MONODROMY OF PICARD-FUCHS DIFFERENTIAL EQUATIONS FOR CALABI-YAU THREEFOLDS 7 0 0 2 YAO-HANCHEN,YIFANYANG,ANDNORIKOYUI n a Abstract. In this paper we are concerned with the monodromy of Picard- J Fuchs differentialequations associatedwithone-parameter familiesofCalabi- 5 Yauthreefolds. Ourresultsshowthatinthehypergeometriccasesthematrix 1 representationsofmonodromyrelativetotheFrobeniusbasescanbeexpressed in terms of the geometric invariants of the underlying Calabi-Yau threefolds. ] Thisphenomenon isalsoverifiednumericallyforotherfamiliesofCalabi-Yau G threefoldsinthepaper. Furthermore,wediscoverthatunderasuitablechange A ofbasesthemonodromygroupsarecontainedincertaincongruencesubgroups ofSp(4,Z)offiniteindexandwhoselevelsarerelatedtothegeometricinvari- h. ants oftheCalabi-Yauthreefolds. t a m [ 1. Introduction 3 Let M be a family of Calabi-Yau n-folds parameterized by a complex variable v z z P1(C), and ω be the unique holomorphic differential n-form on M (up to 5 z z ∈ 7 a scalar). Then the standard theory of Gauss-Manin connections asserts that the 6 periods 5 0 ωz 6 Zγz 0 satisfycertainlineardifferentialequations,calledthePicard-Fuchsdifferentialequa- / tions, where γ are r-cycles on M . h z z t When n=1,Calabi-Yauonefolds arejust elliptic curves. A classicalexample of a Picard-Fuchs differential equations is m z : (1) (1 z)θ2f zθf f =0, θ =zd/dz, v − − − 4 i X satisfied by the periods r ∞ dx a f(z)= x(x 1)(x z) Z1 − − of the family of elliptic curves Ez :y2 =px(x 1)(x z). − − When n=2, Calabi-Yaumanifolds are either 2-dimensionalcomplextori orK3 surfaces. When the Picardnumber of a one-parameterfamily of K3 surfaces is 19, Date:16January2007. 2000 Mathematics Subject Classification. Primary 14J32, 34M35, 14D05, 32S40; Secondary 14J15,14Q15,11F46. Keywordsandphrases. Calabi–Yauthreefold,Picard–Fuchsdifferentialequation,monodromy group,Frobeniusbasis,hypergeometricdifferentialequation, conifoldsingularity. Y.-H. Chen and Yifan Yang were supported by Grant 94-2115-M-009-012 of the National Science Council (NSC) of the Republic of China (Taiwan). N. Yui was supported in part by DiscoveryGrantoftheNaturalSciencesandEngineeringResearchCouncil(NSERC)ofCanada. 1 2 YAO-HANCHEN,YIFANYANG,ANDNORIKOYUI the Picard-Fuchsdifferentialequationhas order3. One ofthe simplest examples is x41+x42+x43+x44−z−1x1x2x3x4 =0⊂P3, whose Picard-Fuchs differential operator is (2) θ3 4z(4θ+1)(4θ+2)(4θ+3). − Another well-known example is (3) (1 34z+z2)θ3+(3z2 51z)θ2+(3z2 27z)θ+(z2 5z), − − − − which is the Picard-Fuchs differential operator for the family of K3 surfaces 1 (1 XY)Z zXYZ(1 X)(1 Y)(1 Z)=0. − − − − − − (See [7].) This differential equation appeared in Ap´ery’s proof of irrationality of ζ(3). (See [5].) When n=3 and Calabi–Yauthreefolds have the Hodge number h2,1 equal to 1, the Picard-Fuchs differential equations have order 4. One of the most well-known examples of such Calabi–Yau threefolds is the quintic threefold x5+x5+x5+x5+x5 z 1x x x x x =0 P4. 1 2 3 4 5− − 1 2 3 4 5 ⊂ In [9], it is shown that the Picard-Fuchs differential operator for this family of Calabi–Yau threefolds is (4) θ4 5z(5θ+1)(5θ+2)(5θ+3)(5θ+4). − (Actually, it is the mirror partner of the quintic Calabi–Yau threefolds that has Hodge number h2,1 = 1 and hence the Picard–Fuch differential equation is of or- der 4. But the mirror pair of Calabi–Yau threefolds share the same “principle periods”. This means that the Picard–Fuchs differential equation of the original quintic Calabi–Yauthreefoldof order 204 contains the above order4 equation as a factor and the factors corresponding to the remaining 200 “semiperiods”. Inthis articleweareconcernedwiththe monodromyaspectofthe Picard-Fuchs differential equations. Let L: r (z)θn+r (z)θn 1+ +r (z), r C(z), n n 1 − 0 i − ··· ∈ be a differential operator with regular singularities. Let z be a singular point 0 and S be the solution space of L at z . Then analytic continuation along a closed 0 curveγ circlingz givesrisetoanautomorphismofS,calledmonodromy. Ifabasis 0 f ,...,f ofS ischosen,thenwehaveamatrixrepresentationofthemonodromy. 1 n { } Suppose that f becomes a f + +a f after completing the loopγ, that is, if i i1 1 in n ··· f a ... a f 1 11 1n 1 . . . . . . . . ,  .   . .  .  7−→ f a ... a f  n  n1 nn n      then the matrix representation of the monodromy along γ relative to the basis f is the matrix (a ). The group of all such matrices are referred to as the i ij { } monodromy group relative to the basis f of the differential equation. Clearly, i { } two different choices of bases may result in two different matrix representations for the same monodromy. However, it is easily seen that they are connected by conjugation by the matrix of basis change. Thus, the monodromy group is defined up to conjugation. In the subsequent discussions, for the ease of exposition, we MONODROMY OF CALABI-YAU DIFFERENTIAL EQUATIONS 3 may often drop the phrase “up to conjugation” about the monodromy groups, when there is no danger of ambiguities. It is knownthat for one-parameterfamilies of Calabi-Yauvarieties of dimension one and two (i.e., elliptic curves and K3 surfaces, respectively), the monodromy groups are very often congruence subgroups of SL(2,R). For instance, the mon- odromygroupof(1)isΓ(2),whilethoseof(2)and(3)areΓ (2)+ω andΓ (6)+ω , 0 2 0 6 respectively,where ω denotesthe Atkin-Lehner involution. (Technicallyspeaking, d the monodromy groups of (2) and (3) are subgroups of SL(3,R) since the order of the differential equations is 3. But because (2) and (3) are symmetric squares of second-order differential equations, we may describe the monodromy in terms of the second-order ones.) Moreover, suppose that y (z) = 1+ is the unique 0 ··· holomorphic solution at z = 0 and y (z) = y (z)logz+g(z) is the solution with 1 0 logarithmic singularity. Set τ = cy (z)/y (z) for a suitable complex number c. 1 0 Then z, as a function of τ, becomes a modular function, and y (z(τ)) becomes a 0 modularformofweight1fortheorder2casesandofweight2fortheorder3cases. For example, a classical result going back to Jacobi states that 1 1 θ4 θ2 = F , ;1; 2 , 3 2 1 2 2 θ4 (cid:18) 3(cid:19) where θ (τ)=q1/8 qn(n+1)/2, θ (τ)= qn2/2, q =e2πiτ, 2 3 n Z n Z X∈ X∈ or equivalently, that the modular form y(τ) = θ2, as a function of z(τ) = θ4/θ4, 3 2 3 satisfies (1). Here F denotes the Gauss hypergeometric function. 2 1 InthispaperwewilladdressthemonodromyproblemforCalabi-Yauthreefolds. Atfirst,giventhe experiencewiththe elliptic curveandK3surfacecases,onemay be tempted to guess that the monodromygroupofsuch a differential equationwill be the symmetric cube of some congruence subgroup of SL(2,R). After all, there is aresultby Stiller [23](see also[27]) assertingthatif t(τ) is a non-constantmod- ular function and F(τ) is a modular form of weight k on a subgroup of SL(2,R) commensurable with SL(2,Z), then F,τF,...,τkF, as functions of t, are solutions of a (k +1)-st order linear differential equation with algebraic functions of t as coefficients. However, this is not the case in general. A quick way to see this is that the coefficients of the symmetric cube of a second order differential equation y +r (t)y +r (t)y = 0 is completely determined by r and r , but the coeffi- ′′ 1 ′ 0 1 0 cients of the Picard-Fuchs differential equations, including (4), do not satisfy the required relations. (The exact relations can be computed using Maple’s command symmetric power.) Nevertheless, in the subsequent discussion we will show that, with a suitable choice of bases, the monodromy groups for Calabi-Yau threefolds arecontainedincertaincongruencesubgroupsofSp(4,Z)whoselevelsaresomehow described in terms of the geometric invariants of the manifolds in question. This isprovedrigorouslyfor the hypergeometriccasesandverifiednumericallyforother (e.g., non-hypergeometric) cases. Furthermore, our computation in the hypergeo- metric cases shows that the matrix representation of the monodromy around the finite singular point (different from the origin) relative to the Frobenius basis at the origin can be expressed completely using the geometric invariants of the asso- ciated Calabi-Yau threefolds. This phenomenon is also verified numerically in the non-hypergeometriccases. Althoughitishighlyexpectedthatgeometricinvariants 4 YAO-HANCHEN,YIFANYANG,ANDNORIKOYUI will enter into the picture, in reality, geometry will dominate the entire picture in the sense that every entry of the matrix is expressed exclusively in terms of the geometric invariants. Themonodromyproblemingeneralhasbeenaddressedbyanumberofauthors. Papers relevant to our consideration include [6], [9], [11], [16], and [26], to name a few. In [6], Beukers and Heckman studied monodromy groups for the hypergeo- metric functions F . They showed that the Zariski closure of the monodromy n n 1 groupsof(4)isSp(4,−C). The sameistrue forotherPicard-Fuchsdifferentialequa- tions for Calabi-Yau threefolds that are hypergeometric. In [9], Candelas et al. obtained precise matrix representations of monodromy for (4). Then Klemm and Theisen [16] applied the same method as that of Candelas et al. to deduce mon- odromy groups for three other hypergeometric cases. In [11] Doran and Morgan determined the monodromy groups for all the hypergeometric cases. Their matrix representations also involve geometric invariants of the Calabi-Yau threefolds. For Picard-Fuchs differential equations of non-hypergeometric type, there is not much known in literature. In [26] van Enckevort and van Straten computed the mon- odromy matrices numerically for a large class of differential equations. In many cases, they are able to find bases such that the monodromy matrices have rational entries. We will discuss the above results in more detail in Sections 3–5. Our motivations of this paper may be formulated as follows. Modular functions andmodularformshavebeenextensivelyinvestigatedovertheyears,andthereare greatbodyofliteraturesonthesesubjects. Asweillustratedabove,themonodromy groups of Picard-Fuchs differential equations for families of elliptic curves and K3 surfaces are congruence subgroups of SL(2,R). This modularity property can be used to study properties of the differential equations and the associatedmanifolds. Forinstance,in[18]LianandYaugaveauniformproofoftheintegralityofFourier coefficients of the mirror maps for several families of K3 surfaces using the fact that the monodromy groups are congruence subgroups of SL (R). For such an 2 application, it is important to express monodromy groups in a proper way so that properties ofthe associateddifferential equations can be more easily discussed and obtained. Thus, the main motivation of our investigation is to find a good rep- resentation for monodromy groups from which further properties of Picard-Fuchs differential equations for Calabi-Yau threefolds can be derived. The terminology “modularity” has been used for many different things. One aspect of the modularity that we would like to address is the modularity question of the Galois representations attached to Calabi–Yau threefolds, assuming that Calabi–Yau threefolds in question are defined over Q. Let X be a Calabi–Yau threefold defined over Q. We consider the L-series associated to the third ´etale cohomology group of X. It is expected that the L-series should be determined by somemodular (automorphic)forms. The examples ofCalabi–Yauthreefolds we treatinthispaperarethosewiththethirdBettinumberequalto4. Itappearsthat Calabi–Yau threefolds with this property are rather scarce. Batyrev and Straten [4] considered 13 examples of Calabi–Yau threefolds with Picard number h1,1 = 1. Then their mirror partners will fulfill this requirement. (We note that more examples of such Calabi–Yau threefolds were found by Borcea [8].) All these 13 Calabi–Yau threefolds are defined as complete intersections of hypersurfaces in weighted projective spaces, and they have defining equations defined over Q. MONODROMY OF CALABI-YAU DIFFERENTIAL EQUATIONS 5 To address the modularity, we ought to have some “modular groups”, and this paper offers candidates for appropriate modular groups via the monodromy group ofthe associatedPicard–Fuchsdifferentialequation(oforder4). Inthese cases,we expect that modular forms of more variables,e.g., Siegel modular forms associated to the modular groups for our congruence subgroups would enter the scene. In general, the third Betti numbers of Calabi–Yau threefolds are rather large, and consequently, the dimension of the associated Galois representations would be rather high. To remedy this situation, we first decompose Calabi–Yau threefolds into motives, and then consider the motivic Galois representations and their mod- ularity. Especially,when the principalmotives (e.g., the motives that are invariant under the mirror maps) are of dimension 4, the modularity question for such mo- tives should be accessible using the method developed for the examples discussed in this paper. The modularity questions will be treated in subsequent papers. 2. Statements of results Tostateourfirstresult,letusrecallthatamongallthePicard-Fuchsdifferential equationsforCalabi-Yauthreefolds,thereare14equationsthatarehypergeometric of the form θ4 Cz(θ+A)(θ+1 A)(θ+B)(θ+1 B). − − − Their geometric descriptions and references are given in the following Table 1. # A B C Description H3 c H c Ref 2 3 · 1 1/5 2/5 3125 X(5) P4 5 50 200 [9] ⊂ − 2 1/10 3/10 8 105 X(10) P4(1,1,1,2,5) 1 34 288 [20] · ⊂ − 3 1/2 1/2 256 X(2,2,2,2) P7 16 64 128 [19] ⊂ − 4 1/3 1/3 729 X(3,3) P5 9 54 144 [19] ⊂ − 5 1/3 1/2 432 X(2,2,3) P6 12 60 144 [19] ⊂ − 6 1/4 1/2 1024 X(2,4) P5 8 56 176 [19] ⊂ − 7 1/8 3/8 65536 X(8) P4(1,1,1,1,4) 2 44 296 [20] ⊂ − 8 1/6 1/3 11664 X(6) P4(1,1,1,1,2) 3 42 204 [20] ⊂ − 9 1/12 5/12 126 X(2,12) P5(1,1,1,1,4,6) 1 46 484 [11] ⊂ − 10 1/4 1/4 4096 X(4,4) P5(1,1,1,1,2,2) 4 40 144 [17] ⊂ − 11 1/4 1/3 1728 X(4,6) P5(1,1,1,2,2,3) 6 48 156 [17] ⊂ − 12 1/6 1/4 27648 X(3,4) P5(1,1,1,1,1,2) 2 32 156 [17] ⊂ − 13 1/6 1/6 28 36 X(6,6) P5(1,1,2,2,3,3) 1 22 120 [17] · ⊂ − 14 1/6 1/2 6912 X(2,6) P5(1,1,1,1,1,3) 4 52 256 [17] ⊂ − Somecommentsmightbeinorderforthenotationsinthetable. Weemploytheno- tations of van Enckevortand van Straten [26]. X(d ,d ,...,d ) Pn(w ,...,w ) 1 2 k 0 n ⊂ stands for a complete intersection of k hypersurfaces of degree d ,...,d in the 1 k weighted projective space with weight (w , ,w ). For instance, X(3,3) P5 is 0 n ··· ⊂ a complete intersection of two cubics in the ordinary projective 5-space P5 defined by Y3+Y3+Y3 3φY Y Y =0 3φY Y Y +Y3+Y3+Y3 =0 . { 1 2 3 − 4 5 6 }∩{− 1 2 3 4 5 6 } 6 YAO-HANCHEN,YIFANYANG,ANDNORIKOYUI Slightly moregenerally,X(4,4) P5(1,1,2,1,1,2)denotes a complete intersection ⊂ of two quartics in the weighted projective 5-space P5(1,1,2,1,1,2) and may be defined by the equations Y4+Y4+Y2 4φY Y Y =0 4φY Y Y +Y4+Y4+Y2 =0 . { 1 2 3 − 4 5 6 }∩{− 1 2 3 4 5 6 } WenotethatalltheseexamplesofCalabi–YauthreefoldsM havethePicardnumber h1,1(M)= 1. Let (H) be the ample generator of the Picard group Pic(M) Z. O ≃ The basic invariants for such a Calabi–Yau threefold M are the degree d := H3, the second Chern number c H and the Euler number c (the Euler characteristic 2 3 · of M). The equations are numbered in the same way as in [1]. In [9], using analytic properties of hypergeometric functions, Candelas et al. provedthat with respectto a certain basis,the monodromymatrices aroundz =0 and z =1/3125 for the quintic threefold case (Equation 1 from Table 1) are 51 90 25 0 1 0 0 0 − 0 1 0 0 0 1 0 1   and  , 100 175 49 0 0 0 1 0 −  75 125 35 1 0 0 0 1 − −    respectively. (Note that these two matrices are both in Sp(4,Z).) Applying the same method as that of Candelas et al., Klemm and Theisen [16] also obtained the monodromy of the one-parameter families of Calabi–Yau threefolds for Equa- tions 2, 7, and 8. Presumably, their method should also work for several other hypergeometriccases. However,the method fails when the indicial equation of the singularity has repeated roots. To be more precise, it does not work for Equa- ∞ tions 3–6, 10, 13 and 14. Moreover, the method uses the explicit knowledge that the singular point z = 1/C is of conifold type. (Note that in geometric terms, a conicalsingularityis a regularsingularpoint whoseneighborhoodlookslike a cone with a certain base. For instance, a 3-dimensional conifold singularity is locally isomorphicto XY ZT =0or equivalently,to X2+Y2+Z2+T2 =0. Reflecting − to the Picard-Fuchs differential equations, this means that the local monodromy is unipotentofindex1.) Thus,itcannotbeappliedimmediatelytostudymonodromy of general hypergeometric differential equations. In [11] Doran and Morgan proved that if the characteristic polynomial of the monodromy around is ∞ x4+(k 4)x3+(6 2k+d)x2+(k 4)x+1, − − − thenthereis abasissuchthatthe monodromymatricesaroundz =0 andz =1/C are 1 1 0 0 1 0 0 0 0 1 d 0 k 1 0 0 (5)   and − , 0 0 1 1 1 0 1 0 − 0 0 0 1  1 0 0 1   −      respectively. It turns out that these numbers d and k both have geometric inter- pretation. Namely, the number d = H3 is the degree of the associated threefolds and k = c H/12+H3/6 is the dimension of the linear system H . Doran and 2 · | | Morgan’s representation has the advantage that the geometric invariants can be read off from the matrices directly (although there is no way to extract the Euler number c from the matrices), but has the disadvantage that the matrices are no 3 longer in the symplectic group (in the strict sense). MONODROMY OF CALABI-YAU DIFFERENTIAL EQUATIONS 7 Before we state our Theorem 1, let us recall the definition of Frobenius basis. Since the only solution of the indicial equation at z = 0 for each of the cases is 0 with multiplicity 4, the monodromy around z = 0 is maximally unipotent. (See [20]formoredetail.) ThenthestandardmethodofFrobeniusimpliesthatatz =0 there are four solutions y , j =0,...,, with the property that j y =1+ , y =y logz+g , 0 1 0 1 ··· (6) 1 1 1 y = y log2z+g logz+g , y = y log3z+ g log2z+g logz+g , 2 0 1 2 3 0 1 2 3 2 6 2 where g are all functions holomorphic and vanishing at z = 0. We remark that i these solutions satisfy the relation y y y y 0 3 = 1 2 , y y y y (cid:12) 0′ 3′(cid:12) (cid:12) 1′ 2′(cid:12) (cid:12) (cid:12) (cid:12) (cid:12) and therefore the monodromy m(cid:12) atrices(cid:12)rel(cid:12)ative to(cid:12)the ordered basis y0,y2,y3,y1 (cid:12) (cid:12) (cid:12) (cid:12) { } are in Sp(4,C), as predicted by [6]. Now we can present our first theorem. Theorem 1. Let L:θ4 Cz(θ+A)(θ+1 A)(θ+B)(θ+1 B) − − − be one of the 14 hypergeometric equations, and H3, c H, and c be geometric 2 3 · invariants of the associated Calabi-Yau threefolds given in the table above. Let y , j = 0,...,3, be the Frobenius basis specified by (6). Then with respect to the j ordered basis y /(2πi)3,y /(2πi)2,y /(2πi),y , the monodromy matrices around 3 2 1 0 { } z =0 and z =1/C are 1 1 1/2 1/6 1+a 0 ab/d a2/d 0 1 1 1/2 b 1 b2/d ab/d (7)   and  − − − , 0 0 1 1 0 0 1 0 0 0 0 1   d 0 b 1 a    − − −      respectively, where c a= 3 ζ(3), b=c H/24, d=H3. (2πi)3 2· Remark 1. We remark that by conjugating by the matrix d 0 b a 0 d d/2 d/6+b  , 0 0 1 1 0 0 0 1      we do recover Doran and Morgan’s representation (5). Thus, our Theorem 1 stren- thens the results of Doran and Morgan [11]. Although the referee suggested that Theorem 1 might be a reformulation of the results of Doran and Morgan. However, we do not believe that is the case. For one thing, the argument of Doran–Morgan is purely based on Linear Algebra. It might be possible to derive our Theorem 1 com- bining the results of Doran–Morgan and those of Kontsevich; we will not address this question here, but left to future investigations. The appearance of the geometric invariants c , c , H and d is not so surprising. 2 3 In [9], it was shown that the conifold period, defined up to a constant as the holomorphic solution f(z) = a (z 1/C)+a (z 1/C)2 + at z = 1/C that 1 2 − − ··· 8 YAO-HANCHEN,YIFANYANG,ANDNORIKOYUI appearsintheuniquesolutionf(z)log(z 1/C)+g(z)withlogarithmicsingularity − at z =1/C, is asymptotically H3 c H c (8) log3z+ 2· logz+ 3 ζ(3)+ 6(2πi)3 48πi (2πi)3 ··· near z = 0. (See also [15].) Therefore, it is expected that the entries of the mon- odromymatricesshouldcontaintheinvariants. However,itisstillquiteremarkable that the matrix is determined completely by the invariantsalone. We have numer- icallyverifiedthe phenomenonforotherfamilies ofCalabi-Yauthreefolds, andalso forgeneraldifferentialequations ofCalabi-Yautype. (See [1] forthe definition ofa differentialequationofCalabi-Yautype. SeealsoSection5below.) Itappearsthat ifthedifferentialequationhasatleastonesingularitywithexponents0,1,1,2,then there is always a singularity whose monodromy relative to the Frobenius basis is of the form stated in the theorem. Thus, this gives a numerical method to identify the possible geometric origin of a differential equation of Calabi-Yau type. We emphasize that our proof of Theorem 1 is merely a verification. That is, we can prove it, but unfortunately it does not give any geometric insight why the matrices are in this special form. Acutally, the referee has pointed out that sucha geometricinterpretationseems to exist by Kontsevich. In the framework of “homological mirror symmetry” of Kontsevich, the first matrix in Theorem 1 would be the matrix associated to ten- soringbythehyperplanelinebundleintheboundedderivedcategoryofsheaveson the Calabi–Yau variety. In general, the matrices in Theorem 1 describe the coho- mology action of certain Fourier–Mukai functors. In particular, this explains why thematricesaredeterminedbytopologicalinvariantsoftheunderlyingCalabi–Yau manifolds. ThepaperofvanEnckevortandvanStraten[26]addressedmonodromy calculations of fourth order equations of Calabi–Yau type based on homological mirror symmetry. The reader is referred to the article [26] for full details about geometric interpretations of matrices. We wonder, though, if the Kontsevich’s results fully explain why there are no “non-geometric” numbers in the second ma- trices. To be more precise, here is our question. Since the second matrix M is unipotent of rank 1, we know that the rows of M Id are all scalar multiples of a − fixedrowvector. We probablycandeduce fromKontsevich’sresultthatthe fourth rowis ( d,0, b, a),but why the firstthree rowsof M Id are a/d, b/a,and − − − − − − 0 times this vector (but not other “non-geometric” scalars? Now conjugating the matrices by 0 0 1 0 0 0 0 1 (9)  , 0 d d/2 b −  d 0 b a − − −    we can bring the matrices into the symplectic group Sp(4,Z). The results are 1 1 0 0 1 0 0 0 0 1 0 0 0 1 0 1   and   d d 1 0 0 0 1 0 0 k 1 1 0 0 0 1  − −        for z = 0 and z = 1/C, respectively, where k = 2b+d/6. Since the monodromy group is generated by these two matrices, we see that the group is contained in MONODROMY OF CALABI-YAU DIFFERENTIAL EQUATIONS 9 the congruence subgroup Γ(d,gcd(d,k)), where the notation Γ(d ,d ) with d d 1 2 2 1 | represents 1 ∗ ∗ ∗ 0 Γ(d1,d2)=γ ∈Sp(4,Z):γ ≡00 0∗ 1∗ 0∗ mod d1  ∗ ∗ ∗   1 ∗ ∗ ∗  0 1 \γ ∈Sp(4,Z):γ ≡00 00 1∗ 0∗1 mod d2  ∗  We remark that the entriesof the matrices in Γ(d1,d2) satisfycertain congruence relations inferred from the symplecticity of the matrices. To be more explicit, let us recall that the symplectic group is characterized by the property that A B γ = Sp(2n,C), C D ∈ (cid:18) (cid:19) where A, B, C, and D are n n blocks, if and only if × Dt Bt γ−1 = Ct −At . (cid:18)− (cid:19) Thus, for a a a a 11 12 13 14 a a a a 21 22 23 24   a a a a 31 32 33 34 a a a a   41 42 43 44   to be in Γ(d ,d ), the integers a should satisfy the implicit conditions 1 2 ij a a a a 1, a a a a a , a a a a a mod d , 22 44 24 42 23 14 22 12 24 43 14 42 12 44 1 − ≡ ≡ − ≡ − and a a mod d . 12 43 2 ≡− We now summarize our finding in the following theorem. Theorem 2. Let θ4 Cz(θ+A)(θ+1 A)(θ+B)(θ+1 B) − − − be one of the 14 hypergeometric equations. Let y , j = 0,...,3 be the Frobenius j basis. Then relative to the ordered basis y H3 H3 c H H3 c H c 1 2 2 3 , y , y + y · y , y · y ζ(3)y , 2πi 0 2(2πi)2 2 4πi 1− 24 0 −6(2πi)3 3− 48πi 1−(2πi)3 0 the monodromy matrices around z =0 and z =1/C are 1 1 0 0 1 0 0 0 0 1 0 0 0 1 0 1 (10)   and   d d 1 0 0 0 1 0 0 k 1 1 0 0 0 1  − −        10 YAO-HANCHEN,YIFANYANG,ANDNORIKOYUI with d=H3, k =H3/6+c H/12, respectively. They are contained in the congru- 2 · ence subgroups Γ(d ,d ) for the 14 cases in the table below. 1 2 # A B d d # A B d d 1 2 1 2 1 1/5 2/5 5 5 8 1/6 1/3 3 1 2 1/10 3/10 1 1 9 1/12 5/12 1 1 3 1/2 1/2 16 8 10 1/4 1/4 4 4 4 1/3 1/3 9 3 11 1/4 1/3 6 1 5 1/3 1/2 12 1 12 1/6 1/4 2 1 6 1/4 1/2 8 2 13 1/6 1/6 1 1 7 1/8 3/8 2 2 14 1/6 1/2 4 1 Remark. We remark that what we show in Theorem2 is merely the fact that the monodromygroupsarecontainedinthe congruencesubgroupsΓ(d ,d ). Although 1 2 thecongruencesubgroupsΓ(d ,d )areoffiniteindexinSp(4,Z)(seetheappendix 1 2 by Cord Erdenberger for the index formula), the monodromy groups themselves may not be so. At this moment, we cannot say anything definite about these groups, e.g., their finite indexness. In fact, there are two opposing speculations (one by the authors, and the other by Zudilin) about these groups. We believe, based on a result (The- orem13.3)of Sullivan [24], that it might be justified to claim that the monodromy group is an arithmetic subgroup of the congruence subgroup Γ(d ,d ), and hence 1 2 is of finite index. (Andrey Todorov pointed out to us Sullivan’s theorem, though we must confess that we do not fully understand the paper of Sullivan.) As opposed to our belief, Zudilin has indicated to us via e-mail that a heuristic argumentsuggeststhatthe monodromygroupsaretoo“thin”to be offinite index. It would not be of much significance if the hypergeometric equations are the only cases where the monodromy groups are contained in congruence subgroups. Our numerical computation suggests that there are a number of further examples wherethe monodromygroupscontinueto be containedincongruencesubgroupsof Sp(4,Z). However, the general picture is not as simple as that for the hypergeo- metric cases. Asmentionedearlier,ournumericaldatasuggestthatthePicard-Fuchsdifferen- tial equations for Calabi-Yau threefolds known in literature all have bases relative to which the monodromy matrices around the origin and some singular points of conifolds take the form (7) described in Theorem 1. Thus, with respect to the basis given in Theorem 2, the matrices around the origin and the conifold points againhavetheform(10). However,withthisbasischange,themonodromymatrices aroundothersingularitiesmaynotbeinSp(4,Z),butinSp(4,Q)instead,although the entries still satisfy certain congruence relations. Furthermore, in most cases, we can still realize the monodromy groupsin congruencesubgroups of Sp(4,Z), by a suitable conjugation.