"The Smoke Ring," 1987.
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I have never stopped getting letters about it. People have been reading 'Ringworld', and commenting on the assumptions, overt and hidden, and the mathematics and the ecology and the philosophical implications, precisely as if it were a proposed engineering project and they were being paid to work.
His short story, The Fourth Profession, appeared in The 1972 Annual World's Best SF, a compilation on Science Fiction writers.