Littlewood, William
Cambridge Uniiversity Press , 1983
Buku Teks
Klippel, Friederike
Cambridge University Press , 1993
Buku Teks
University of Hawai'i at Manao, 2000
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Artikel Jurnal
Fromkin, Victoria A.
Holt, Rinehart and Winston, 1981
Buku Teks SO