Hi,
I have a basic question on Mueller matrix which I came across upon reading through the Handbook of Optic, chapter 22 polarimetry. It says that :-

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when a polarization element with Mueller matrix M is rotated about the beam of light by an angle theta such that the angle of incident is unchanged, the resulting Mueller matrix M is :

= M(theta)
= R(theta) M R(-theta)
= [matrix elements of R(theta)] [matrix elements of M] X [matrix elements of R(-theta)]
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My question is, what does the "X" mean here..

Sorry if this is too basic, I've just started brushing up my matrix since college.

Appreciate any help.

regards,
--islahna