I have been wondering about this question since I was in school. How can one number tell so much about the whole matrix being invertible or not?
I know the proof of this statement now. But I would like to know the intuition behind this result and why this result is actually true.
The proof I have in mind is:
If A is invertible, then
1=det(I)=det(AA−1)=det(A)⋅det(A−1)
whence det(A)≠0.
Conversely, if det(A)≠0, we have
Aadj(A)=adj(A)A=det(A)I
whence A is invertible.
adj(A) is the adjugate matrix of A.
adj(A)ji=(−1)i+jdet(Aij)
where Aij is the matrix obtained from A by deleting ith row and jth column.
Any other insightful proofs are also welcome.
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