MHB Commutativity in the linear transformation space of a 2 dimensional Vector Space

quarkine
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A variant of a problem from Halmos :
If AB=C and BA=D then explain why (C-D)^2 is commutative with all 2x2 matrices if A and B are 2x2 matrices.
This result does not hold for any other nxn matrices where n > 2. Explain why.

Edit: I tried to show that ((C-D)^2) E - E((C-D)^2) is identically zero. But that didn't work.
A guess is that since the above matrix commute with any 2x2, it has to be of the form bI (where b is a scalar anad I the indentity), which can be confirmed by brute calculation but I am searching for a better way.
 
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quarkine said:
If AB=C and BA=D then explain why (C-D)^2 is commutative with all 2x2 matrices if A and B are 2x2 matrices.
This is known as Hall's identity. The proof, in very brief outline, goes like this. The commutator $[A,B] = AB-BA$ has trace zero. Its characteristic equation is therefore of the form $\lambda^2 = \mathrm{const.}$ It then follows from the Cayley–Hamilton theorem that $[A,B]^2$ is a multiple of the identity.
 
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