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On discontinuous groups operating on the product of the upper half planes. (English) Zbl 0218.10045

##### MSC:
 11F55 Other groups and their modular and automorphic forms (several variables) 11F41 Automorphic forms on $$\mbox{GL}(2)$$; Hilbert and Hilbert-Siegel modular groups and their modular and automorphic forms; Hilbert modular surfaces
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