Is there a relationship to describe the maximum number of non-zero elements for any matrix, A, of size (n,n), such that the matrix product AA is equal to the zero matrix?
Just curious about how to solve this question.
Is there a relationship to describe the maximum number of non-zero elements for any matrix, A, of size (n,n), such that the matrix product AA is equal to the zero matrix?
Just curious about how to solve this question.
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The maximum number is in fact $n^2$, i.e., all coefficients can be non-zero. Consider $$ A=\begin{pmatrix} 1 & -1 \cr 1 & -1 \end{pmatrix}. $$ Then $A^2=0$. Another example is $$ A=\begin{pmatrix} 1 & 1 & -2 \cr 1 & 1 & -2 \cr 1 & 1 & -2 \end{pmatrix}. $$ You can generalise this, if you want.