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asiasociety.org
"The Kingdom of Butuan". Philippine Gold: Treasures of Lost Kingdoms. Asia Society New York. Retrieved March 8, 2019.
The Travels of Pedro Teixeira, tr. and annotated by W.F. Sinclair, London, Hakluyt Society, Series 2, Vol.9, 1902, p.10; H. R. Wagner and Pedro de Unamuno, "The Voyage of Pedro de Unamuno to California in 1587", California Historical Society Quarterly, Vol. 2, No. 2 (Jul., 1923), pp. 140-160, p.142.[4]
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Manit Vallibhotama, "Muang U-Thong", Muang Boran Journal, Volume 14, no.1, January–March 1988, pp.29-44; Sisak Wanliphodom, Suwannaphum yu thi ni, Bangkok, 1998; Warunee Osatharom, Muang Suphan Through Changing Periods, Bangkok, Thammasat University Press, 2004; The Siam Society, Miscellaneous Articles Written for the JSS by His Late Highness Prince Damrong, The Siam Society, Bangkok, B.E. 2505 (1962); William J. Gedney, "A Possible Early Thai Route to the Sea", Journal of the Siam Society, Volume 76, 1988, pp.12-16.[2]