MHB Commutativity in the linear transformation space of a 2 dimensional Vector Space

quarkine
Messages
2
Reaction score
0
A variant of a problem from Halmos :
If AB=C and BA=D then explain why (C-D)^2 is commutative with all 2x2 matrices if A and B are 2x2 matrices.
This result does not hold for any other nxn matrices where n > 2. Explain why.

Edit: I tried to show that ((C-D)^2) E - E((C-D)^2) is identically zero. But that didn't work.
A guess is that since the above matrix commute with any 2x2, it has to be of the form bI (where b is a scalar anad I the indentity), which can be confirmed by brute calculation but I am searching for a better way.
 
Last edited:
Physics news on Phys.org
Can you give your thoughts and/or show what you have tried so our helpers know exactly where you are stuck and how best to help?
 
quarkine said:
If AB=C and BA=D then explain why (C-D)^2 is commutative with all 2x2 matrices if A and B are 2x2 matrices.
This is known as Hall's identity. The proof, in very brief outline, goes like this. The commutator $[A,B] = AB-BA$ has trace zero. Its characteristic equation is therefore of the form $\lambda^2 = \mathrm{const.}$ It then follows from the Cayley–Hamilton theorem that $[A,B]^2$ is a multiple of the identity.
 
I asked online questions about Proposition 2.1.1: The answer I got is the following: I have some questions about the answer I got. When the person answering says: ##1.## Is the map ##\mathfrak{q}\mapsto \mathfrak{q} A _\mathfrak{p}## from ##A\setminus \mathfrak{p}\to A_\mathfrak{p}##? But I don't understand what the author meant for the rest of the sentence in mathematical notation: ##2.## In the next statement where the author says: How is ##A\to...
The following are taken from the two sources, 1) from this online page and the book An Introduction to Module Theory by: Ibrahim Assem, Flavio U. Coelho. In the Abelian Categories chapter in the module theory text on page 157, right after presenting IV.2.21 Definition, the authors states "Image and coimage may or may not exist, but if they do, then they are unique up to isomorphism (because so are kernels and cokernels). Also in the reference url page above, the authors present two...
When decomposing a representation ##\rho## of a finite group ##G## into irreducible representations, we can find the number of times the representation contains a particular irrep ##\rho_0## through the character inner product $$ \langle \chi, \chi_0\rangle = \frac{1}{|G|} \sum_{g\in G} \chi(g) \chi_0(g)^*$$ where ##\chi## and ##\chi_0## are the characters of ##\rho## and ##\rho_0##, respectively. Since all group elements in the same conjugacy class have the same characters, this may be...

Similar threads

Back
Top