8.E: Exercises for Chapter 8
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Calculational Exercises
1. Let A∈C3×3 be given by
A=[10i010−i0−1]
(a) Calculate det(A).
(b) Find det(A4).
2. (a) For each permutation π∈S3 , compute the number of inversions in π, and classify π as being either an even or an odd permutation.
(b) Use your result from Part (a) to construct a formula for the determinant of a 3×3 matrix.
3. (a) For each permutation π∈S4, compute the number of inversions in π, and classify π as being either an even or an odd permutation.
(b) Use your result from Part (a) to construct a formula for the determinant of a 4×4
matrix.
4. Solve for the variable x in the following expression:
det([x−131−x])=det([10−32x−613x−5]).
5. Prove that the following determinant does not depend upon the value of θ:
det([sin(θ)cos(θ)0−cos(θ)sin(θ)0sin(θ)−cos(θ)sin(θ)+cos(θ)1])
6. Given scalars α,β,γ∈F, prove that the following matrix is not invertible:
[sin2(α)sin2(β)sin2(γ)cos2(α)cos2(β)cos2(γ)111]
Hint: Compute the determinant.
Proof-Writing Exercises
1. Let a,b,c,d,e,f∈F be scalars, and suppose that A and B are the following matrices:
A=[ab0c] and B=[de0f]
Prove that AB=BA if and only if det([ba−ced−f])=0.
2. Given a square matrix A, prove that A is invertible if and only if ATA is invertible.
3. Prove or give a counterexample: For any n≥1 and A,B∈(R)n×n, one has
det(A+B)=det(A)+det(B).
4. Prove or give a counterexample: For any r∈R,n≥1 and A∈Rn×n, one has
det(rA)=rdet(A).
Contributors
- Isaiah Lankham, Mathematics Department at UC Davis
- Bruno Nachtergaele, Mathematics Department at UC Davis
- Anne Schilling, Mathematics Department at UC Davis
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