Matrix multiplication is not commutative. That is AB is not equal to BA. But there are exceptions. Which pair of matrices M1M1 and M2M2 commute. a. When they are both symmetric. A matrix is symmetric when it is equal to its transpose. b. When they have the form M1=S+STM1=S+ST and M2=S−STM2=S−ST where SS is any matrix. c. When they have the form M1=I+SM1=I+S and M2=I−SM2=I−S where SS is any matrix. d. When they are both orthogonal (aka orthonormal or unitary). A matrix is orthogonal when its transpose is equal to its inverse.  e. They never commute.

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Matrix multiplication is not commutative. That is AB is not equal to BA. But there are exceptions. Which pair of matrices M1M1 and M2M2 commute.
a.

When they are both symmetric. A matrix is symmetric when it is equal to its transpose.

b.

When they have the form M1=S+STM1=S+ST and M2=S−STM2=S−ST where SS is any matrix.

c.

When they have the form M1=I+SM1=I+S and M2=I−SM2=I−S where SS is any matrix.

d.

When they are both orthogonal (aka orthonormal or unitary). A matrix is orthogonal when its transpose is equal to its inverse. 

e.

They never commute.

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