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Quaternion CR-submanifolds of a quaternion Kähler manifold. (English) Zbl 1006.53052

The quaternionic Kähler manifold \(\widetilde M\) was introduced and studied by S. Ishihara [J. Differ. Geom. 9, 483-500 (1974; Zbl 0297.53014)], the quaternionic CR-submanifold \(M\) of such a manifold by M. Barros, B. Y. Chen and F. Urgano [Kodai Math. J. 4, 399-417 (1981; Zbl 0481.53046)]. A local basis of almost Hermitian structures is denoted by \(\psi_1=I\), \(\psi_2=J\), \(\psi_3=K\). For any \(X\in TM\) and \(N\in T^\perp M\) there are decompositions into tangential and normal components \(\psi_r X=P_rX+ Q_rX\), \(\psi_rN= t_rN+f_rN\), where \(r=1,2,3\). The covariant differentiations are defined by \((\widetilde\nabla P_r)(Y)= \nabla(P_rY) -P_r \nabla_XY\) etc. If \(\widetilde\nabla P_r=0\) the endomorphism \(P_r\) is said to be parallel.
It is proved that 1) \(Q_r\) is parallel if and only if \(t_r\) is parallel, 2) \(M\) is a QR-product (i.e. is locally the Riemannian product of a quaternionic submanifold and a totally real submanifold) if and only if \(P_r\) is parallel, 3) if \(D\) and \(D^\perp\) are orthogonal distributions on \(M\) and \(D^\perp\) is a totally real foliation then the Bott connection of \(D^\perp\) preserves the volume form of \(D\). A classification theorem is deduced for the totally umbilical quaternionic CR-submanifolds of a quaternionic Kähler manifold.

MSC:

53C40 Global submanifolds
53C26 Hyper-Kähler and quaternionic Kähler geometry, “special” geometry
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