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The earliest fleeting reference to square roots of negative numbers can perhaps be said to occur in the work of the Greek mathematician Hero of Alexandria in the 1st century AD , where in his " Stereometrica " he considers , apparently in error , the volume of an impossible frustum of a pyramid to arrive at the term in his calculations , although negative quantities were not conceived of in Hellenistic mathematics and Hero merely replaced it by its positive ( ) .
 Hero of Alexandria, Hero
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Hero of Alexandria, Hero