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The precise statement of the P = NP problem was introduced in 1971 by Stephen Cook in his seminal paper " The complexity of theorem proving procedures " and is considered by many to be the most important open problem in the field .
 Stephen Cook
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Stephen Cook