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In page Emperor of Japan:

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The origins of the Japanese imperial dynasty are obscure, and it bases its position on the claim that it has "reigned since time immemorial"[citation needed]. There are no records of any emperor who was not said to have been a descendant of other, yet there is suspicion that Emperor Keitai (c. AD 500) may have been an unrelated outsider, though the sources (Kojiki, Nihon-Shoki) state that he was a male-line descendant of Emperor Ōjin. However, his descendants, including his successors, were according to records descended from at least one and probably several imperial princesses of the older lineage.