**189-571B:** Higher Algebra II

## Assignment 7

## To be presented: Friday, March 9.

The problem marked with a (*) is for everyone to do and to hand in on
the day of the problem session.
I will return this problem to you on Friday of the same week,
so late problems will not be accepted!

If you are stuck on your problem, come see me during my Monday
office hours (or make an appointment)
to get a hint...

**1**. (*)
Let E be a cyclic field extension of F of degree n, and let
t:E --> E be a generator of its Galoia group.
Let r be an element of F. Let A be the set of all expressions of
the form

x_{0} + x_{1} u + x_{2} u^{2} + ... +
x_{n-1} u^{n-1}

where the x_{i} are in E and u is an indeterminate.
Make A into an algebra by defining addition in the obvious way and
defining the multiplication by the relations

ux = (tx) u, for all x in E.

u^{n} = r.

The algebra thus obtained is called the cyclic
algebra attached to (E,t,r).

a) Show that A is a central simple algebra over F of degree n
(i.e., of dimension n^{2} over F).

b) (Extra credit) Under what condition on r and E is
A a division algebra? You may suppose in this question,
to simplify your analysis, that n is prime, so that
E has no non-trivial subfields containing F.

c) (Extra credit) For each prime p, write down a division
algebra of rank p^{2} over Q, the field of rational numbers.

**2**. (Alexandru Stanculescu)
If G is a finite group and H is a proper subgroup, show that G
is not equal to the union of the conjugates of H.
Let D be a finite division algebra with center F. Show that any two
maximal subfields E_{1} and
E_{2} of D are conjugate in D, i.e.,
there exists a in D such that
E_{1} = a E_{2} a^{-1}.
Conclude that there are no finite non commutative division algebras.
(Wedderburn's theorem).

(Christian Cote)
**3**. Let A be the ring of "Hamilton quaternions over Q".
Find necesary and sufficient conditions on a quadratic extension E of Q
to be a maximal subfield of A.
Write A as a cyclic algebra, as in question 1.