a)
A=⎝⎛−3−6−3121000⎠⎞
A−λI=⎝⎛−3−λ−6−312−λ1000−λ⎠⎞ Characteristic polynomial
det(A−λI)=∣A−λI∣=∣∣−3−λ−6−312−λ1000−λ∣∣
=(−3−λ)∣∣2−λ10−λ∣∣−1∣∣−6−30−λ∣∣+0∣∣−6−32−λ1∣∣
=(−3−λ)(−2λ+λ2−0)−(6λ+0)+0
=6λ−3λ2+2λ2−λ3−6λ=−λ3−λ2
The characteristic equation of A is
∣A−λI∣=0
−λ3−λ2=0The roots are λ1=λ2=0,λ3=−1.
These are the eigenvalues.
b) Find the eigenvectors.
λ=−1
A−λI=⎝⎛−2−6−3131001⎠⎞ R2=R2−3R1
⎝⎛−20−3101001⎠⎞ R3=R3−3R1/2
⎝⎛−20010−1/2001⎠⎞Swap the rows 2 and 3
⎝⎛−2001−1/20010⎠⎞ R2=−2R2
⎝⎛−2001100−20⎠⎞ R1=R1−R2
⎝⎛−2000102−20⎠⎞ R1=R1/(−2)
⎝⎛100010−1−20⎠⎞ Solve the matrix equation
⎝⎛100010−1−20⎠⎞⎝⎛x1x2x3⎠⎞=⎝⎛000⎠⎞If we take x3=t, then x1=t,x2=2t.
Thus
x=⎝⎛t2tt⎠⎞=⎝⎛121⎠⎞t The null space of this matrix is
{⎝⎛121⎠⎞}
λ=0
A−λI=⎝⎛−3−6−3121000⎠⎞ R2=R2−2R1
⎝⎛−30−3101000⎠⎞ R3=R3−R1
⎝⎛−300100000⎠⎞
R1=R1/(−3)
⎝⎛100−1/300000⎠⎞
Solve the matrix equation
⎝⎛100−1/300000⎠⎞⎝⎛x1x2x3⎠⎞=⎝⎛000⎠⎞If we take x2=t,x3=s, then x1=(1/3)t.
Thus
x=⎝⎛(1/3)tts⎠⎞=⎝⎛1/310⎠⎞t+⎝⎛001⎠⎞s
The null space of this matrix is
{⎝⎛1/310⎠⎞,⎝⎛001⎠⎞}
Eigenvalue: −1, multiplicity: 1, eigenvector: ⎝⎛121⎠⎞
Eigenvalue: 0, multiplicity: 2, eigenvectors: ⎝⎛1/310⎠⎞,⎝⎛001⎠⎞
c) A matrix is diagonalizable if and only of for each eigenvalue the dimension of the eigenspace is equal to the multiplicity of the eigenvalue.
For the eigenvalue −1 this is trivially true as its multiplicity is only one and we find one nonzero eigenvector associated to it.
For the eigenvector we find two linearly indepedent eigenvectors.
Therefore the matrix A is diagonalizable.
d)
Form the matrix P, whose column i is i-th eigenvector
P=⎝⎛1211/310001⎠⎞Form the diagonal matrix D whose element at row i, column i is i-th eigenvalue
D=⎝⎛−100000000⎠⎞
detP=∣∣1211/310001∣∣=1∣∣1001∣∣−(1/3)∣∣2101∣∣+0
=1−2/3=1/3=0 The cofactor matrix is
⎝⎛1−1/30−210−11/31/3⎠⎞ The adjugate matrix is
⎝⎛1−2−1−1/311/3001/3⎠⎞ The inverse matrix is the adjugate matrix divided by the determinant.
P−1=⎝⎛3−6−3−131001⎠⎞
A=PDP−1
A2018=PD2018P−1
=⎝⎛1211/310001⎠⎞⎝⎛(−1)201800000000⎠⎞⎝⎛3−6−3−131001⎠⎞
=⎝⎛121000000⎠⎞⎝⎛3−6−3−131001⎠⎞
=⎝⎛363−1−2−1000⎠⎞
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