Übrigens and im Übrigen both mean incidentally/by the way. Are there any differences at all between the two?
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First I want to say there are different "feelings" attached to them, but most of the people are not very sensitive about it. So using the one or the other it will never be very wrong.
The first sentence is what you would throw in during a conversation because it just sprung to your mind: "No offense, but by the way, remember you owe me 5 Euros?"
This nears außerdem or darüber hinaus – generally adding to a topic. The "by the way"-statement is made with übrigens.
Übrigens is much more in use (although not just colloquial), im Übrigen is more formal.