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Suppose that delta is an eigenvalue of an invertible matrix A. Show that 1/delta is an eigenvalue of A inverse.
 one year ago
 one year ago
Suppose that delta is an eigenvalue of an invertible matrix A. Show that 1/delta is an eigenvalue of A inverse.
 one year ago
 one year ago

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TuringTestBest ResponseYou've already chosen the best response.0
write the definition of an eigenvalue
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
So I know what an eigenvalue is, but how would you write the definition of it, what do u mean by that?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
Start with showing what an eigenvalue is: a scalar, \(\delta\) in this case, such that\[AI=\delta A\]Now, since we can assume \(A\) has an inverse \(A^{1}\), multiply both sides by the inverse. What do you get?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
*I meant of course\[AI=\delta I\]multiply both sides by \(A^{1}\) and what do you get?
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
Thanks for the help
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
So I do what u said and that should be the answer? That should prove what we need to prove?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
Not quite, you have not demonstrated what we have set out to prove. why don't you write out what you get by multiplying the definition of the eigenvalue\[AI=\delta I\]by the inverse of the matrix, \(A^{1}\) and show me what you get? I will help you from there.
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
So this is what I did, so here's my work: A^1 AI = delta*I*A^1 I = delta*I*A^1 So I that's what I got, I'm pretty sure the right side can't be simplified any further, so if you could help me with the rest, that'd be great.
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
divide both sides by delta and you're done check it out, you get the definition of the eigenvalue, with the eigenvalue of A^1 as 1/delta
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
So on the right side, we then get I * A^1, but that's just still A^1 right?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
yes\[AI=\delta I\]\[A^{1}AI=II=I^2=I=A^{1}\delta I\]remember that scalars are commutative so we can move delta around\[\delta A^{1}I=I\]\[A^{1}I=\frac1\delta I\]which is the definition of the eigenvalue
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
And one more thing, we also divide I from both sides too right, because that's how we get the one right?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
I is the identity matrix, multiplying or dividing by it changes nothing, just like the scalar number 1
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
Right, so that'll just give us that one.
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
Because it's just I / I.
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
matrices do not become scalars matrix division is defined as multiplication by its inverse the inverse of the identity matrix is still the identity matrix, so\[II^{1}=II=I^2=I\]still the identity matrix, not the scalar number 1 big difference, gotta get that straight in linear algebra
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
Ok, so I think I see, ok so in the end we get this: A^1 * I = 1/delta * I, so then the I's just cancel out since they're on both sides and that is what gives us A^1 = 1/delta right?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
you don't need to cancel the I's, it's just like times 1 if I had 1x=1y would I need to cancel the 1's ? a matrix times the identity matrix is itself, so\[AI=AI^{1}=A\] in the definition of the eigenvalue, I is written explicitly to show that the scalar eigenvalue lambda is multiplying into a matrix\[A=\lambda I\]. if the I's cancel you get\[A=\lambda\]that's wrong because the thing on the right is a scalar and the thing on the left is not Just leave your last line as\[A^{1}=\frac1\delta I\]and that makes clear the properties of the eigenvalue we were looking for
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
All right that makes sense, and can u help out with one more problem related to this same stuff?
 one year ago

TuringTestBest ResponseYou've already chosen the best response.0
I can try, but my connection is horrible right now, I may only be able to pm you
 one year ago

Cali_Native559Best ResponseYou've already chosen the best response.0
Ok, so here's the question: If matrix A has an inverse A^1, use equation 2: Av = lambda*v, to show that A^1 has the same eigenvectors as A. Determine a relationship between the eigenvalues of A and A^1. Illustrate with a suitable example.
 one year ago
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