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Definitions If R is the ring of integers of a real quadratic field , then the Hilbert modular group SL2 ( R ) acts on the product H × H of two copies of the upper half plane H. There are several birationally equivalent surfaces related to this action , any of which may be called Hilbert modular surfaces : There are several variations of this construction : Singularities showed how to resolve the quotient singularities , and showed how to resolve their cusp singularities .
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Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
 Entities:  
Answer:
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