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Cambridge University Press, 2003
Buku Teks
Ross, Martin J.
A Fawcett Crest Book, 1967
Buku Teks SO
edited by Robert A. Goldwin and William A. Schambra
American Enterprise Institute for Public Policy Research, 1985
Buku Teks
Artikel Jurnal
Muhammdijah, 1928
Buku Klasik