coefficients
. The Quantum Lefschetz theorem implies (see [CitationCoates et al. 16, Section D.1]) that the identity component of the J-function of
is equal to
(8--8)
(8--8) and it is known that c1, 0 = 1 and c0, 1 = 0 [CitationStrangeway 15, Section 5.1]. Equating (Equation8–7
(8--7)
(8--7) ) and (Equation8–8
(8--8)
(8--8) ) determines the cl, m:
The Quantum Lefschetz theorem now gives that
9. Numerical calculations of quantum differential operators
As discussed in Section 2, the regularized quantum period of a Fano manifold X satisfies a differential equation:
(9--9)
(9--9) called the regularized quantum differential equation. Here the pm are polynomials and
. The regularized quantum differential equation for X coincides with the (unregularized) quantum differential equation for an anticanonical Calabi–Yau manifold Y⊂X; the study of the regularized quantum period from this point of view was pioneered by Batyrev–Ciocan-Fontanine–Kim–van Straten [CitationBatyrev et al. 98, CitationBatyrev et al. 00]. The differential equation (Equation9–9
(9--9)
(9--9) ) is expected to be Fuchsian, and the local system of solutions to LXf ≡ 0 is expected to be of low ramification in the following sense.
Definition 9.1 ([CitationCoates et al. 12]).
Let a finite set, and
a local system. Fix a basepoint
. For s ∈ S, choose a small loop that winds once anticlockwise around s and connect it to x via a path, thereby making a loop γs about s based at x. Let
denote the monodromy of
along γs. The ramification of
is
The ramification defect of
is the quantity
. Non-trivial irreducible local systems
have non-negative ramification defect; this gives a lower bound for the ramification of
. A local system of ramification defect zero is called extremal.
Definition 9.2.
The ramification (respectively, ramification defect) of a differential operator LX is the ramification (respectively, ramification defect) of the local system of solutions LXf ≡ 0.
Definition 9.3.
The quantum differential operator for a Fano manifold X is the operator such that
which is of lowest order in D and, among all such operators of this order, is of lowest degree in t. (This defines LX only up to an overall scalar factor, but this suffices for our purposes.)
Suppose that each of the polynomials p0, …, pN are of degree at most r, and write:
The differential equation
gives a system of linear equations for the coefficients akl which, given sufficiently many terms of the Taylor expansion of
, becomes over-determined. Given a priori bounds on N and r, therefore, we could compute the quantum differential operator LX by calculating sufficiently many terms in the Taylor expansion. In general, we do not have such bounds, but nonetheless by ensuring the linear system for (akl) is highly over-determined we can be reasonably confident that the operator LX which we compute is correct. In addition, since LX is expected to correspond under mirror symmetry to a Picard–Fuchs differential equation for the mirror family, LX is expected to be of Fuchsian type. This is an extremely delicate condition on the coefficients (akl), and it can be checked by exact computation.
We computed candidate quantum differential operators LX for all four-dimensional Fano manifolds of Fano index r > 1, and checked the Fuchsian condition in each case. The operators LX, together with their ramification defects and the log-monodromy data {log Ts: s ∈ S} in Jordan normal form, can be found in Appendix B. In 24 cases, the local system of solutions to the regularized quantum differential equation is extremal, and in the remaining 11 cases it is of ramification defect 1.
To compute the ramification of LX, we follow Kedlaya [CitationKedlaya 10, Section 7.3]. This involves only linear algebra over a splitting field for pN(t)—recall that every singular point of LX occurs at a root of pN(t)—and thus can be implemented using exact (not numerical) computer algebra. For this we use Steel’s symbolic implementation of in the computational algebra system Magma [CitationBosma et al. 97, CitationSteel 10].
The situation in lower dimensions
The classification of three-dimensional Fano manifolds is known [CitationIskovskih 77, CitationIskovskih 78, CitationIskovskih 79, CitationMori and Mukai 82, CitationMori and Mukai 83, CitationMori and Mukai 86, CitationMori and Mukai 03, CitationMori and Mukai 04], and the quantum periods of all three-dimensional Fano manifolds have been computed [CitationCoates et al. 16]. In each case, the regularized quantum differential equation (Equation9–9(9--9)
(9--9) ) is extremal, and coincides with the Picard–Fuchs differential equation associated with a Laurent polynomial
in three variables.Footnote1 This latter phenomenon is a manifestation of Mirror Symmetry, and when it occurs then we say that the Laurent polynomial f is a mirror to the corresponding Fano manifold X. The classification of two-dimensional Fano manifolds, which are called del Pezzo surfaces, is classical, and it was proved in [CitationAkhtar et al. 16] that del Pezzo surfaces correspond under Mirror Symmetry to a distinguished family of Laurent polynomials in two variables called maximally mutable Laurent polynomials. This mirror correspondence extends to two-dimensional Fano orbifolds too [CitationAkhtar et al. 16, CitationOneto and Petracci, CitationCavey and Prince 17], and this inspired the classification of del Pezzo surfaces with
singularities by Corti–Heuberger [CitationCorti and Heuberger 17, CitationKasprzyk et al. 17]. Kasprzyk and Tveiten have defined what it means for a Laurent polynomial in any number of variables to be maximally mutable [CitationKasprzyk and Tveiten], and each of the three-dimensional Fano manifolds corresponds under mirror symmetry to a maximally mutable Laurent polynomial in three variables. It would be very interesting to find out whether the correspondence between maximally mutable Laurent polynomials and Fano manifolds (or, more precisely, Fano varieties with an appropriate class of mild singularities) persists to higher dimensions. Furthermore, Golyshev has observed a connection, which holds in low dimensions, between the ramification defect of the regularized quantum differential operator LX of a Fano manifold X and the dimension of the primitive part of the middle-dimensional cohomology of X. The regularized quantum differential operators for three-dimensional Fano manifolds are all extremal, which is consistent with the fact that this primitive part automatically vanishes in odd dimensions. It will be interesting to see how much of this picture persists beyond dimension 3.
Source code
This paper is accompanied by full source code, written in Magma. See the included file README.txt for usage instructions. The source code, but not the text of this paper, is released under a Creative Commons CC0 license [CitationCCO]: see the included file COPYING.txt for details. If you make use of the source code in an academic or commercial context, please acknowledge this by including a reference or citation to this paper.
Acknowledgments
We thank Alessio Corti for a number of very useful conversations.
Additional information
Funding
Notes
1 The proof of extremality involves applying the generalized Griffiths–Dwork algorithm of Lairez [CitationLairez 16] to f.
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Appendix A. Regularized quantum period sequences
In this appendix, we record the description, degree, and Picard rank ρX for each of the four-dimensional Fano manifolds X considered in this paper, together with the first few Taylor coefficients αd of the regularized quantum period:
The tables are divided by Fano index r. We include only coefficients αd with
, since coefficients αd with
are zero. Notation is as follows:
denotes n-dimensional complex projective space;
Qn denotes a quadric hypersurface in
;
FI4k is as in Section 5 above;
V4k is as in Section 6.1 above;
MW4k is as in Section 6.2 above;
BØS4k is as in Section 7 above;
Strk is as in Section 8 above;
S2k denotes the del Pezzo surface of degree k;
F1 denotes the Hirzebruch surface
;
V3k denotes the three-dimensional Fano manifold of Picard rank 1, Fano index 1, and degree k;
B3k denotes the three-dimensional Fano manifold of Picard rank 1, Fano index 2, and degree 8k;
denotes the kth entry in the Mori–Mukai list of three-dimensional Fano manifolds of Picard rank ρ [CitationMori and Mukai 82, CitationMori and Mukai 83, CitationMori and Mukai 86, CitationMori and Mukai 03, CitationMori and Mukai 04]. We use the ordering as in [CitationCoates et al. 16], which agrees with the original papers of Mori–Mukai except when ρ = 4.
We preferto express manifolds as products of lower-dimensional manifolds where possible, so, for example, is the product
, but we refer to this space as
rather than
. The tables for Fano index r with r ∈ {2, 3, 4, 5} are complete. The table for r = 1 is very far from complete.
Table A.1. Four-dimensional Fano manifolds with Fano index r = 5.
Table A.2. Four-dimensional Fano manifolds with Fano index r = 4.
Table A.3. Four-dimensional Fano manifolds with Fano index r = 3.
Table A.4. Four-dimensional Fano manifolds with Fano index r = 2.
Table A.5. Certain four-dimensional Fano manifolds with Fano index r = 1.
It appears from Table A.5 as if the regularized quantum period might coincide for the pairs and
. This is not the case. The coefficients α8, α9 in these cases are:
Table
Thus 10 terms of the Taylor expansion of the regularized quantum period suffice to distinguish all of the four-dimensional Fano manifolds considered in this paper.
Appendix B. Quantum differential operators for four-dimensional Fano manifolds of index r > 1: Numerical results
In this appendix, we record the quantum differential operators for all four-dimensional Fano manifolds of Fano index r > 1. These were computed numerically, as described in Section 9, from 500 terms of the Taylor expansion of the quantum period. They pass a number of strong consistency checks, and so we are reasonably confident that they are correct, but this has not been rigorously proven. We record also the local log-monodromies and ramification defect for the quantum local system, that is, for the local system of solutions to the regularized quantum differential equation. These are derived using exact computer algebra from the (numerically computed) operators LX, as described in Section 9.
B1. ![](//:0)
. [description p. 2, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B2. Q4. [description p. 2, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B3. FI41. [description p. 3, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B4. FI42. [description p. 3, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B5. FI43. [description p. 3, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B6. FI44. [description p. 3, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B7. FI45. [description p. 3, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B8. ![](//:0)
. [description p. 3, regularized quantum period p. 15]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B9. V42. [description p. 3, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B10. V44. [description p. 3, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B11. V46. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B12. V48. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B13. V410. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B14. V412. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B15. V414. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B16. V416. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B17. V418. [description p. 4, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B18. MW41. [description p. 5, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B19. MW42. [description p. 5, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B20. MW43. [description p. 5, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B21. MW44. [description p. 5, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B22. MW45. [description p. 6, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B23. MW46. [description p. 6, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B24. MW47. [description p. 6, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B25. MW48. [description p. 6, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B26. MW49. [description p. 6, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B27. MW410. [description p. 6, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B28. MW411. [description p. 7, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B29. MW412. [description p. 10, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B30. MW413. [description p. 10, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B31. MW414. [description p. 10, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B32. MW415. [description p. 10, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B33. MW416. [description p. 11, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.
B34. MW417. [description p. 11, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The ramification defect of LX is 1.
B35. MW418. [description p. 11, regularized quantum period p. 16]
The quantum differential operator is
The local log-monodromies for the quantum local system:
The operator LX is extremal.