ISBN 978-0-521-53567-0. (原始内容存档于2023-05-12) (英语). Knowledge is a pure public good: once something is known, that knowledge can be used by anyone, and its use by any
JSTOR 2114256 Jochen Streb, et al. "Technological and geographical knowledgespillover in the German empire 1877–1918", Economic History Review, May 2006