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Working out a rigorous foundation for calculus occupied mathematicians for much of the century following Newton and Leibniz , and is still to some extent an active area of research today .
 Newton, Leibniz
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Newton, Leibniz