Solving Nilpotent Matrices: Show det(I-A)=det(I+A)=1

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SUMMARY

The discussion centers on proving that for a nilpotent matrix A, the determinants satisfy the equations det(I-A) = det(I+A) = 1. It is established that since A is nilpotent, det(A) = 0 and det(I) = 1. The trace of A, tr(A), is zero, leading to the conclusion that tr(I-A) and tr(I+A) both equal n. The hint provided suggests utilizing the property (I + A)² to further explore the determinants.

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  • Understanding of nilpotent matrices and their properties
  • Familiarity with determinants and their calculations
  • Knowledge of matrix traces and their implications
  • Basic concepts of power series and logarithms in matrix analysis
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  • Explore the power series expansion for the logarithm of matrices
  • Investigate the implications of the Cayley-Hamilton theorem on matrix determinants
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Homework Statement



Show that if A is nilpotent then det(I-A)=det(I+A)=1.

Homework Equations



I know that det(A)=0 if A is nilpotent and det(I)=1, so this seems like it follows logically. I also know that the tr(A)=0 and that tr(I-A)=tr(I+A)=n, and that the characteristic polynomial of A is xn.

However, I am lost as to how to start this solution because I don't know what I can say about the det(I-A)? Any hints about where I could start?
 
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Use

\det M = e^{\text{Tr} \ln M}

and the fact that the power series for \ln (I\pm A) is finite.
 
welcome to pf!

hi kmjonec! welcome to pf! :smile:

hint: (I + A)2 :wink:
 

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