溯源 [ㄙㄨˋ ㄩㄢˊ; su4 yuan2]
  to investigate the origin of sth
  to trace a river upstream back to its source
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  遡源
  溯源
  さくげん
  そげん
  noun (common) (futsuumeishi)
  noun or participle which takes the aux. verb suru
  returning to the origin
  going back to the beginning
  retracing
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