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More complicated , generally non-linear Bäcklund transforms , such as in the sine-Gordon equation , are of great interest in the theory of solitons and integrable systems .
 person: Bäcklund
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: Bäcklund 
Answer:
 person: Bäcklund