Blog Archive

Saturday, May 24, 2025

LaTeX Beamer presentation template for my Talk

Here is the LaTeX Beamer presentation template for my talk:

A Modern Approach to Classical Mathematics: A Mathematical Feast

 

Monday, May 19, 2025

Functions with Compact Support and Algebraic Structures

 

Functions with Compact Support

Let G be a group and X a topological space. Then HomSet(X,G) is a group under pointwise multiplication:

(fg)(x)=f(x)g(x),f1(x)=f(x)1,e(x)=e.

Define

D(f)={xX:f(x)e},supp(f)=D(f).

Lemma 1. UV=UV.

Proof. It is easy to see that

UKUi(K)

The closure operator () is left adjoint to the inclusion of closed sets into all subsets, hence preserves unions (colimits).

It follows that

supp(fg)=D(fg)D(f)D(g)=supp(f)supp(g).

Lemma 2. The union of two compact subsets of X is compact.

Proof. If {Ui}iI covers UV, it covers each of U and V. By compactness extract finite subcovers for U and V, then take their union.

Lemma 3. A closed subset of a compact space is compact.

Proof. If {Ui} covers FK, then {XF}{Ui} covers K. Extract a finite subcover and discard XF.

Corollary. If f,g have compact support then so does fg.

Proposition.

K={fHomSet(X,G):supp(f) is compact}

is a subgroup.

Proof. The constant map e has empty (hence compact) support. Closure under products follows from the corollary. For inverses note supp(f1)=supp(f).


Ring‐valued functions

If R is a ring, pointwise addition and multiplication make HomSet(X,R) into a ring. Defining D(f) and supp(f) as above, one shows:

  • supp(f+g)supp(f)supp(g).

  • supp(fg)supp(f)supp(g).

Hence

K={f:XR:supp(f) is compact}

is an ideal, and if X itself is compact, a subring.


Module‐valued functions

If M is an R‐module then HomSet(X,M) is an R‐module pointwise, and

supp(f+g)supp(f)supp(g),supp(rf)supp(f).

Thus

K={f:XM:supp(f) is compact}

is an R‐submodule.


Example: infinite coproduct

Let I be discrete. Then the only compact subsets of I are the finite ones, and we could construct the coproduct as:

iIMi:={(xi)iIiIMi|(xi)iI has compact support}

 

 

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