In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, major diagonal, or good diagonal) of a Matrix \(A\) is the list of entries \(A_{i,j}\) where \(i=j\). All off-diagonal elements are zero in a diagonal matrix. The following three matrices have their main diagonals indicated by red ones:

\(\begin{bmatrix}1&0&0\\0&1&0\\0&0&1\end{bmatrix}\qquad\)\(\begin{bmatrix}1&0&0&0\\0&1&0&0\\0&0&1&0\end{bmatrix}\qquad\)\(\begin{bmatrix}1&0&0\\0&1&0\\0&0&1\\0&0&0\end{bmatrix}\qquad\)\(\begin{bmatrix}1&0&0&0\\0&1&0&0\\0&0&1&0\\0&0&0&1\end{bmatrix}\qquad\)

(“Main Diagonal” 2022)

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