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In 1920 , Thoralf Skolem simplified a previous result by Leopold Löwenheim , leading to the Löwenheim–Skolem theorem and , in 1930 , to the notion of a Herbrand universe and a Herbrand interpretation that allowed ( un ) satisfiability of first-order formulas ( and hence the validity of a theorem ) to be reduced to ( potentially infinitely many ) propositional satisfiability problems .
 person: Thoralf Skolem; person: Leopold Löwenheim; person: Löwenheim–Skolem; person: Herbrand; person: Herbrand
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: Thoralf Skolem, Leopold Löwenheim, Löwenheim–Skolem, Herbrand, Herbrand 
Answer:
 person: Thoralf Skolem; person: Leopold Löwenheim; person: Löwenheim–Skolem; person: Herbrand; person: Herbrand