Leiden University
Leiden University, 2008
Buku Teks
Universitas Indonesia, 2012
Majalah, Jurnal, Buletin
Awad, Elias M., author
Prentice-Hall, 2004
Buku Teks
Ayer, A. J. (Alfred Jules), 1910-1989, author
Penguin Books, 1956
Buku Teks
Chisholm, Roderick M., author
Prentice Hall International, 1989
Buku Teks