Nonexistence of Proper Holomorphic Maps Between Certain Classical Bounded Symmetric Domains

The author, motivated by his results on Hermitian metric rigidity, conjectured in 4] that a proper holomorphic mapping ƒ: Ω → Ω′ from an irreducible bounded symmetric domain Ω of rank ≥ 2 into a bounded symmetric domain Ω′ is necessarily totally geodesic provided that r′:= rank(Ω′) ≤ rank(Ω):= r. The Conjecture was resolved in the affirmative by I.-H. Tsai 8]. When the hypothesis r′ ≤ r is removed, the structure of proper holomorphic maps ƒ: Ω → Ω′ is far from being understood, and the complexity in studying such maps depends very much on the difference r′ − r, which is called the rank defect. The only known nontrivial non-equidimensional structure theorems on proper holomorphic maps are due to Z.-H. Tu 10], in which a rigidity theorem was proven for certain pairs of classical domains of type I, which implies nonexistence theorems for other pairs of such domains. For both results the rank defect is equal to 1, and a generalization of the rigidity result to cases of higher rank defects along the line of arguments of 10] has so far been inaccessible. In this article, the author produces nonexistence results for infinite series of pairs of (Ω, Ω′) of irreducible bounded symmetric domains of type I in which the rank defect is an arbitrarily prescribed positive integer. Such nonexistence results are obtained by exploiting the geometry of characteristic symmetric subspaces as introduced by N. Mok and I.-H Tsai 6] and more generally invariantly geodesic subspaces as formalized in 8]. Our nonexistence results motivate the formulation of questions on proper holomorphic maps in the non-equirank case. Project supported by a CERG of the Research Grants Council of Hong Kong, China.