Show exp(A) is invertible for a matrix A

1. May 4, 2013

chipotleaway

1. The problem statement, all variables and given/known data

Define $e^A=I_n+A+\frac{1}{2!}A^2+\frac{1}{3!}A^3+...+\frac{1}{2012!}A2012$
where A is an nxn matrix such that $A^{2013}=0$. Show that $e^A$ is invertible and find an expression for $(e^A)^{-1}$ in terms of A.

3. The attempt at a solution

To first show that it's invertible, can I just show the function $e^x$ has an inverse?

2. May 4, 2013

Dick

Where x is a real number? That would work fine if A were diagonal. I don't think assuming $A^{2013}=0$ makes it any easier. I would just try to show $e^{sA}e^{tA}=e^{(s+t)A}$ first. It's really just the binomial theorem. Then put s=1 and t=(-1).

3. May 5, 2013

HallsofIvy

Staff Emeritus
Do you know that $x^n- 1= (x- 1)(1+ x+ x^2+ \cdot\cdot\cdot+ x^{n-1})$?

If not try proving it by induction on n. Notice that proof works for matrices as well as numbers.

4. May 5, 2013