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I have a problem that I would like to check:

Expand $(A+B)^3$ where $A$ and $B$ are matrices.

Is this right? $$ A^3+A^2B+ABA+AB^2+BA^2+BAB+B^2A+B^3 $$ Thanks.

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Yes, you've got it.

It is probably easiest to start off with $$ (A+B)^2=A(A+B)+B(A+B)=A^2+AB+BA+B^2. $$ From here, we find $$ (A+B)^3=(A+B)(A^2+AB+BA+B^2)=A^3+A^2B+ABA+AB^2+BA^2+BAB+B^2A+B^3, $$ as you claimed.

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