Spring, 2012 CIS 515. Fundamentals of Linear Algebra and Optimization Jean Gallier

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1 Spring 0 CIS 55 Fundamentals of Linear Algebra and Optimization Jean Gallier Homework 5 & 6 + Project 3 & 4 Note: Problems B and B6 are for extra credit April 7 0; Due May 7 0 Problem B (0 pts) Let A be any real or complex n n matrix and let be any operator norm Prove that for every m m I + A k k! e A k= Deduce from the above that the sequence (E m ) of matrices E m = I + converges to a limit denoted e A and called the exponential of A Problem B (Extra Credit 60 pts) Recall that the affine maps ρ: R R defined such that ( ) ( ) ( ) ( ) x cos θ sin θ x w ρ = + y sin θ cos θ y where θ w w R are rigid motions (or direct affine isometries) and that they form the group SE() Given any map ρ as above if we let ( cos θ sin θ R = sin θ cos θ ) X = m k= A k k! w ( ) x and W = y then ρ can be represented by the 3 3 matrix ( ) cos θ sin θ w R W A = = sin θ cos θ w ( w w )

2 in the sense that iff ( ) ρ(x) = ( R W 0 ρ(x) = RX + W ) ( ) X (a) Consider the set of matrices of the form 0 θ u θ 0 v where θ u v R Verify that this set of matrices is a vector space isomorphic to (R 3 +) This vector space is denoted by se() Show that in general BC CB if B C se() (b) Given a matrix 0 θ u B = θ 0 v prove that if θ = 0 then 0 u e B = 0 v 0 0 and that if θ 0 then u cos θ sin θ sin θ + v (cos θ ) e B θ θ = u sin θ cos θ ( cos θ + ) + v sin θ θ θ 0 0 Hint Prove that and that B 3 = θ B e B = I 3 + sin θ θ B + cos θ B θ (c) Check that e B is a direct affine isometry in SE() Prove that the exponential map exp: se() SE() is surjective If θ kπ (k Z) how do you need to restrict θ to get an injective map? Problem B3 ( pts) Consider the affine maps ρ: R R defined such that ( ) ( ) ( ) ( ) x cos θ sin θ x w ρ = α + y sin θ cos θ y w

3 where θ w w α R with α > 0 These maps are called (direct) affine similitudes (for short similitudes) The number α > 0 is the scale factor of the similitude These affine maps are the composition of a rotation of angle θ a rescaling by α > 0 and a translation (a) Prove that these maps form a group that we denote by SIM() Given any map ρ as above if we let ( cos θ sin θ R = sin θ cos θ ) X = ( ) x and W = y then ρ can be represented by the 3 3 matrix ( ) α cos θ α sin θ w αr W A = = α sin θ α cos θ w ( w w ) in the sense that iff ( ) ρ(x) = ( αr W 0 ρ(x) = αrx + W ) ( ) X (b) Consider the set of matrices of the form λ θ u θ λ v where θ λ u v R Verify that this set of matrices is a vector space isomorphic to (R 4 +) This vector space is denoted by sim() (c) Given a matrix prove that Hint Write with ( ) λ θ Ω = θ λ ( cos θ e Ω = e λ sin θ Ω = λi + θj J = ( ) 0 0 ) sin θ cos θ 3

4 Observe that J = I and prove by induction on k that Ω k = ( (λ + iθ) k + (λ iθ) k) I + ( (λ + iθ) k (λ iθ) k) J i (d) As in (c) write let and let Prove that where Ω 0 = I Prove that where (e) Prove that ( ) λ θ Ω = θ λ U = B = B n = e B = V = I + k V = I + k ( ) u v ( ) Ω U 0 0 ( ) Ω n Ω n U 0 0 ( ) e Ω V U 0 Ω k (k + )! Ω k (k + )! = 0 e Ωt dt Use this formula to prove that if λ = θ = 0 then else V = λ + θ V = I ( ) λ(e λ cos θ ) + e λ θ sin θ θ( e λ cos θ) e λ λ sin θ θ( e λ cos θ) + e λ λ sin θ λ(e λ cos θ ) + e λ θ sin θ Observe that for λ = 0 the above gives back the expression in B(b) for θ 0 Conclude that if λ = θ = 0 then e B = ( ) I U 0 4

5 else with and V = λ + θ e B = ( cos θ e Ω = e λ sin θ and that e B SIM() with scale factor e λ ( ) e Ω V U 0 ) sin θ cos θ ( ) λ(e λ cos θ ) + e λ θ sin θ θ( e λ cos θ) e λ λ sin θ θ( e λ cos θ) + e λ λ sin θ λ(e λ cos θ ) + e λ θ sin θ (f) Prove that the exponential map exp: sim() SIM() is surjective (g) Similitudes can be used to describe certain deformations (or flows) of a deformable body B t in the plane Given some initial shape B in the plane (for example a circle) a deformation of B is given by a piecewise differentiable curve D : [0 T ] SIM() where each D(t) is a similitude (for some T > 0) The deformed body B t at time t is given by B t = D(t)(B) The surjectivity of the exponential map exp: sim() SIM() implies that there is a map log: SIM() sim() although it is multivalued The exponential map and the log function allows us to work in the simpler (noncurved) Euclidean space sim() For instance given two similitudes A A SIM() specifying the shape of B at two different times we can compute log(a ) and log(a ) which are just elements of the Euclidean space sim() form the linear interpolant ( t) log(a ) + t log(a ) and then apply the exponential map to get an interpolating deformation t e ( t) log(a )+t log(a ) t [0 ] Also given a sequence of snapshots of the deformable body B say A 0 A A m where each is A i is a similitude we can try to find an interpolating deformation (a curve in SIM()) by finding a simpler curve t C(t) in sim() (say a B-spline) interpolating log A log A log A m Then the curve t e C(t) yields a deformation in SIM() interpolating A 0 A A m () (75 pts) Write a program interpolating between two deformations () (5 pts) Write a program using your cubic spline interpolation program from the first project to interpolate a sequence of deformations given by similitudes A 0 A A m 5

6 Problem B4 (40 pts) Recall that the coordinates of the cross product u v of two vectors u = (u u u 3 ) and v = (v v v 3 ) in R 3 are (u v 3 u 3 v u 3 v u v 3 u v u v ) (a) If we let U be the matrix 0 u 3 u U = u 3 0 u u u 0 check that the coordinates of the cross product u v are given by 0 u 3 u v u v 3 u 3 v u 3 0 u v = u 3 v u v 3 u u 0 v 3 u v u v (b) Show that the set of matrices of the form 0 u 3 u U = u 3 0 u u u 0 is a vector space isomorphic to (R 3 +) This vector space is denoted by so(3) Show that such matrices are never invertible Find the kernel of the linear map associated with a matrix U Describe geometrically the action of the linear map defined by a matrix U Show that when restricted to the plane orthogonal to u = (u u u 3 ) if u is a unit vector then U behaves like a rotation by π/ Problem B5 (00 pts) (a) For any matrix 0 c b A = c 0 a b a 0 if we let θ = a + b + c and a ab ac B = ab b bc ac bc c prove that A = θ I + B AB = BA = 0 6

7 From the above deduce that A 3 = θ A (b) Prove that the exponential map exp: so(3) SO(3) is given by or equivalently by if θ 0 with exp(0 3 ) = I 3 exp A = e A = cos θ I 3 + sin θ ( cos θ) A + B θ θ e A = I 3 + sin θ ( cos θ) A + A θ θ (c) Prove that e A is an orthogonal matrix of determinant + ie a rotation matrix (d) Prove that the exponential map exp: so(3) SO(3) is surjective For this proceed as follows: Pick any rotation matrix R SO(3); () The case R = I is trivial () If R I and tr(r) then { } θ exp (R) = sin θ (R RT ) + cos θ = tr(r) (Recall that tr(r) = r + r + r 3 3 the trace of the matrix R) Note that both θ and π θ yield the same matrix exp(r) (3) If R I and tr(r) = then prove that the eigenvalues of R are that R = R and that R = I Prove that the matrix S = (R I) is a symmetric matrix whose eigenvalues are 0 Thus S can be diagonalized with respect to an orthogonal matrix Q as 0 0 S = Q 0 0 Q Prove that there exists a skew symmetric matrix 0 d c U = d 0 b c b 0 7

8 so that U = S = (R I) Observe that (c + d ) bc bd U = bc (b + d ) cd bd cd (b + c ) and use this to conclude that if U = S then b + c + d = Then show that 0 d c exp (R) = (k + )π d 0 b k Z c b 0 where (b c d) is any unit vector such that for the corresponding skew symmetric matrix U we have U = S (e) To find a skew symmetric matrix U so that U = S = (R I) as in (d) we can solve the system b bc bd bc c cd = S bd cd d We immediately get b c d and then since one of b c d is nonzero say b if we choose the positive square root of b we can determine c and d from bc and bd Implement a computer program to solve the above system Problem B6 (Extra Credit 00 pts) (a) Consider the set of affine maps ρ of R 3 defined such that ρ(x) = RX + W where R is a rotation matrix (an orthogonal matrix of determinant +) and W is some vector in R 3 Every such a map can be represented by the 4 4 matrix ( ) R W 0 in the sense that iff ( ) ρ(x) = ( R W 0 ρ(x) = RX + W ) ( ) X Prove that these maps are affine bijections and that they form a group denoted by SE(3) (the direct affine isometries or rigid motions of R 3 ) 8

9 (b) Let us now consider the set of 4 4 matrices of the form ( ) Ω W B = 0 0 where Ω is a skew-symmetric matrix 0 c b Ω = c 0 a b a 0 and W is a vector in R 3 Verify that this set of matrices is a vector space isomorphic to (R 6 +) This vector space is denoted by se(3) Show that in general BC CB (c) Given a matrix as in (b) prove that where Ω 0 = I 3 Given B n = B = ( ) Ω W 0 0 ( ) Ω n Ω n W c b Ω = c 0 a b a 0 let θ = a + b + c Prove that if θ = 0 then e B = ( I3 W 0 ) and that if θ 0 then where (d) Prove that and e B = V = I 3 + k ( ) e Ω V W 0 Ω k (k + )! e Ω = I 3 + sin θ ( cos θ) Ω + Ω θ θ V = I 3 + ( cos θ) (θ sin θ) Ω + Ω θ θ 3 9

10 Hint Use the fact that Ω 3 = θ Ω (e) Prove that e B is a direct affine isometry in SE(3) Prove that V is invertible and that Z = I Ω + ( θ sin θ ) Ω θ ( cos θ) for θ 0 Hint Assume that the inverse of V is of the form Z = I 3 + aω + bω and show that a b are given by a system of linear equations that always has a unique solution Prove that the exponential map exp: se(3) SE(3) is surjective Remark: As in the case of the plane rigid motions in SE(3) can be used to describe certain deformations of bodies in R 3 Problem B7 (80 pts) Let A be a real matrix ( ) a A = a a a () Prove that the squares of the singular values σ σ of A are the roots of the quadratic equation X tr(a A)X + det(a) = 0 () If we let prove that µ(a) = a + a + a + a a a a a cond (A) = σ σ = µ(a) + (µ(a) ) / (3) Consider the subset S of invertible matrices whose entries a i j are integers such that 0 a ij 00 Prove that the functions cond (A) and µ(a) reach a maximum on the set S for the same values of A Check that for the matrix we have A m = µ(a m ) = ( 00 ) det(a m ) =

11 and cond (A m ) (4) Prove that for all A S if det(a) then µ(a) Conclude that the maximum of µ(a) on S is achieved for matrices such that det(a) = ± Prove that finding matrices that maximize µ on S is equivalent to finding some integers n n n 3 n 4 such that 0 n 4 n 3 n n 00 n + n + n 3 + n = n n 4 n n 3 = You may use without proof that the fact that the only solution to the above constraints is the multiset { } (5) Deduce from part (4) that the matrices in S for which µ has a maximum value are ( ) ( ) ( ) ( ) A m = and check that µ has the same value for these matrices Conclude that max A S cond (A) = cond (A m ) (6) Solve the system ( ) ( ) x = Perturb the right-hand side b by δb = x ( ) ( ) and solve the new system where y = (y y ) Check that A m y = b + δb ( ) δx = y x = 003 Compute x δx b δb and estimate c = δx x ( ) δb b

12 Ckeck that c cond (A m ) = Problem B8 (0 pts) Let E be a real vector space of finite dimension n Say that two bases (u u n ) and (v v n ) of E have the same orientation iff det(p ) > 0 where P the change of basis matrix from (u u n ) and (v v n ) namely the matrix whose jth columns consist of the coordinates of v j over the basis (u u n ) (a) Prove that having the same orientation is an equivalence relation with two equivalence classes An orientation of a vector space E is the choice of any fixed basis say (e e n ) of E Any other basis (v v n ) has the same orientation as (e e n ) (and is said to be positive or direct) iff det(p ) > 0 else it is said to have the opposite orientation of (e e n ) (or to be negative or indirect) where P is the change of basis matrix from (e e n ) to (v v n ) An oriented vector space is a vector space with some chosen orientation (a positive basis) (b) Let B = (u u n ) and B = (v v n ) be two orthonormal bases For any sequence of vectors (w w n ) in E let det B (w w n ) be the determinant of the matrix whose columns are the coordinates of the w j s over the basis B and similarly for det B (w w n ) Prove that if B and B have the same orientation then det B (w w n ) = det B (w w n ) Given any oriented vector space E for any sequence of vectors (w w n ) in E the common value det B (w w n ) for all positive orthonormal bases B of E is denoted and called a volume form of (w w n ) λ E (w w n ) (c) Given any Euclidean oriented vector space E of dimension n for any n vectors w w n in E check that the map x λ E (w w n x) is a linear form Then prove that there is a unique vector denoted w w n such that λ E (w w n x) = (w w n ) x for all x E The vector w w n is called the cross-product of (w w n ) It is a generalization of the cross-product in R 3 (when n = 3)

13 Problem B9 (40 pts) Given p vectors (u u p ) in a Euclidean space E of dimension n p the Gram determinant (or Gramian) of the vectors (u u p ) is the determinant u u u u u p u u u u u p Gram(u u p ) = u p u u p u u p () Prove that Gram(u u n ) = λ E (u u n ) Hint If (e e n ) is an orthonormal basis and A is the matrix of the vectors (u u n ) over this basis det(a) = det(a A) = det(a i A j ) where A i denotes the ith column of the matrix A and (A i A j ) denotes the n n matrix with entries A i A j () Prove that u u n = Gram(u u n ) Hint Letting w = u u n observe that and show that λ E (u u n w) = w w = w w 4 = λ E (u u n w) = Gram(u u n w) = Gram(u u n ) w Problem B0 (60 pts) () Implement the Gram-Schmidt orthonormalization procedure and the modified Gram-Schmidt procedure You may use the pseudo-code showed in () () Implement the method to compute the QR decomposition of an invertible matrix You may use the following pseudo-code: function qr(a: matrix): [Q R] pair of matrices begin n = dim(a); R = 0; (the zero matrix) Q(: ) = A(: ); R( ) = sqrt(q(: ) Q(: )); Q(: ) = Q(: )/R( ); for k := to n do 3

14 w = A(: k + ); for i := to k do R(i k + ) = A(: k + ) Q(: i); w = w R(i k + )Q(: i); endfor; Q(: k + ) = w; R(k + k + ) = sqrt(q(: k + ) Q(: k + )); Q(: k + ) = Q(: k + )/R(k + k + ); endfor; end Test it on various matrices including those involved in Project (3) Given any invertible matrix A define the sequences A k Q k R k as follows: A = A Q k R k = A k A k+ = R k Q k for all k where in the second equation Q k R k is the QR decomposition of A k given by part () Run the above procedure for various values of k (50 00 ) and various real matrices A in particular some symmetric matrices; also run the Matlab command eig on A k and compare the diagonal entries of A k with the eigenvalues given by eig(a k ) What do you observe? How do you explain this behavior? TOTAL: points + 60 extra 4