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In page Family of Vladimir the Great:

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Vladimir reportedly had two unnamed Czech wives: "by one Czech [woman] he had a son Vÿsheslav; by another, Svyatoslav and Mstislav."[1][2] His children by these marriages were probably Svyatoslav of the Derevlyans,[citation needed] killed during the 1015 internecine war,[citation needed] and Mstislav of Chernigov, prince of Tmutorakan.[3] Some historians think this Mstislav was the son of Rogned' instead, but Omeljan Pritsak (1995) reasoned that it was not,[3] and Martin (2007) concluded that Rogned's son Mstislav had probably died before 988, as he is not mentioned in the list of land distributions.[2]