# (January 14, 2009) End k (V ) End k (V/W )

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1 (January 14, 29) [16.1] Let p be the smallest prime dividing the order of a finite group G. Show that a subgroup H of G of index p is necessarily normal. Let G act on cosets gh of H by left multiplication. This gives a homomorphism f of G to the group of permutations of [G : H] = p things. The kernel ker f certainly lies inside H, since gh = H only for g H. Thus, p [G : ker f]. On the other hand, f(g) = [G : ker f] = G / ker f and f(g) divides the order p! of the symmetric group on p things, by Lagrange. But p is the smallest prime dividing G, so f(g) can only have order 1 or p. Since p divides the order of f(g) and f(g) divides p, we have equality. That is, H is the kernel of f. Every kernel is normal, so H is normal. /// [16.2] Let T Hom k (V ) for a finite-dimensional k-vectorspace V, with k a field. Let W be a T -stable subspace. Prove that the minimal polynomial of T on W is a divisor of the minimal polynomial of T on V. Define a natural action of T on the quotient V/W, and prove that the minimal polynomial of T on V/W is a divisor of the minimal polynomial of T on V. Let f(x) be the minimal polynomial of T on V, and g(x) the minimal polynomial of T on W. (We need the T -stability of W for this to make sense at all.) Since f(t ) = on V, and since the restriction map is a ring homomorphism, End k (V ) End k (W ) (restriction of)f(t) = f(restriction of T ) Thus, f(t ) = on W. That is, by definition of g(x) and the PID-ness of k[x], f(x) is a multiple of g(x), as desired. Define T (v + W ) = T v + W. Since T W W, this is well-defined. Note that we cannot assert, and do not need, an equality T W = W, but only containment. Let h(x) be the minimal polynomial of T (on V/W ). Any polynomial p(t ) stabilizes W, so gives a well-defined map p(t ) on V/W. Further, since the natural map End k (V ) End k (V/W ) is a ring homomorphism, we have p(t )(v + W ) = p(t )(v) + W = p(t )(v + W ) + W = p(t )(v + W ) Since f(t ) = on V, f(t ) =. By definition of minimal polynomial, h(x) f(x). /// [16.3] Let T Hom k (V ) for a finite-dimensional k-vectorspace V, with k a field. Suppose that T is diagonalizable on V. Let W be a T -stable subspace of V. Show that T is diagonalizable on W. Since T is diagonalizable, its minimal polynomial f(x) on V factors into linear factors in k[x] (with zeros exactly the eigenvalues), and no factor is repeated. By the previous example, the minimal polynomial g(x) of T on W divides f(x), so (by unique factorization in k[x]) factors into linear factors without repeats. And this implies that T is diagonalizable when restricted to W. /// [16.4] Let T Hom k (V ) for a finite-dimensional k-vectorspace V, with k a field. Suppose that T is diagonalizable on V, with distinct eigenvalues. Let S Hom k (V ) commute with T, in the natural sense that ST = T S. Show that S is diagonalizable on V. The hypothesis of distinct eigenvalues means that each eigenspace is one-dimensional. We have seen that commuting operators stabilize each other s eigenspaces. Thus, S stabilizes each one-dimensional λ- eigenspaces V λ for T. By the one-dimensionality of V λ, S is a scalar µ λ on V λ. That is, the basis of eigenvectors for T is unavoidably a basis of eigenvectors for S, too, so S is diagonalizable. /// 1

2 Paul Garrett: (January 14, 29) [16.5] Let T Hom k (V ) for a finite-dimensional k-vectorspace V, with k a field. Suppose that T is diagonalizable on V. Show that k[t ] contains the projectors to the eigenspaces of T. Though it is only implicit, we only want projectors P which commute with T. Since T is diagonalizable, its minimal polynomial f(x) factors into linear factors and has no repeated factors. For each eigenvalue λ, let f λ (x) = f(x)/(x λ). The hypothesis that no factor is repeated implies that the gcd of all these f λ (x) is 1, so there are polynomials a λ (x) in k[x] such that 1 = λ a λ (x) f λ (x) For µ λ, the product f λ (x)f µ (x) picks up all the linear factors in f(x), so f λ (T )f µ (T ) = Then for each eigenvalue µ (a µ (T ) f µ (T )) 2 = (a µ (T ) f µ (T )) (1 a λ (T ) f λ (T )) = (a µ (T ) f µ (T )) λ µ Thus, P µ = a µ (T ) f µ (T ) has P 2 µ = P µ. Since f λ (T )f µ (T ) = for λ µ, we have P µ P λ = for λ µ. Thus, these are projectors to the eigenspaces of T, and, being polynomials in T, commute with T. For uniqueness, observe that the diagonalizability of T implies that V is the sum of the λ-eigenspaces V λ of T. We know that any endomorphism (such as a projector) commuting with T stabilizes the eigenspaces of T. Thus, given an eigenvalue λ of T, an endomorphism P commuting with T and such that P (V ) = V λ must be on T -eigenspaces V µ with µ λ, since P (V µ ) V µ V λ = And when restricted to V λ the operator P is required to be the identity. Since V is the sum of the eigenspaces and P is determined completely on each one, there is only one such P (for each λ). /// [16.6] Let V be a complex vector space with a (positive definite) inner product. Show that T Hom k (V ) cannot be a normal operator if it has any non-trivial Jordan block. The spectral theorem for normal operators asserts, among other things, that normal operators are diagonalizable, in the sense that there is a basis of eigenvectors. We know that this implies that the minimal polynomial has no repeated factors. Presence of a non-trivial Jordan block exactly means that the minimal polynomial does have a repeated factor, so this cannot happen for normal operators. /// [16.7] Show that a positive-definite hermitian n-by-n matrix A has a unique positive-definite square root B (that is, B 2 = A). Even though the question explicitly mentions matrices, it is just as easy to discuss endomorphisms of the vector space V = C n. By the spectral theorem, A is diagonalizable, so V = C n is the sum of the eigenspaces V λ of A. By hermitianness these eigenspaces are mutually orthogonal. By positive-definiteness A has positive real eigenvalues λ, which therefore have real square roots. Define B on each orthogonal summand V λ to be the scalar λ. Since these eigenspaces are mutually orthogonal, the operator B so defined really is hermitian, as we now verify. Let v = λ v λ and w = µ w µ be orthogonal decompositions of two vectors into eigenvectors v λ with eigenvalues λ and w µ with eigenvalues µ. Then, using the orthogonality of eigenvectors with distinct eigenvalues, Bv, w = B v λ, w µ = λv λ, w µ = λ v λ, w λ λ µ λ µ λ 2

3 Paul Garrett: (January 14, 29) = λ v λ, λw λ = µ v µ, λ λw λ = v, Bw Uniqueness is slightly subtler. Since we do not know a priori that two positive-definite square roots B and C of A commute, we cannot immediately say that B 2 = C 2 gives (B + C)(B C) =, etc. If we could do that, then since B and C are both positive-definite, we could say (B + C)v, v = Bv, v + Cv, v > so B + C is positive-definite and, hence invertible. Thus, B C =. But we cannot directly do this. We must be more circumspect. Let B be a positive-definite square root of A. Then B commutes with A. Thus, B stabilizes each eigenspace of A. Since B is diagonalizable on V, it is diagonalizable on each eigenspace of A (from an earlier example). Thus, since all eigenvalues of B are positive, and B 2 = λ on the λ-eigenspace V λ of A, it must be that B is the scalar λ on V λ. That is, B is uniquely determined. /// [16.8] Given a square n-by-n complex matrix M, show that there are unitary matrices A and B such that AMB is diagonal. We prove this for not-necessarily square M, with the unitary matrices of appropriate sizes. This asserted expression is called a Cartan decomposition of M. M = unitary diagonal unitary First, if M is (square) invertible, then T = MM is self-adjoint and invertible. From an earlier example, the spectral theorem implies that there is a self-adjoint (necessarily invertible) square root S of T. Then 1 = S 1 T S 1 = (S 1 M)( 1 SM) so k 1 = S 1 M is unitary. Let k 2 be unitary such that D = k 2 Sk 2 is diagonal, by the spectral theorem. Then M = Sk 1 = (k 2 Dk 2)k 1 = k 2 D (k 2k 1 ) expresses M as as desired. M = unitary diagonal unitary In the case of m-by-n (not necessarily invertible) M, we want to reduce to the invertible case by showing that there are m-by-m unitary A 1 and n-by-n unitary B 1 such that ( ) M A 1 MB 1 = where M is square and invertible. That is, we can (in effect) do column and row reduction with unitary matrices. Nearly half of the issue is showing that by left (or right) multiplication by a suitable unitary matrix A an arbitrary matrix M may be put in the form ( ) M11 M AM = 12 with s below the r th row, where the column space of M has dimension r. To this end, let f 1,..., f r be an orthonormal basis for the column space of M, and extend it to an orthonormal basis f 1,..., f m for the 3

4 Paul Garrett: (January 14, 29) whole C m. Let e 1,..., e m be the standard orthonormal basis for C m. Let A be the linear endomorphism of C m defined by Af i = e i for all indices i. We claim that this A is unitary, and has the desired effect on M. That is has the desired effect on M is by design, since any column of the original M will be mapped by A to the span of e 1,..., e r, so will have all s below the r th row. A linear endomorphism is determined exactly by where it sends a basis, so all that needs to be checked is the unitariness, which will result from the orthonormality of the bases, as follows. For v = i a if i and w = i b if i, Av, Aw = i a i Af i, j b j Af j = i a i e i, j b j e j = i a i b i by orthonormality. And, similarly, a i b i = i i a i f i, j b j f j = v, w Thus, Av, Aw = v, w. To be completely scrupulous, we want to see that the latter condition implies that A A = 1. We have A Av, w = v, w for all v and w. If A A 1, then for some v we would have A Av v, and for that v take w = (A A 1)v, so contradiction. That is, A is certainly unitary. (A A 1)v, w = (A A 1)v, (A A 1)v > If we had had the foresight to prove that row rank is always equal to column rank, then we would know that a combination of the previous left multiplication by unitary and a corresponding right multiplication by unitary would leave us with ( ) M with M square and invertible, as desired. /// [16.9] Given a square n-by-n complex matrix M, show that there is a unitary matrix A such that AM is upper triangular. Let {e i } be the standard basis for C n. To say that a matrix is upper triangular is to assert that (with left multiplication of column vectors) each of the maximal family of nested subspaces (called a maximal flag) V = V 1 = Ce 1 Ce 1 + Ce 2... Ce Ce n 1 V n = C n is stabilized by the matrix. Of course MV MV 1 MV 2... MV n 1 V n is another maximal flag. Let f i+1 be a unit-length vector in the orthogonal complement to MV i inside MV i+1 Thus, these f i are an orthonormal basis for V, and, in fact, f 1,..., f t is an orthonormal basis for MV t. Then let A be the unitary endomorphism such that Af i = e i. (In an earlier example and in class we checked that, indeed, a linear map which sends one orthonormal basis to another is unitary.) Then AMV i = V i so AM is upper-triangular. /// [16.1] Let Z be an m-by-n complex matrix. Let Z be its conjugate-transpose. Show that det(1 m ZZ ) = det(1 n Z Z) 4

5 Write Z in the (rectangular) Cartan decomposition Paul Garrett: (January 14, 29) Z = ADB with A and B unitary and D is m-by-n of the form d 1 d 2 D =... dr... where the diagonal d i are the only non-zero entries. We grant ourselves that det(xy) = det(x) det(y) for square matrices x, y of the same size. Then det(1 m ZZ ) = det(1 m ADBB D A ) = det(1 m ADD A ) = det(a (1 m DD ) A ) = det(aa ) det(1 m DD ) = det(1 m DD ) = i (1 d i d i ) Similarly, det(1 n Z Z) = det(1 n B D A ADB) = det(1 n B D DB) = det(b (1 n D D) B) = det(b B) det(1 n D D) = det(1 n D D) = i (1 d i d i ) which is the same as the first computation. /// 5

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