Mathematical rank: any quadratic rank x, AX=XA, and find a, why can the x of the middle frame be set like this?
Chinese is called matrix. Matrix multiplication is feasible only when the number of columns in the left matrix is the same as the number of rows in the right matrix. A is a second-order square matrix, AX can be operated to indicate that X is a matrix with two rows, and XA can be operated to indicate that X has two columns. Taken together, x is a second-order square matrix, so it can be set as follows.