# Circled minus sign

1. Jul 25, 2014

### thegreenlaser

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?

2. Jul 25, 2014

### thegreenlaser

Did some more searching and found the answer here. One of the answers there says:

3. Jul 26, 2014

### HallsofIvy

Thanks for posting that. So "$A\oplus B$" is the "orthogonal complement" of B in A.