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Suppose \({A^n} = {\bf{0}}\) for some \({\bf{n}} > {\bf{1}}\). Find an inverse for \(I - A\).

Short Answer

Expert verified

The inverse of \(I - A\) is \(I + A + {A^2} + ... + {A^{n - 1}}\).

Step by step solution

01

Use the matrix computation in Exercise 3

That is \(\left( {I - A} \right)\left( {I + A + {A^2}} \right) = I - {A^3}\). In general,

\(\left( {I - A} \right)\left( {I + A + {A^2} + ... + {A^{n - 1}}} \right) = I - {A^n}\)

02

Suppose \({A^n} = {\bf{0}}\)

Substitute \({A^n} = 0\) in the above equation to obtain:

\(\left( {I - A} \right)\left( {I + A + {A^2} + ... + {A^{n - 1}}} \right) = I\)

This implies \(I - A\) is invertibleand \({\left( {I - A} \right)^{ - 1}} = I + A + {A^2} + ... + {A^{n - 1}}\)

03

Conclusion

Hence, theinverse of \(I - A\) is \(I + A + {A^2} + ... + {A^{n - 1}}\).

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