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His main contributions belong to the following fields of mathematics : Approximation theory In this field , Severini proved a generalized version of the Weierstrass approximation theorem .
 Severini, Weierstrass
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Severini, Weierstrass