Linear Algebra proof (nonsingular matrices)


by seang
Tags: algebra, linear, matrices, nonsingular, proof
seang
seang is offline
#1
Sep14-06, 08:29 PM
P: 185
Let A and B be n x n matrices and let C = AB. Prove that if B is singular then C must be singular.

I have no idea how to prove this. I also don't understand how you can make such a claim without making some stipulations about A. I mean, if A were the 0 matrix, then C doesn't equal AB. And if A is singular, couldn't C also be singular? I was trying to prove this using row equivalence but I couldn't get there. Thanks
Phys.Org News Partner Science news on Phys.org
Cougars' diverse diet helped them survive the Pleistocene mass extinction
Cyber risks can cause disruption on scale of 2008 crisis, study says
Mantis shrimp stronger than airplanes
AKG
AKG is offline
#2
Sep14-06, 08:34 PM
Sci Advisor
HW Helper
P: 2,589
I mean, if A were the 0 matrix, then C doesn't equal AB.
What? C = AB by hypothesis, so if A = 0, then C = 0B = 0.
And if A is singular, couldn't C also be singular?
Yes, but that has nothing to do with anything.

Do you know what it means for a matrix to be singular?
seang
seang is offline
#3
Sep14-06, 09:44 PM
P: 185
I think so. I think it means that it doesn't have an inverse. Doesn't it also mean that there is a 0 in the diagonal? I'm not good at writing proofs.

fourier jr
fourier jr is offline
#4
Sep14-06, 10:26 PM
P: 943

Linear Algebra proof (nonsingular matrices)


Quote Quote by seang
Let A and B be n x n matrices and let C = AB. Prove that if B is singular then C must be singular.
...in other words, if C=AB is invertible then B is invertible. that's how i would do it. if i had to do it exactly as stated i might use contradiction. suppose B is singular & AB is invertible, that is, [tex](AB)^{-1} = B^{-1}A^{-1}[/tex]. maybe it's easier that way.
AKG
AKG is offline
#5
Sep14-06, 10:30 PM
Sci Advisor
HW Helper
P: 2,589
Singular means there's no inverse, correct. It doesn't mean there's a zero on the diagonal, and there are singular matrices with no zeroes on the diagonal.

If B is singular, what can you say about the solutions to Bx = 0?
seang
seang is offline
#6
Sep14-06, 10:39 PM
P: 185
the only solution is 0
AKG
AKG is offline
#7
Sep14-06, 10:42 PM
Sci Advisor
HW Helper
P: 2,589
If B is non-singular, what can you say abou the solutions to Bx = 0?
seang
seang is offline
#8
Sep14-06, 10:44 PM
P: 185
its zero? I might see where this is going
AKG
AKG is offline
#9
Sep15-06, 12:29 AM
Sci Advisor
HW Helper
P: 2,589
I don't mean to confuse you too much. If B is non-singular, then Bx = 0 has only one solution, x=0, so post 8 is correct. If B is singular, then Bx = 0 has infinitely many non-zero solutions, so post 7 is incorrect. In fact, B is singular iff Bx = 0 has infinitely many non-zero solutions. This means that B is non-singular iff Bx = 0 has only the zero-solution. Don't you have any theorems like these?
seang
seang is offline
#10
Sep15-06, 12:41 AM
P: 185
Yes, I actually misread post 5, I thought you had wrote nonsingular. I know the theorems. This is just the first course where I have to write proofs since 7th grade, also, I'm not particularly good at math and am taking linear algebra for mostly applications. (I don't deny that studying the proofs and theory will be a strong foundations for the applications.)

So where do I start? a hint?
HallsofIvy
HallsofIvy is offline
#11
Sep15-06, 10:52 AM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,894
You can also do this by looking at determinants:det(C)= det(AB)= det(A)det(B)
tot
tot is offline
#12
May1-10, 02:40 PM
P: 40
is it just me or is the math department lame.
Why do we need so many contradicting words for the same thing
correct me if I am wrong
"non-singular"="One single trival solution"= "invertible"
"singular" = "many solutions" ="not invertible"


Register to reply

Related Discussions
Linear Algebra- Matrices Calculus & Beyond Homework 3
Linear Algebra Question - Matrices Calculus & Beyond Homework 0
Linear Algebra (Matrices) Calculus & Beyond Homework 1
LINEAR ALGEBRA: Consider 2X2 Matrices - What are the subspaces? Calculus & Beyond Homework 10
Linear Algebra question concerning matrices Precalculus Mathematics Homework 2