Meaning of ##\ominus## in Hilbert Spaces

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SUMMARY

The symbol ##\ominus## in the context of Hilbert spaces represents the operation of taking the orthogonal complement of a subspace. Specifically, if ##H## is a Hilbert space and ##A## is a linear subspace of ##H##, then ##H \ominus A## denotes the set of all vectors in ##H## that are orthogonal to every vector in ##A##. This is confirmed by the relationship where if ##H = A \oplus B##, then ##H \ominus A = B##. Additionally, ##\ominus## can also refer to the symmetric difference of sets.

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thegreenlaser
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This should be really easy, but I can't seem to find the answer. What does the symbol ##\ominus## mean in the context of Hilbert spaces? As in "##H \ominus A##" where H is a Hilbert space and A is presumably a subspace or subset of H. I'm guessing it's like the inverse of a direct sum, ##\oplus##? As in, if ##H = A \oplus B##, then ##H \ominus A = B##. Is that correct?
 
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Did some more searching and found the answer here. One of the answers there says:

If ##A \subset B## are linear subspaces of a Hilbert space, ##B \ominus A = \{x \in B: (x,y) = 0 \text{ for all }y \in A\}##. ##\ominus## is also used for the symmetric difference of sets.
 
Thanks for posting that. So "A\oplus B" is the "orthogonal complement" of B in A.
 

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