It appears to be just a multiplication sign. But it is peculiar that they would only use it in that one place.
All this, [matrix elements of R(theta)] [matrix elements of M] X [matrix elements of R(-theta)], which I would think should be [matrix elements of R(theta)] X [matrix elements of M] X [matrix elements of R(-theta)] just means the same as the previous line: multiply those three matrices together. |