Let n be a positive integer and let F be a field. Let A∈Mn×n(F) be a matrix for which there exists a matrix B∈Mn×n(F) satisfying I+A+AB=O. Show that A is nonsingular.
Since I+A+AB=O, we can get
I+A(I+B)=0
A[−(I+B)]=I
I know it seems −(I+B) is the inverse of A, however, I am not sure how to get
−(I+B)A=I. I may choose a wrong way to solve this problem. I am really stuck with this question.
Answer
In+A+AB=OIn+A(In+B)=OA(In+B)=−In. Take determinant of both sides gives det(A)det(In+B)=(−1)n, hence det(A) is non-zero and A is invertible.
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