**189-570A:** Higher Algebra I

## Take-Home Final: Frequently Asked Questions

From **Lorna McNair**:
I am experiencing
a few difficulties with the question involving the differential
galois theory paper. The question asks us to supply all the details of the
proof but I dont know what a few of the terms mean. The terms in question
are " two-step" " Integral Heisenberg group" and "unipotent".

The integral Heisenberg group is the group of 3 x 3 upper-triangular matrices with integer entries and ones on the diagonal.
A unipotent element is a matrix all of whose eigenvalues are equal to one, and a unipotent group is a grop consisting only of such elements.
Finally, a two-step group is a solvable group whose commutator subgroup is abelian.

From **Austin Roche**: In 1(b) the restriction that the characters in question be _complex_ is
relaxed. So can the associated field be any field, or must it be
commutative?

All fields
are assumed to be commutative, unless it is otherwise explicitly mentionned.
Also, the field in question 1(b) is assumed to be of characteristic zero.

From **Ramon Casanova**: A question about problem 2a)
on the take-home exam. It says that if
f(x) in Z[x] has a root modulo every prime p, then it has a root in Q. I
remembered from number theory that either 2,3, or 6 must be a quadratic
residue modulo every prime p, so the polynomial:
(x^2 - 2)(x^2 - 3)(x^2 - 6) must have a root modulo every prime p, but this
has no rational roots. I was wondering if perhaps I was missing something or
if the question needs to be changed a bit.

This is completely true. So I goofed! What I REALLY wanted to ask was the
following:
Show that an irreducible poynomial cannot have a root mod p for all p.

From ** Amelie Schinck**:
I was just wondering if we can take for
granted in the exam that GL_2(F_p) and GL_3(F_2) are simple groups?
(since it has been seen in previous algebra courses
and it takes a while to prove it).

Actually, you can show this without too much effort using what we did in class, so I appreciate if a few lines of proof are included. You always get extra brownie points for short, elegant proofs as opposed to long exhausting ones, in
questions of this sort.
But be careful, by the way: GL_{2}(F_{p}) is not simple!

From **Sidney Trudeau**:
On the differential galois theory stuff, although no specific mention is
made in the paragraph, I assume D = d/dt ?

Yes, that's right.

From **alot of different people in the class**:
There seems to be some uncertainty
about what is being asked of you in the question
on differential Galois theory. This question should be taken in the spirit
of an essay question.
The goal is for you to convince me that
you've read the article and understood enough of it to be able to carry
out some some of the calculations (those that are claimed to be
"easy verifications" in that paragraph). With what you have learned of Galois
theory in the course, you are in a situation where you should be able to
take a text like this, read it, and understand it.
Reading the mathematics literature, and being able to extract the essential
meaning of a text even if you have not been introduced formally to all the
words and concepts, is an important skill that comes in very useful
when writing a thesis, just as
much as the problem-solving skills that are usually
emphasized on mathematics exams!

From **Lorna McNair** comes the question about what "Zariski closure" means. The Zariski topology on GL_{n}(C) is the topology in which the closed sets are defined to be the zero sets of polynomial equations with complex coefficients.
The Zariski closure of a set
is then the smallest Zariski-closed set which contains it.