Halbertsma, Herrius (1982). "Summary". Frieslands Oudheid(PDF) (Thesis) (in Dutch). Groningen: Rijksuniversiteit Groningen. pp. 791–798. OCLC746889526. Archived from the original(PDF) on October 1, 2013. Retrieved April 17, 2010.
Halbertsma, Herrius (1982). "Summary". Frieslands Oudheid(PDF) (Thesis) (in Dutch). Groningen: Rijksuniversiteit Groningen. pp. 791–798. OCLC746889526. Archived from the original(PDF) on October 1, 2013. Retrieved April 17, 2010.
worldcat.org
Halbertsma, Herrius (1982). "Summary". Frieslands Oudheid(PDF) (Thesis) (in Dutch). Groningen: Rijksuniversiteit Groningen. pp. 791–798. OCLC746889526. Archived from the original(PDF) on October 1, 2013. Retrieved April 17, 2010.