Artikel Jurnal
Backman, Fredrik, author
Noura Books, 2016
Buku Teks
Teresa Stoppani, Giorgio Ponzo, George Themistokleous.
Routledge, 2017
Buku Teks
Chalmers, A.F., author
University of Green Sland Press, 1982
Buku Teks
Trese, Leo J., author
Notre Dame, 1964
Buku Teks