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The airspheres resemble in some respects the orbit-sized ring of breathable atmosphere created by Larry Niven in " The Integral Trees " , but spherical not toroidal , require a force field to retain their integrity , and arose by artificial rather than natural processes .
 Larry Niven, The Integral Trees
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Larry Niven, The Integral Trees