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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 .
 Thoralf Skolem, Leopold Löwenheim, Löwenheim–Skolem, Herbrand, Herbrand
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Thoralf Skolem, Leopold Löwenheim, Löwenheim–Skolem, Herbrand, Herbrand