# Minimal Polynomial

• Mar 25th 2009, 08:27 AM
azdang
Minimal Polynomial
Here is my problem:

Let A be a complex n x n matrix with minimal polynomial q(x)=the sum from j=0 to m of $\alpha_j x^j$ where $m\leq n$ and $\alpha_m$ = 1.

Show: If A is non-singular then $\alpha_0$ does not equal 0.

So, I get that 0=q(A)= $\alpha_0 I_n + \alpha_1 A + \alpha_2 A^2 +...+A^m$, but I'm not sure what to do here. I assume we will have to use the fact that A is non-singular, but I'm not sure how. Does it maybe involve multiplying both sides by x on the right side? Any hints would be much appreciated! :)
• Mar 25th 2009, 11:30 AM
siclar
If $\alpha_0$ is zero, then $0=\alpha_1 A + \alpha_2 A^2 +...+A^m=A(\alpha_1 I + \alpha_2 A +...+A^{m-1})
$
. But since $A$ is invertible...
• Mar 27th 2009, 08:40 AM
azdang
I have tried before to assume that $\alpha_0$ is 0, but I can't figure out what to do after. Because A is invertible, we can multiply both sides by $A^{-1}$ so we get:

0= $\alpha_1+\alpha_2 A+...+A^{m-1}$.

Buth I haven't been able to understand how that poses a contradiction.

Does it imply that all the alphas would have to be 0? Which would be aproblem because $\alpha_m$ was given to be equal to 1.
• Mar 27th 2009, 08:50 AM
azdang
Oh!!! Does 0=http://www.mathhelpforum.com/math-he...111e8367-1.gif imply that the there would have to be a polynomial of lower degree (m-1)? Which would be impossible because we were talking about the minimal polynomial to begin with.
• Mar 27th 2009, 12:40 PM
ThePerfectHacker
Quote:

Originally Posted by azdang
Oh!!! Does 0=http://www.mathhelpforum.com/math-he...111e8367-1.gif imply that the there would have to be a polynomial of lower degree (m-1)? Which would be impossible because we were talking about the minimal polynomial to begin with.

Yes.