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by itself; automatically; naturally
電気が一人でに消えたので不思議に思った。
The light went out by itself so I thought it was strange.
傷は一人でに治るから心配しなくていい。
The wound will heal by itself so you don't have to worry.
問題が一人でに解決することは珍しい。
It's rare for problems to solve themselves.