On Chen invariants and inequalities in quaternionic geometry
© Vîlcu; licensee Springer 2013
Received: 26 September 2012
Accepted: 9 February 2013
Published: 22 February 2013
In this paper we give a survey of main results concerning Chen inequalities for submanifolds in quaternionic space forms and propose some open problems in the field for further research.
AMS Subject Classification:53C15, 53C25, 53C40.
KeywordsChen’s invariant squared mean curvature shape operator quaternionic space form slant submanifold
The theory of Chen invariants, initiated by Prof. B.-Y. Chen in a seminal paper published in 1993 , is presently one of the most interesting research topic in differential geometry of submanifolds. The author’s original motivation to introduce new types of Riemannian invariants, known as δ-invariants or Chen invariants, was the need to provide answers to an open question raised by Chern concerning the existence of minimal immersions into a Euclidean space of arbitrary dimension . In fact, due the lack of control of the extrinsic properties of the submanifolds by the known intrinsic invariants, no solutions to Chern’s problem were known before the invention of Chen invariants (see ). Therefore, in , Chen obtained a necessary condition for the existence of minimal isometric immersion from a given Riemannian manifold M into Euclidean space and established a sharp inequality for a submanifold in a real space form using the scalar curvature and the sectional curvature (both being intrinsic invariants) and squared mean curvature (the main extrinsic invariant). On the other hand, in , Chen obtained inequalities between the k-Ricci curvature, the squared mean curvature and the shape operator for submanifolds in real space forms with arbitrary codimensions. These inequalities are also sharp, and many nice classes of submanifolds realize equality in all above inequalities. Since then many papers concerning Chen invariants and inequalities have appeared in the literature for different classes of submanifolds in various ambient spaces, like complex space forms [5–7], cosymplectic space forms [8–10], Sasakian space forms [11–13], locally conformal Kähler space forms [14–16], generalized complex space forms [17–19], locally conformal almost cosymplectic manifolds [20, 21], -contact space forms [22, 23], Kenmotsu space forms [24, 25], S-space forms [26, 27], T-space forms ; see also  and references therein.
In the last decade, Chen-like inequalities were extended in the quaternionic setting. It is the main purpose of this paper to present the evolution of these inequalities for submanifolds in quaternionic space forms, to survey some recent results on this topic and to propose a set of natural problems in the field.
2.1 Riemannian invariants
In this subsection we recall some basic concepts concerning Chen invariants, using mainly .
For an integer , we denote by the set of k-tuples of integers ≥2 satisfying , . We denote by the set of unordered k-tuples with for a fixed n.
We note that such a curvature is called a k-Ricci curvature.
where L runs over all k-plane sections in and X runs over all unit vectors in L.
It is well known that all these above invariants have many interesting applications to several fields of mathematics (see ).
2.2 Quaternionic Kähler manifolds
The geometry of Riemannian structures, complex structures, almost contact structures, hypercomplex structures, quaternionic and Cauchy-Riemann structures belongs to the general theory of the G-structures, a domain of great interest in modern differential geometry, in global analysis and mathematical physics. Quaternionic manifolds correspond to the reduction of the structural group at . From the metric viewpoint, the most interesting is the case of quaternionic Kähler manifolds, which corresponds to the reduction of the holonomy at a subgroup of . They appear in Berger’s list for possible holonomy groups of irreducible Riemannian manifolds . We give in this subsection a quick review of basic definitions and properties concerning the differential geometry of manifolds endowed with quaternionic structures. For details, see .
where Id denotes the identity tensor field of type on M and the indices are taken from modulo 3. Then the bundle σ is called an almost quaternionic structure on M and is called a canonical local basis of σ. Moreover, is said to be an almost quaternionic manifold. It is easy to see that any almost quaternionic manifold is of dimension 4m, .
for all vector fields X, Y on and any canonical local basis of σ. Moreover, is said to be an almost quaternionic Hermitian manifold.
for any vector field X on , where the indices are taken from modulo 3.
for all vector fields X, Y, Z on and any local basis of σ. We note that some interesting characterizations of quaternionic space forms were obtained in .
for all and .
The submanifold M is called totally geodesic if the second fundamental form vanishes identically and totally umbilical if there is a real number λ such that for any tangent vectors X, Y on M. If , then the submanifold M is said to be minimal.
3 Fundamental inequalities involving Chen invariants and the squared mean curvature
The submanifold M is said to satisfy Chen’s equality if the equality case of (15) holds identically.
In quaternionic Kähler ambient, the first classes of submanifolds which have been introduced and studied were quaternionic  and totally real submanifolds . A submanifold M in a quaternionic Kähler manifold is called a quaternionic submanifold (resp. a totally real submanifold) if each tangent space of M is carried into itself (resp. into the normal space) by each section in σ. An n-dimensional totally real submanifold of a quaternionic space form is said to be a Lagrangian submanifold if . It is well known that a quaternionic submanifold is totally geodesic, so the first class of interest from Chen’s inequalities viewpoint is given by the totally real submanifolds. In , the validity of the inequality (15) was proved for totally real submanifolds in a quaternionic space form of quaternionic sectional curvature 4c, and Chen’s equality was interpreted in terms of eigenvalues and eigenspaces of the Weingarten operators of the submanifold. As a consequence, Hong and Houh obtained the following interesting result.
Theorem 3.1 
Every Lagrangian submanifold of a quaternionic space form satisfying Chen’s equality is minimal.
In , Şahin introduced the concept of slant submanifolds as a natural generalization of both quaternionic and totally real submanifolds. A submanifold M of a quaternionic Kähler manifold is said to be a slant submanifold if for each non-zero vector X tangent to M at p, the angle between and , is constant, i.e., it does not depend on choice of and . We can easily see that quaternionic submanifolds are slant submanifolds with and totally-real submanifolds are slant submanifolds with . A slant submanifold of a quaternionic Käler manifold is said to be proper (or θ-slant proper) if it is neither quaternionic nor totally real.
In , the present author obtained the generalization of the first Chen inequality to the case of slant submanifolds in quaternionic space forms as follows.
Theorem 3.2 
for any k-tuples . Immersion of a submanifold which realizes equality in this inequality at every point is said to be an ideal immersion.
In , Yoon proved that the inequality (19) is also true for totally real submanifolds in a quaternionic space forms of quaternionic sectional curvature 4c. This inequality was later generalized for slant submanifolds in quaternionic space forms as follows.
Theorem 3.3 
for any k-tuples .
4 Fundamental inequalities involving the Ricci curvature and the squared mean curvature
which is known as the Chen-Ricci inequality. In , Liu obtained the same inequality for totally real submanifolds in quaternionic space forms.
A submanifold M of a quaternion Kähler manifold is said to be a quaternionic CR-submanifold if there exists two orthogonal complementary distributions D and on M such that D is invariant under quaternionic structure and is totally real (see ). An estimation of the Ricci curvature of a quaternionic CR-submanifold in a quaternionic space form has been established in , as follows.
Theorem 4.1 
Let M be an n-dimensional quaternionic CR-submanifold of a quaternionic space form . Then:
- (ii)If , then a unit tangent vector X at p satisfies the equality case of (24) (respectively (25)) if and only if (respectively ), where is the relative null space of M at the point defined by
We note that an optimal inequality concerning the Ricci curvature for quaternionic CR-submanifolds of quaternionic space forms with a semi-symmetric metric connection was obtained in . It is clear that, although quaternionic CR-submanifolds are also the generalization of both quaternionic and totally real submanifolds, there exists no inclusion between the two classes of quaternionic CR-submanifolds and slant submanifolds. The following estimation of the Ricci curvature for slant submanifolds in quaternionic space forms was firstly proved in , and an alternative nice proof can be found in .
Theorem 4.2 
- (i)For each unit vector , we have(26)
If , then a unit tangent vector X at p satisfies the equality case of (26) if and only if X belongs to the relative null space of M at p.
The equality case of (26) holds identically for all unit tangent vectors at p if and only either p is a totally geodesic point or and p is a totally umbilical point.
In , Oprea proved using optimization methods that the inequality (23) is not optimal for Lagrangian submanifolds in complex space forms and obtained an improved Chen-Ricci inequality. In , Deng obtained the proof of the improved inequality as an application of suitable algebraic inequalities. This allows one to determine the equality condition in an explicit form. Recently, in , the Chen-Ricci inequality was improved for Lagrangian submanifolds in quaternionic space forms as follows.
Theorem 4.3 
p is a totally geodesic point or
- (ii)and p is an H-umbilical point with , , i.e., there exists an orthonormal basis of such that the second fundamental form takes the following simple form:
for some suitable functions and satisfying , .
5 Fundamental inequalities involving k-Ricci curvature, squared mean curvature and shape operator
at p, where denotes the identity map of .
The inequality (28) was extended in the setting of a totally real submanifold in a quaternionic space form by Liu and Dai in  and recently, in , both inequalities (28) and (29) were generalized for proper slant submanifolds in quaternionic space forms as follows.
Theorem 5.1 
Theorem 5.2 
- (i)If , then the shape operator at the mean curvature satisfies(31)
If , then at p.
- (iii)A unit vector satisfies(32)
- (iv)The identity(33)
holds at p if and only if p is a totally geodesic point.
6 Some open problems
In this section we propose to investigate the following open problems related to the Chen invariants and ideal immersions in quaternionic space forms.
Problem 6.1 QR-submanifolds were introduced by Bejancu  as a generalization of the real hypersurfaces of a quaternionic Kähler manifold. In fact, a real submanifold M of a quaternionic Kähler manifold is called a QR-submanifold if there exists a vector subbundle D of the normal bundle such that and , for all , and for any local basis of σ, where is the complementary orthogonal bundle to D in . The problem is to extend the inequalities (15), (19), (23), (28) and (29) for QR-submanifolds in quaternionic space forms.
Problem 6.2 To generalize the Theorems 3.2, 3.3, 4.1, 4.2, 5.1 and 5.2 for semi-slant submanifolds in quaternionic space forms.
We note that the concept of a semi-slant submanifold in quaternionic geometry was introduced by Şahin  as follows: a real submanifold M of a quaternionic Kähler manifold is said to be a semi-slant submanifold if there exist two orthogonal vector subbundles μ and of the normal bundle such that , is anti-invariant with respect to and is slant with respect to for all , and for any local basis of σ. It is easy to see that this notion is natural because QR-submanifolds and in particular real hypersurfaces are examples of semi-slant submanifolds of a quaternionic Kähler manifold and therefore fulfill the main purpose for which semi-slant submanifolds were introduced in Kähler geometry by Papaghiuc . We also remark that recently, in , Shukla and Rao defined another concept of a semi-slant submanifold of a quaternionic Kähler manifold by analogy with the definition of Papaghiuc, but that class of submanifolds, although generalizes slant submanifolds, does not contain real hypersurfaces as a subclass.
for all vector fields X, Y on , where the indices are taken from modulo 3 and denotes the tangential component of . The problem is to extend the improved Chen-Ricci inequality (27) to quaternionic slant submanifolds in quaternionic space forms and to investigate the equality case of the inequality.
Problem 6.4 In , Oprea improved the inequality (15) for Lagrangian submanifolds in complex space forms and recently, in , Chen and Dillen obtained improved general inequalities which involve the squared mean curvature and the invariants for Lagrangian submanifolds in complex space forms, giving also necessary and sufficient condition for a Lagrangian submanifold to attain the equality for arbitrary . The problem is to obtain improved general inequalities for Lagrangian submanifolds in quaternionic space forms and to completely classify Lagrangian submanifolds which realize the equality case of these inequalities.
Problem 6.5 In  it was proved that there do not exist quaternionic slant immersions of minimal codimension in quaternionic projective space with unfull first normal bundle. An interesting problem is to investigate the existence of quaternionic slant immersions of minimal codimension in a quaternionic projective space satisfying the equality case of (19) such that either or and at least one of is 2. We note that the answer of the corresponding problem in Kählerian geometry is negative .
Problem 6.6 To completely classify -ideal slant submanifolds and -ideal Lagrangian submanifolds in quaternionic space forms.
Problem 6.7 To obtain Chen-like inequalities for the Casorati curvatures of slant submanifolds in quaternionic space forms and to completely classify Casorati ideal submanifolds.
It is known that the Casorati curvature of a submanifold in a Riemannian manifold is an extrinsic invariant defined as the normalized square of the length of the second fundamental form. Moreover, this notion extends the concept of the principal direction of a hypersurface of a Riemannian manifold to submanifolds of a Riemannian manifold and it was preferred by Casorati over the traditional Gauss curvature because corresponds better with the common intuition of curvature . Therefore it is of great interest to obtain optimal inequalities for the Casorati curvatures of submanifolds in different ambient spaces. We note that in , Decu, Haesen and Verstraelen obtained some optimal inequalities involving the scalar curvature and the Casorati curvature of a Riemannian submanifold in a real space form and the holomorphic sectional curvature and the Casorati curvature of a Kähler hypersurface in a complex space form. Moreover, the same authors proved in  an inequality in which the scalar curvature is estimated from above by the normalized Casorati curvatures.
This paper is a contribution to the International Conference on Applied Analysis and Algebra (ICAAA2012) (20-24 June 2012, Istanbul, Turkey).
Dedicated to Prof. Ravi P Agarwal on his 65th birth anniversary.
This work was supported by CNCS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0118.
- Chen B-Y: Some pinching and classification theorems for minimal submanifolds. Arch. Math. 1993, 60: 568–578. 10.1007/BF01236084View ArticleGoogle Scholar
- Chern SS: Minimal Submanifolds in a Riemannian Manifold. University of Kansas Press, Lawrence; 1968.Google Scholar
- Suceavă B, Vâjiac M: Remarks on Chen’s fundamental inequality with classical curvature invariants in Riemannian spaces. An. Ştiint. Univ. Al. I. Cuza Iaşi. Mat. 2008, 54(1):27–37.Google Scholar
- Chen B-Y: Relations between Ricci curvature and shape operator for submanifolds with arbitrary codimensions. Glasg. Math. J. 1999, 41: 33–41. 10.1017/S0017089599970271MathSciNetView ArticleGoogle Scholar
- Chen B-Y: A general inequality for submanifolds in complex-space-forms and its applications. Arch. Math. 1996, 67(6):519–528. 10.1007/BF01270616MathSciNetView ArticleGoogle Scholar
- Chen B-Y: An optimal inequality for CR-warped products in complex space forms involving CR δ -invariant. Int. J. Math. 2012., 23(3): Article ID 1250045Google Scholar
- Oiagă A, Mihai I: B.-Y. Chen inequalities for slant submanifolds in complex space forms. Demonstr. Math. 1999, 32: 835–846.Google Scholar
- Kim J-S, Choi J: A basic inequality for submanifolds in a cosymplectic space form. Int. J. Math. Math. Sci. 2003, 9: 539–547.View ArticleGoogle Scholar
- Liu X, Su W: Shape operator of slant submanifolds in cosymplectic space forms. Studia Sci. Math. Hung. 2005, 42(4):387–400.MathSciNetGoogle Scholar
- Yoon DW: Inequality for Ricci curvature of slant submanifolds in cosymplectic space forms. Turk. J. Math. 2006, 30: 43–56.Google Scholar
- Cioroboiu D, Oiagă A: B.Y. Chen inequalities for slant submanifolds in Sasakian space forms. Rend. Circ. Mat. Palermo 2003, 52: 367–381. 10.1007/BF02872761MathSciNetView ArticleGoogle Scholar
- Defever F, Mihai I, Verstraelen L: B.-Y. Chen’s inequality for C -totally real submanifolds of Sasakian space forms. Boll. Unione Mat. Ital, B 1997, 11(2):365–374.MathSciNetGoogle Scholar
- Mihai I: Ricci curvature of submanifolds in Sasakian space forms. J. Aust. Math. Soc. 2002, 72(2):247–256. 10.1017/S1446788700003888MathSciNetView ArticleGoogle Scholar
- Carriazo A, Kim YH, Yoon DW: Some inequalities on totally real submanifolds in locally conformal Kähler space forms. J. Korean Math. Soc. 2004, 41(5):795–808. 10.4134/JKMS.2004.41.5.795MathSciNetView ArticleGoogle Scholar
- Hong S, Matsumoto K, Tripathi MM: Certain basic inequalities for submanifolds of locally conformal Kähler space forms. SUT J. Math. 2005, 41(1):75–94.MathSciNetGoogle Scholar
- Kim J-S, Yoon DW: Inequality for totally real warped products in locally conformal Kaehler space forms. Kyungpook Math. J. 2004, 44(4):585–592.MathSciNetGoogle Scholar
- Alegre P, Carriazo A, Kim YH, Yoon DW: B.-Y. Chen’s inequality for submanifolds of generalized space forms. Indian J. Pure Appl. Math. 2007, 38(3):185–201.MathSciNetGoogle Scholar
- Kim J-S, Song YM, Tripathi MM: B.-Y. Chen inequalities for submanifolds in generalized complex space forms. Bull. Korean Math. Soc. 2003, 40: 411–423.MathSciNetView ArticleGoogle Scholar
- Mihai A:Shape operator for slant submanifolds in generalized complex space forms. Turk. J. Math. 2003, 27(4):509–523.MathSciNetGoogle Scholar
- Arslan K, Ezentas R, Mihai I, Murathan C, Özgür C: B.Y. Chen inequalities for submanifolds in locally conformal almost cosymplectic manifolds. Bull. Inst. Math. Acad. Sin. 2001, 29(3):231–242.Google Scholar
- Yoon DW: Inequality for Ricci curvature of certain submanifolds in locally conformal almost cosymplectic manifolds. Int. J. Math. Math. Sci. 2005, 10: 1621–1632.View ArticleGoogle Scholar
- Arslan K, Ezentas R, Mihai I, Murathan C, Özgür C:Certain inequalities for submanifolds in -contact space forms. Bull. Aust. Math. Soc. 2001, 64(2):201–212. 10.1017/S0004972700039873View ArticleGoogle Scholar
- Tripathi MM, Kim JS: C -totally real submanifolds in -contact space forms. Bull. Aust. Math. Soc. 2003, 67(1):51–65. 10.1017/S0004972700033517MathSciNetView ArticleGoogle Scholar
- Arslan K, Ezentas R, Mihai I, Murathan C, Özgür C: Ricci curvature of submanifolds in Kenmotsu space forms. Int. J. Math. Math. Sci. 2002, 29(12):719–726. 10.1155/S0161171202012863MathSciNetView ArticleGoogle Scholar
- Carriazo A, Fernandez LM, Hans-Uber MB: B.Y. Chen’s inequality for S -space-forms: applications to slant immersions. Indian J. Pure Appl. Math. 2003, 34(9):1287–1298.MathSciNetGoogle Scholar
- Fernández LM, Hans-Uber MB: New relationships involving the mean curvature of slant submanifolds in S -space-forms. J. Korean Math. Soc. 2007, 44(3):647–659. 10.4134/JKMS.2007.44.3.647MathSciNetView ArticleGoogle Scholar
- Kim J-S, Dwivedi MK, Tripathi MM: Ricci curvature of integral submanifolds of an S -space form. Bull. Korean Math. Soc. 2007, 44(3):395–406. 10.4134/BKMS.2007.44.3.395MathSciNetView ArticleGoogle Scholar
- Aktan N, Zeki Sarikaya M, Ozusaglam E: B.Y. Chen’s inequality for semi-slant submanifolds in T -space forms. Balk. J. Geom. Appl. 2008, 13(1):1–10.Google Scholar
- Chen B-Y Topics in Differential Geometry. In δ-Invariants, Inequalities of Submanifolds and Their Applications. Ed. Acad. Române, Bucharest; 2008:29–155.Google Scholar
- Chen B-Y: Pseudo-Riemannian Geometry, δ-Invariants and Applications. World Scientific, Hackensack; 2011.View ArticleGoogle Scholar
- Berger M: Sur les groupes d’holonomie homogène des variétés à connexion affine et des variétés riemanniennes. Bull. Soc. Math. Fr. 1955, 83: 279–330.Google Scholar
- Ishihara S: Quaternion Kählerian manifolds. J. Differ. Geom. 1974, 9: 483–500.MathSciNetGoogle Scholar
- Besse A: Einstein Manifolds. Springer, Berlin; 1987.View ArticleGoogle Scholar
- Salamon S: Differential geometry of quaternionic manifolds. Ann. Sci. Éc. Norm. Super. 1986, 19: 31–55.MathSciNetGoogle Scholar
- Adachi T, Maeda S: Some characterizations of quaternionic space forms. Proc. Jpn. Acad., Ser. A, Math. Sci. 2000, 76(10):168–172. 10.3792/pjaa.76.168MathSciNetView ArticleGoogle Scholar
- Gray A:A note on manifolds whose holonomy group is a subgroup of . Mich. Math. J. 1969, 16: 125–128.View ArticleGoogle Scholar
- Chen B-Y, Houh CS: Totally real submanifolds of a quaternion projective space. Ann. Mat. Pura Appl. 1979, 120(1):185–199. 10.1007/BF02411943MathSciNetView ArticleGoogle Scholar
- Hong Y, Houh CS: Lagrangian submanifolds of quaternion Kaehlerian manifolds satisfying Chen’s equality. Beiträge Algebra Geom. 1998, 39(2):413–421.MathSciNetGoogle Scholar
- Şahin B: Slant submanifolds of quaternion Kaehler manifolds. Commun. Korean Math. Soc. 2007, 22(1):123–135. 10.4134/CKMS.2007.22.1.123MathSciNetView ArticleGoogle Scholar
- Vîlcu GE: B.-Y. Chen inequalities for slant submanifolds in quaternionic space forms. Turk. J. Math. 2010, 34(1):115–128.Google Scholar
- Chen B-Y: Some new obstructions to minimal and Lagrangian isometric immersions. Jpn. J. Math. 2000, 26(1):105–127.Google Scholar
- Yoon DW: A basic inequality of submanifolds in quaternionic space forms. Balk. J. Geom. Appl. 2004, 9(2):92–102.Google Scholar
- Liu X: On Ricci curvature of totally real submanifolds in a quaternion projective space. Arch. Math. 2002, 38(4):297–305.MathSciNetGoogle Scholar
- Barros M, Chen B-Y, Urbano F: Quaternion CR-submanifolds of quaternion manifolds. Kodai Math. J. 1981, 4: 399–417. 10.2996/kmj/1138036425MathSciNetView ArticleGoogle Scholar
- Mihai I, Al-Solamy F, Shahid MH: On Ricci curvature of a quaternion CR-submanifold in a quaternion space form. Rad. Mat. 2003, 12(1):91–98.MathSciNetGoogle Scholar
- Decu S: Optimal inequalities for submanifolds in quaternion-space-forms with semi-symmetric metric connection. Bull. Transilv. Univ. Braşov, Ser. III 2009, 2(51):175–184.MathSciNetGoogle Scholar
- Shahid MH, Al-Solamy F: Ricci tensor of slant submanifolds in a quaternion projective space. C. R. Math. Acad. Sci. Paris 2011, 349(9):571–573. 10.1016/j.crma.2011.01.013MathSciNetView ArticleGoogle Scholar
- Oprea, T: On a geometric inequality. arXiv:math.DG/0511088Google Scholar
- Deng S: An improved Chen-Ricci inequality. Int. Electron. J. Geom. 2009, 2(2):39–45.MathSciNetGoogle Scholar
- Deng S: Improved Chen-Ricci inequality for Lagrangian submanifolds in quaternion space forms. Int. Electron. J. Geom. 2012, 5(1):163–170.MathSciNetGoogle Scholar
- Liu X, Dai W: Ricci curvature of submanifolds in a quaternion projective space. Commun. Korean Math. Soc. 2002, 17(4):625–633.MathSciNetView ArticleGoogle Scholar
- Vîlcu GE: Slant submanifolds of quaternionic space forms. Publ. Math. (Debr.) 2012, 81(3–4):397–413.View ArticleGoogle Scholar
- Bejancu A: QR-submanifolds of quaternion Kaehler manifolds. Chin. J. Math. 1986, 14: 81–94.MathSciNetGoogle Scholar
- Papaghiuc N: Semi-slant submanifolds of a Kaehlerian manifold. An. Ştiint. Univ. Al. I. Cuza Iaşi 1994, 40(1):55–61.MathSciNetGoogle Scholar
- Shukla SS, Rao PK: Ricci curvature of quaternion slant submanifolds in quaternion space forms. Acta Math. Acad. Paedagog. Nyházi. 2012, 28(1):69–81.MathSciNetGoogle Scholar
- Tripathi MM: Improved Chen-Ricci inequality for curvature-like tensors and its application. Differ. Geom. Appl. 2011, 29(5):685–698. 10.1016/j.difgeo.2011.07.008View ArticleGoogle Scholar
- Mihai A, Rădulescu I: An improved Chen-Ricci inequality for Kaehlerian slant submanifolds in complex space forms. Taiwan. J. Math. 2012, 16(2):761–770.Google Scholar
- Oprea T: Chen’s inequality in the Lagrangian case. Colloq. Math. 2007, 108: 163–169. 10.4064/cm108-1-15MathSciNetView ArticleGoogle Scholar
- Chen B-Y, Dillen F: Optimal general inequalities for Lagrangian submanifolds in complex space forms. J. Math. Anal. Appl. 2011, 379(1):229–239. 10.1016/j.jmaa.2010.12.058MathSciNetView ArticleGoogle Scholar
- Li G, Wu C: Slant immersions of complex space forms and Chen’s inequality. Acta Math. Sci., Ser. B, Engl. Ed. 2005, 25(2):223–232.MathSciNetGoogle Scholar
- Chen B-Y, Dillen F, Verstraelen L, Vrancken L:Totally real submanifolds of satisfying a basic equality. Arch. Math. 1994, 63(6):553–564. 10.1007/BF01202073MathSciNetView ArticleGoogle Scholar
- Chen B-Y, Prieto-Martín A, Wang X:Lagrangian submanifolds in complex space forms satisfying an improved equality involving . Publ. Math. (Debr.) 2013, 82(1):1–25.Google Scholar
- Chen B-Y, Vrancken L: Lagrangian submanifolds satisfying a basic equality. Math. Proc. Camb. Philos. Soc. 1996, 120(2):291–307. 10.1017/S0305004100074867MathSciNetView ArticleGoogle Scholar
- Chen B-Y, Vrancken L: Lagrangian submanifolds of the complex hyperbolic space. Tsukuba J. Math. 2002, 26(1):95–118.MathSciNetGoogle Scholar
- Casorati F: Mesure de la courbure des surfaces suivant l’idée commune. Ses rapports avec les mesures de courbure gaussienne et moyenne. Acta Math. 1890, 14(1):95–110. 10.1007/BF02413317MathSciNetView ArticleGoogle Scholar
- Decu S, Haesen S, Verstraelen L: Optimal inequalities involving Casorati curvatures. Bull. Transilv. Univ. Braşov, Ser. B 2007, 14(49):85–93. suppl.MathSciNetGoogle Scholar
- Decu S, Haesen S, Verstraelen L: Optimal inequalities characterising quasi-umbilical submanifolds. JIPAM. J. Inequal. Pure Appl. Math. 2008., 9(3): Article ID 79Google Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.