Vakil
Posted on September 15, 2018Here are my exercise solutions and notes for the beginning of Vakil’s Foundations of Algebraic Geometry.
Chapter 1: Category Theory
1.2.A.
The set
in a groupoid with one object is a group, and vice versa.
Pick any groupoid with two or more objects?
1.2.B. Identity and associativity under the operation of composition follows from the definition of morphisms in a category. Thus the set of invertible automorphisms forms a group.
The automorphism groups in the category of sets are the symmetric groups. Assuming vector spaces are finite dimensional, their automorphism groups are general linear groups. For infinite dimensional vector spaces more complicated groups would arise.
If is isomorphic to
then
is isomorphic to
Suppose is an isomorphism
Let
Observe that
is a morphism
with inverse
So
Let be defined by
Note that this map is surjective by the argument above. Moreover, for
, observe that
Thus
is a homomorphism. Finally, suppose that
Then
Thus
has trivial kernel and is hence injective.
The point of the “isomorphic, but not canonically isomorphic” comment is that although the automorphism groups are pairwise isomorphic, there is not necessarily a “canonical” way of identifying the groups; that is, there might be distinct isomorphisms between the same two automorphism groups. This is classically the case with pointed fundamental groups in the situation that Vakill describes.
1.3.A. Let be two initial objects. Let
,
be the only morphisms. Observe that both
and
are morphisms
, thus they must be equal since
is initial. Similarly for
Thus
are isomorphisms.
A similar argument follows for final.
1.3.B. In Sets and Top the initial object is and the final object is a singleton
In Rings the initial object is
and the final object is the zero ring.
1.3.C. Let be the natural homomorphism. Observe that
if and only if
for some
Thus
, and
injective if and only if there exist no zero divisors in
1.3.D. Suppose is an
-algebra homomorphism sending each element of
to a unit in
Observe that if
for some
, then
Thus
Hence there exists a unique map
such that
(see Lemma 1.2 in my [Fulton notes][BroFulton]).
1.3.E. Clearly as defined in the hint is an abelian group under the defined addition, with identity
for all
Moreover, the defined ring action defined satisfies the axioms for
to be an
-module.
1.3.F.
(a) Define by
Observe that
Therefore
is a group homomorphism, and thus clearly an
-module homomorphism.
Suppose that Then
for some
Therefore
for all
and thus
Hence
Thus
Let Let
and
Then observe that
Thus
is surjective.
(b) The same argument above works equivalently for arbitrary direct sums.
(c) Let for some prime
Then
See the corresponding exercise in my solutions to Fulton’s algebraic curve.