# Real Analysis Prelim Questions Day 1 August 27, 2013

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1 Real Analysis Prelim Questions Day 1 August 27, 2013 are 5 questions. TIME LIMIT: 3 hours Instructions: Measure and measurable refer to Lebesgue measure µ n on R n, and M(R n ) is the collection of measurable subsets. Please provide complete proofs and justifications for answers. 1. Let 1 p i <, 1 i k, and 1 q < be such that k i=1 Prove that if f i L p i (R n ), 1 i k, then k i=1 f i L q (R n ) and 1 p i = 1. q k f i L q i=1 k fi L p i. i=1 2. For A R 2 and x R, define A x := {y R (x, y) A}. (i) Show that if A M(R 2 ), then A x M(R) for almost every x R. (ii) Show that if A µ2 = 0, then A x µ1 = 0 for almost every x R. 3. Prove the L 1 (R n ) norm triangle inequality: f + g L 1 f L 1 + g L 1. (Hint: first prove it for R-valued functions, and then consider the case of C-valued functions.) 4. Prove the continuity of the integral: If f L 1 (R), then f(x + h) f(x) dx = 0. lim h 0 R 5. (i) Define what it means for a function f : R n [0, ] = [0, ) {+ } to be measurable. 1

2 (ii) Prove or find a counterexample of: If an f as in (i) also satisfies f L 1 (R n ) <, then f takes values in [0, ) almost everywhere. (iii) Prove or find a counterexample of: If an f as in (i) takes values in [0, ) almost everywhere, then f L 1 (R n ) <. 2

3 Complex Analysis Questions Day 1 August 27, 2013 are five questions. TIME LIMIT: 3 hours 6. Suppose that f(z) is holomorphic on D = {z : z < 1}. Suppose also that for all z D, Rf(z) > 0 and f(0) = 1. Prove that for all z D we have f(z) 1 + z 1 z. 7. Suppose that Ω is an open connected set containing 0 and that f(z) is holomorphic on Ω. Show that if f(1/n) e n for n = 1, 2,..., then f(z) is identically zero on Ω. 8. Let {f k (z)} be a sequence of functions holomorphic in the complex plane C, which converges uniformly on compact subsets of C to a polynomial P (z) of positive degree n. Prove that if k is sufficiently large, then f k (z) has at least n zeros (counting multiplicities). 9. Compute, using the residue theorem and including complete justifications, cos x x 2 + x dx Let P (z) be a non-constant complex polynomial, all of whose zeros lie in a half-plane {z : Rz < a} where a is a real number. Show that all the zeros of P (z) lie in this same half-plane. (Hint: compute the logarithmic derivative of P (z). 3

4 Algebra I Questions Day 2 August 28, 2013 are five questions. TIME LIMIT: 3 hours 11. a) Show that A 4 has no subgroup of order 6, and explain why this implies that the converse of Lagrange s theorem is false. b) Find the commutator subgroups of S 4 and A 4 and of S n and A n if n 5, and justify your answers. (Hint: you can do this without actually computing any commutators.) 12. Let H be a proper subgroup of a finite group G. Show that G can not equal the union of conjugates of H. 13. a) Assume without proof that all groups of orders dividing 36 but strictly smaller than 36 are solvable, and use this to conclude that all groups of order 36 are solvable. b) Show that if G is any group of order p 2 q for distinct primes p, q then at least one of its Sylow subgroups must be normal. (Keep in mind that your argument should also work in the case p = 2 and q = 3.) 14. Recall that an element x of a ring A is called nilpotent if x n = 0 for some positive integer n. a) Suppose A is a commutative ring with 1 0 satisfying the property that A m (the localization of A outside of m) has no nonzero nilpotent elements for any maximal ideal m. Prove that A has no nonzero nilpotent elements. b) Would the analogous result still be true if the phrase no nonzero nilpotent elements was replaced by the phrase no zero-divisors, (where by definition a zero divisor can not equal 0)? Prove it or give a counterexample. 15. a) Prove that any prime ideal in an Artin ring is a maximal ideal. If you 4

5 use any properties about Artin rings other than the definition of Artin rings in your answer, also include an explanation of why those properties are true. b) Prove that an Artin ring has only a finite number of prime ideals. 5

6 Algebra II Questions Day 2 August 28, 2013 are six questions. TIME LIMIT: 3 hours 16. a) Let k be a finite field with q elements. Prove that x qn x = f d (x) (1) d n f d where the inside product is over all monic irreducible polynomials of degree d over k, and the outside product is over all positive integers d dividing k. b) Give an example of a field k and a polynomial f(x) k[x] of degree bigger than 1 that is irreducible over k, but is neither separable nor purely inseparable over k if it is possible to do so. If it is not possible to do so, briefly explain why not. Be sure to justify all your claims. 17. Let k be a field, f(x) an irreducible polynomial in k[x], and K a finite normal extension of k. If g(x), h(x) are monic irreducible polynomials in K[x] that divide f(x), show there exists an automorphism σ of K over k such that g(x) = h σ (x). b) Give an example that shows the conclusion need not be valid if we drop the hypothesis that K is normal over k. 18. a) Let Φ n (x) denote the n th cyclotomic polynomial. Prove that Φ n (x) is irreducible over Q. b) Prove that the Galois group of Q(ζ n ) over Q is isomorphic to (Z/nZ) (where ζ n represents a primitive n th root of 1 in characteristic 0. c) Is it also true that the Galois group of F p (ζ n ) over F p must be isomorphic to (Z/nZ) (where ζ n now represents a primitive n th root of 1 in characteristic p in a case where p n )? Prove it or give a counterexample. 19. To answer this question you can use the additive form of Hilbert s Theorem 90 without proof if you want to. 6

7 Let k be a field of characteristic p 0. Prove each of the following. a) Let K be a cyclic extension of k of degree p. Then K = k(α) for some α K that is a root of a polynomial x p x c for some c k. b) Conversely, for any c k, the polynomial x p x c either has one root in k, in which case, all its roots are in k, or it is irreducible. Moreover, in the latter case, k(α) is Galois and cyclic of degree p over k. 20. Let k be a field of characteristic 2, 3. Let f(x) be an irreducible cubic polynomial over k and g(x) be an irreducible quadratic polynomial over k of the form g(x) = x 2 c for c k. Assume that [k( 1/2 ) : k] = 2 and k( 1/2 ) k(c 1/2 ) where denotes the discriminant of f. Let α be a root of f(x) and let β be a root of g(x) in an algebraic closure. Prove a) The splitting field of f(x)g(x) over k has degree 12 over k. b) Let γ = α + β. Then [k(γ) : k] = 6. 7

8 Topology Questions Day 3 August 29, 2013 are five questions. TIME LIMIT: 3 hours 21. Let A X. Let C be a connected subspace of X that intersects both A and X A. Prove that C intersects the boundary of A, BdA. 22. Let Y X and assume that both X and Y are connected. Suppose that X Y = A B with Ā B = φ = A B. Prove Y A is connected. 23. Prove that if Y is compact, then the projection v 1 : X Y X maps closed sets to closed sets. 24. Let {X α } be an indexed family of non-empty spaces. Suppose α X α is locally compact. Prove that each X α is locally compact and X α is compact for all but finitely many values of α. 25. Prove that a closed countable subset of a complete metric space has an isolated point. 8

9 Geometry Questions Day 3 August 29, 2013 are five questions. TIME LIMIT: 3 hours 26. A form β is closed if dβ = 0. The form β is exact if there exists a form γ such that β = dγ. In R 3 let α = xdy dz be a two form. (a) Determine whether α is exact. Either exhibit a one form γ such that α = dγ or show that no such form exists. (b) Given the submanifold M = {(x, y, z) x y 2 z 2 = 1} let β be the form α restricted to the submanifold M. Determine whether β is exact by either exhibiting a one form whose exterior derivative is β or proving that no such form exists. 27. A derivation δ p at the point p on a manifold M is a linear functional on C (M) which also satisfies δ p (fg) = δ p (f)g(p) + f(p)δ p (g). (a) Prove that δ p is identically 0 on all constant functions. (b) List the properties of a C bump function. (c) Assuming the existence of C (R n ) bump functions prove the following: If f(x) = g(x) for all x such that x p < ɛ for some positive ɛ then δ p f = δ p g. (d) Show that the space of derivations δ p on R n is isomorphic to the space of directional derivatives D vp at p where v p is a vector in the tangent space at p. 28. Consider the set G defined as the set of invertible matrices which satisfy ( ) 0 1 G = {A A T JA = J} where J = 1 0 It is easily shown that G forms a group called the group of symplectic ma- 9

10 trices. (a) Prove that G is a smooth submanifold of R 4 b) Describe the elements in the tangent space to G at the identity element e and determine the dimension of the submanifold. ( ) 1 a (c) If Y is the left invariant vector field on G whose value at g = is 0 1 Y g = what is the value of Y at e? Y e =. ( ) 2 1 2a 0 2 (d) What is the dimension of the space of left invariant vector fields on G? 29. For which values of a is M a = {(x, y) y 2 = x(x 1)(x a)} a submanifold? What is the dimension of the submanifold? 30. Let M, N and X be smooth manifolds. Let π : M N be a surjective submersion and let F : M X be a smooth function. If G: N X satisfies G(y) = F (x) for all x such that π(x) = y and F = G π then G is a smooth function from N X. Prove this statement. (Use the properties of a surjection or the constant rank theorem.) 10

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