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… is from page 226 of the late Harold Demsetz [2]’s 1969 paper “Perfect Competition, Regulation, and the Stock Market” as this paper is reprinted in volume II of the 1989 collection of some of Demsetz’s most important works, Efficiency, Competition, and Policy [3]:
Complete absence of imperfections is consistent with efficiency only if the cost of accomplishing this objective is zero.