Kähler manifolds and their relatives

Let M1 and M2 be two Kähler manifolds. We call M1 and M2 relatives if they share a non-trivial Kähler submanifold S, namely, if there exist two holomorphic and isometric immersions (Kähler immersions) h1 : S → M1 and h2 : S → M2. Moreover, two Kähler manifolds M1 and M2 are said to be weakly relatives if there exist two locally isometric (not necessarily holomorphic) Kähler manifolds S1 and S2 which admit two Kähler immersions into M1 and M2 respectively. The notions introduced are not equivalent (cfr. Example 2.3). Our main results in this paper are Theorem 1.2 and Theorem 1.4. In the first theorem we show that a complex bounded domainD ⊂ C with its Bergman metric and a projective Kähler manifold (i.e. a projective manifold endowed with the restriction of the Fubini–Study metric) are not relatives. In the second theorem we prove that a Hermitian symmetric space of noncompact type and a projective Kähler manifold are not weakly relatives. Notice that the proof of the second result does not follows trivially from the first one. We also remark that the above results are of local nature, i.e. no assumptions are used about the compactness or completeness of the manifolds involved.