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## Main Question or Discussion Point

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?