[Math] What does this symbol mean? (looks like a lower-right corner — subject: manifolds theory)

differential-geometrynotation

Below I have posted an excerpt of Lee's Book "Introduction to Smooth Manifolds" (page 371). I don't know what the symbol means that looks like a lower-right corner, and I cannot find it via the index, nor does any alternative source for the definition of the divergence operator have that symbol. If anybody could give a hint what it denotes I can maybe find the passage in the book where it is explained, that would be a HUGE help!!!

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Best Answer

It is the interior product (Wikipedia page, MathWorld page). It's defined on page 334-336 of Lee's book:

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