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He is best known for his work on ultraproduct s , in particular for " Łoś ' theorem " , which states that any first-order formula is true in an ultraproduct if and only if it is true in " most " factors ( see ultraproduct for details ) .
 Łoś
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
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Answer:
 Łoś