SQUARE null matrix is also a diagonal matrix. Null matrix can be square or rectangular matrix and diagonal matrix are types of square matrix. Null matrix is one whose all elements are equal to 0. Diagonal matrix is a square matrix whose non diagonal elements are equal to 0 and diagonal elements may/may not be equal to zero. So, we can say that,null matrix can be diagonal matrix but null matrix should be square. For more details, you may refer
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