Algebra Qualifying Examination
10 January 2011
Instructions:
•There are 8 problems worth a total of 100 points. Individual point values are listed
by each problem.
•Credit awarded for your answers will be based upon the correctness of your answers
as well as the clarity and main steps of your reasoning. “Rough working” will
not receive credit: Answers must be written in a structured and understandable
manner.
•You may use a calculator to check your computations (but may not be used as a
step in your reasoning).
•Every effort is made to ensure that there are no typographical errors or omissions.
If you suspect there is an error, check with the exam administrator. Do not
interpret the problem in a way that makes it trivial.
1. (13 points)
•Show that the set theoretic union of two subgroups cannot be a subgroup,
unless one of the subgroups is contained in the other.
•Show that every vector space over GF (2) of finite dimension d > 1 can be
written as the set theoretic union of three of its proper subspaces.
2. (14 points)
Let Abe a free Abelian group of finite rank.
•(a) Let Bbe a subgroup of Asuch that A=B+pA for some prime number
p. Prove that Bis a subgroup of finite index in A.
•(b) Let Bbe a subgroup of Asuch that A=B+pA for any prime number
p. Prove that A=B.
3. (11 points) Prove that a group of order 616 is solvable.
4. (11 points) Suppose Tis a linear operator on an n-dimensional vector space V
over a field Fsuch that for any non-zero vin Vthe set {Ti(v) : i= 0, ..., n −1}is
linearly independant. Show that the characteristic polynomial of Tis irreducible
over F.
5. (16 points)
(a) Let Fbe a field and let R=F[t] be the polynomial ring. Let R0be the ring
extension R[x]/(tx−1) obtained by adjoining an inverce of tto R. Prove that
this ring can be identified as the ring F[x, x−1] of Laurent polynomials which
are finite linear combinations of powers of x, negative exponents included.
(b) Let Fas before be a field. Prove that the ring F[x, x−1] of Laurent polyno-
mials is a principal ideal domain.
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