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Folkvid is only known from the Bagler sagas, where he is mentioned by virtue of his marriage to Cecilia Sigurdsdotter, an illegitimate daughter of Norwegian king Sigurd Munn. The forced marriage was arranged by the earl Erling Skakke. It was annulled in 1184 by the archbishop of Nidaros, on the initiative of Cecilia's half-brother, king Sverre I of Norway. Most Swedish historians have questioned the historical existence of Folkvid, probably due to lack of reference in Swedish sources from the time.
- Knut Peter Lyche Arstad. "Håkon Galen". Norsk biografisk leksikon. Retrieved June 1, 2017.
- Knut Peter Lyche Arstad. "Knut Håkonsson". Norsk biografisk leksikon. Retrieved June 1, 2017.
- Knut Helle. "Cecilia Sigurdsdatter". Norsk biografisk leksikon. Retrieved June 1, 2017.
- Jan E Almquist (1954) Lagsagor och domsagor i Sverige (Stockholm: Norstedts förlag)