@book {2000,
	title = {Frieslands oudheid. Het rijk van de Friese koningen, opkomst en ondergang},
	year = {2000},
	pages = {408},
	publisher = {Matrijs},
	organization = {Matrijs},
	address = {Utrecht},
	issn = {90-5345-167-6},
	author = {Halbertsma, Herre}
}
@inbook {1989,
	title = {Herinneringen aan St. Willibrord in Friesland},
	booktitle = {Willibrord: Apostel der Niederlande, Gr{\"u}nder der Abtei Echternach: Gedenkgabe zum 1250. Todestag des angels{\"a}chsischen Missionars},
	year = {1989},
	pages = {42-68},
	publisher = {Imprimerie Saint-Paul},
	organization = {Imprimerie Saint-Paul},
	address = {Luxembourg},
	issn = {2-87963-079-7},
	author = {Halbertsma, Herre},
	editor = {Kiesel, Georges and Schroeder, Jean}
}
