Tuesday, 28 March 2017

linear algebra - Showing that a Matrix is Nonsingular




Let n be a positive integer and let F be a field. Let AMn×n(F) be a matrix for which there exists a matrix BMn×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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