Question #151741
If T:R^3 tends to R^3 is defined by T(x, y, z)=(2x+y-2z, 2x+3y-4z, x+y-z) Find all eigen value of T and find a basis of each eigen space. Is T is diagonalizable?
1
Expert's answer
2020-12-21T17:21:56-0500

Solution: Given that "T(x, y, z)=(2x+y-2z, 2x+3y-4z, x + y-z)"

First we find matrix representation of T relative to the standard basis B={(1,0,0),(0,1,0),(0,0,1)}

So matrix obtained is A=[212234111]A=\left[\begin{array}{ccc}2&1&-2\\2&3&-4\\1&1&-1\\\end{array}\right]

To find eigenvalues , we consider characteristic equation,

2λ1223λ4111λ=0\left|\begin{array}{ccc}2-\lambda&1&-2\\2&3-\lambda&-4\\1&1&-1-\lambda\\\end{array}\right|=0


we know that characteristic equation is given by


λ3(Trace of A)λ2+(Sum of minors along diagonal)λdet(A)=0\lambda^3-(Trace~of~A )\lambda^2+(Sum ~of~minors~along~diagonal)\lambda-det(A)=0


Trace of A=sum of diagonal elements=2+3-1=4

Sum of minors along diagonal=1+0+4=5

det(A)=2

λ3(Trace of A)λ2+(Sum of minors along diagonal)λdet(A)=0\therefore\lambda^3-(Trace~of~A )\lambda^2+(Sum ~of~minors~along~diagonal)\lambda-det(A)=0 is written as


λ3(4)λ2+(5)λ2=0\lambda^3-(4 )\lambda^2+(5)\lambda-2=0

λ34λ2+5λ2=0\lambda^3-4 \lambda^2+5\lambda-2=0 is called the characteristic equation.


Put λ=1\lambda=1 the characteristic equation satisfies. Therefore \lambda=1 is one of the eigen value of the characteristic equation.

Now we find other eigen values by synthetic division method.

1|1 -4 5 -2

| 1 -3 2

-------------------------------------------

| 1 -3 2 0


λ23λ+2=0\lambda^2-3\lambda+2=0

λ22λλ+2=0\lambda^2-2\lambda-\lambda+2=0

λ(λ2)1(λ2)=0\lambda(\lambda-2)-1(\lambda-2)=0

(λ1)(λ2)=0(\lambda-1)(\lambda-2)=0

λ=1,λ=2\lambda=1,\lambda=2

Therefore the eigenvalues are λ=1,1,2\lambda=1,1,2


To find eigenspace corresponding to λ=1:\lambda=1:

Put λ=1\lambda=1 in the equation (AλI)X=0 where X isvector X=[xyz](A-\lambda I)X=0~ where ~X ~is vector ~X=\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right]

[2λ1223λ4111λ][xyz]=[000]\therefore\left[\begin{array}{ccc}2-\lambda&1&-2\\2&3-\lambda&-4\\1&1&-1-\lambda\\\end{array}\right]\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right] =\left[\begin{array}{ccc}0\\0\\0\\\end{array}\right]


[112224112][xyz]=[000]\therefore\left[\begin{array}{ccc}1&1&-2\\2&2&-4\\1&1&-2\\\end{array}\right]\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right] =\left[\begin{array}{ccc}0\\0\\0\\\end{array}\right]

Solving this by by Gauss elimination method, we get y and zy~and~z as free variable.

X=[xyz]=[y+2zyz]=y[110]+z[201]=y[v1]+z[v2]\therefore X=\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right]=\left[\begin{array}{ccc}-y+2z\\y\\z\\\end{array}\right]=y\left[\begin{array}{ccc}-1\\1\\0\\\end{array}\right] +z\left[\begin{array}{ccc}2\\0\\1\\\end{array}\right] =y[v1]+z[v2]


Where v1=[110] and v2=[201]Where~v1=\left[\begin{array}{ccc}-1\\1\\0\\\end{array}\right] ~and~v2=\left[\begin{array}{ccc}2\\0\\1\\\end{array}\right]  are two eigen vectors corresponding to λ=1~are~two~eigen~vectors~corresponding~ to~\lambda=1


To find eigenspace corresponding to λ=2:\lambda=2:

Put λ=2\lambda=2 in the equation (AλI)X=0 where X isvector X=[xyz](A-\lambda I)X=0~ where ~X ~is vector ~X=\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right]

[2λ1223λ4111λ][xyz]=[000]\therefore\left[\begin{array}{ccc}2-\lambda&1&-2\\2&3-\lambda&-4\\1&1&-1-\lambda\\\end{array}\right]\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right] =\left[\begin{array}{ccc}0\\0\\0\\\end{array}\right]

[012214113][xyz]=[000]\therefore\left[\begin{array}{ccc}0&1&-2\\2&1&-4\\1&1&-3\\\end{array}\right]\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right] =\left[\begin{array}{ccc}0\\0\\0\\\end{array}\right]

Solving this by by Gauss elimination method, we get zz as free variable.


X=[xyz]=[z2zz]=z[121]=z[v3]\therefore X=\left[\begin{array}{ccc}x\\y\\z\\\end{array}\right]=\left[\begin{array}{ccc}z\\2z\\z\\\end{array}\right]=z\left[\begin{array}{ccc}1\\2\\1\\\end{array}\right] =z[v3]


Where v3=[121] is the eigen vectors corresponding to λ=2Where~v3=\left[\begin{array}{ccc}1\\2\\1\\\end{array}\right] ~is~the~eigen~vectors~corresponding~ to~\lambda=2 .


Now, to check T is diagonalizable :

To check T is diagonalizable or not, it is sufficient to prove that AP=PDAP=PD

where A=[212234111], P=[v1 v2 v3]=[121102011]where~A=\left[\begin{array}{ccc}2&1&-2\\2&3&-4\\1&1&-1\\\end{array}\right],~P=[v1~v2~v3]=\left[\begin{array}{ccc}-1&2&1\\1&0&2\\0&1&1\\\end{array}\right]


 and D=[100010002]=diagonal elements of eigen values.~and~D=\left[\begin{array}{ccc}1&0&0\\0&1&0\\0&0&2\\\end{array}\right]=diagonal ~elements~ of~eigen~values.


Now, AP=[212234111][121102011]                 [matrix multiplication]AP=\left[\begin{array}{ccc}2&1&-2\\2&3&-4\\1&1&-1\\\end{array}\right]\left[\begin{array}{ccc}-1&2&1\\1&0&2\\0&1&1\\\end{array}\right] ~~~~~~~~~~~~~~~~~[matrix~ multiplication]

=[122104012]..................................(1)=\left[\begin{array}{ccc}-1&2&2\\1&0&4\\0&1&2\\\end{array}\right]..................................(1)

PD=[121102011][100010002]PD=\left[\begin{array}{ccc}-1&2&1\\1&0&2\\0&1&1\\\end{array}\right]\left[\begin{array}{ccc}1&0&0\\0&1&0\\0&0&2\\\end{array}\right]


=[122104012]..................................(2)=\left[\begin{array}{ccc}-1&2&2\\1&0&4\\0&1&2\\\end{array}\right]..................................(2)

from (1) and (2) ,AP=PDfrom~(1)~and~(2)~,AP=PD

Hence T is diagonalizable.



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