Sheaf of rings on a discrete set. I was reading through some notes for an exam and one exericse ask

Mauricio Hayden

Mauricio Hayden

Answered question

2022-05-29

Sheaf of rings on a discrete set.
I was reading through some notes for an exam and one exericse asks me to prove the following
There is a unique sheaf of rings making a topological set X with discrete topology a ringed space.
I tried doing it but I feel I'm missing something, using the definition of presheaf and than of sheaf doesn't seem to bring me any result. How can I solve such a problem? I leave you my definitions of presheaf and sheaf.
A presheaf F (of rings) on a topological space X consists of the data:
for every open set U X a ring F ( U ) (think of this as the ring of functions on U),
for every inclusion U V of open sets in X a ring homomorphism ρ V , U : F ( V ) F ( U ) called the restriction map (think of this as the usual restriction of functions to a subset), such that
F ( ) = 0
ρ U , U is the identity map of F ( U ) for all U,
for any inclusion U V W of open sets in X we have ρ V , U ρ W , V = ρ W , U
The elements of F ( U ) are usually called the sections of F over U, and the restriction maps ρV,U are written as φ φ | U
A presheaf F is called a sheaf of rings if it satisfies the following gluing property:
if X is an open set, { U i : i I } an arbitrary open cover of U and φ i F ( U i ) sections for all i such that φ i | U i U j = φ j | U i U j for all i, i , j I, then there is a unique φ F ( U ) such that φ | U i = φ i for all i.
EDIT: This is what is given as the definition of a K-ringed space:
A ringed spaces equipped with a sheaf of rings such that the elements of O X ( U ) are actual functions from U to a fixed ring K;
EDIT: It turns out that the actual definition is
A ringed spaces equipped with a sheaf of rings such that the elements of O X ( U ) are actual functions from U to a fixed ring K and O X ( U ) is not only a subring of the ring of functions from U→K but a sub−K−algebra of it;
What does this change?

Answer & Explanation

nicoupsqb

nicoupsqb

Beginner2022-05-30Added 5 answers

Being a K-ringed space has a very specific meaning, namely that the associated sheaf O X ( U ) is a K-subalgebra of the K-algebra Mor(U,K). In particular this means that O X ( U ) contains the constant functions.
A set X with the discrete topology has the trivial open covering
X = x X x ,
where we identify x with the set { x }. The sheaf axioms then give that we can decompose any sheaf F over an open subset U as
F ( U ) x U F ( x ) .
After all the intersection of x y for any distinct elements is empty. For F(x) to sheaf for a K-ringed space we need it to be a K-subalgebra of K x := Mor ( x , K ) and must contain the constant functions. But all elements of Mor(x,K) are constant functions and the set can be identified with K. We must thus have F ( x ) = K x for all x X. It follows that for a K-ringed space with the specified conditions we must have
O X ( U ) u U K .
It is easy to see existence of a presheaf defined that way and the sheaf axioms hold because the restriction maps come from dropping terms in the product.

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