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QUESTION
State a condition on the matrix \textbf{A} for the set of $n$
inhomogeneous equations in $n$ unknowns ${\bf AX}={\bf 0}$
to have a non-zero solution.
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ANSWER
There is a non-zero solution of ${\bf AX}={\bf 0}$ if $\det{\bf
A}=0.$
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