Intereting Posts

How to find an approximation to $1 – \left( \frac{13999}{14000}\right )^{14000}$?
Cyclic rearrangements of periods of the decimal expansions of certain rationals
Real Analysis Proofs: Additive Functions
estimations of solutions
Expressing a Non Negative Integer as Sums of Two Squares
What is the negation of this statement?
How to find the multiplicity of eigenvalues?
Not sure which test to use?
Proving that $\sigma_7(n) = \sigma_3(n) + 120 \sum_{m=1}^{n-1} \sigma_3(m)\sigma_3(n-m)$ without using modular forms?
Lindelöf'ize a space?
Is there an absolute notion of the infinite?
What is a physical interpretation of a skew symmetric bilinear form?
Finding number of ways of selecting 6 gloves each of different colour from 18 gloves?
Signed angle between 2 vectors?
Easiest way to solve system of linear equations involving singular matrix

Let $X$ be a set. We can turn $\mathcal P(X)$ (the power set of $X$) into a category by taking inclusion maps as morphisms. Now consider a function $f : X \to Y$, which induces the functor $f^{-1} : \mathcal P(Y) \to \mathcal P(X)$. Now we have the identities

$$

f^{-1} \left( \bigcup_{\alpha} V_{\alpha} \right) = \bigcup_{\alpha} f^{-1}(V_{\alpha}), \quad f^{-1} \left( \bigcap_{\alpha} V_{\alpha} \right) = \bigcap_{\alpha} f^{-1}(V_{\alpha}).

$$

A limit/colimit in the category $\mathcal P(X)$ is a intersection/union of subsets of $X$, so the first equation says that $f^{-1}$ commutes with all limits/colimits. If our categories are nice enough (which we assumed, since everything here is small, c.f. this Wikipedia page), the Special Adjoint Functor Theorem tells us that $f^{-1}$ should have a left-adjoint and a right-adjoint functor. This :

$$

f(U) \subseteq V \quad \Longleftrightarrow \quad U \subseteq f^{-1}(V)

$$

tells us that

$$

\mathrm{Hom}_Y(f(U),V)) \simeq \mathrm{Hom}_X(U, f^{-1}(V))

$$

so I assumed that $f : \mathcal P(X) \to \mathcal P(Y)$ mapping $U \mapsto f(U)$ was the left-adjoint functor I was looking for, so that $f^{-1}$ has a left-adjoint and preserves all limits, we are happy.

Now the thing is I can’t find the right adjoint ; it’s not $f$, and I’ve tried several other things, doesn’t work.

Question : First of all, did I understand all the above correctly, or did I make a mistake somewhere? I am not quite acquainted with the concepts of limits/colimits and the theorem, I am still learning this stuff.

- Grp as a reflexive/coreflexive subcategory of Mon
- Left Adjoint of a Representable Functor
- How are the cardinalities of the object images of adjoint functors related?
- Adjoint Functor Theorem
- Equivalence of the definition of Adjoint Functors via Universal Morphisms and Unit-Counit
- (co)reflector to the forgetful functor $U:\mathbf{CMon} \to \mathbf{ Mon}$

Second of all, assuming the answer to the first question is yes, what is the right-adjoint of $f^{-1}$?

- Is learning haskell a bad thing for a beginner mathematician?
- Monos in $\mathsf{Mon}$ are injective homomorphisms.
- Is every equivalence of monoidal categories a monoidal equivalence?
- What's an Isomorphism?
- Properties of $\mathbf{Cat}$
- Does $f \leq f \circ f^\dagger \circ f$ hold in an arbitrary allegory?
- Is duality an exact functor on Banach spaces or Hilbert spaces?
- When is the kernel pair of a finite presentation of algebraic structures finitely generated?
- Why is every category not isomorphic to its opposite?
- What am I working with?

Powersets, as posets / categories, are self-dual via taking complements. This implies that the right adjoint is the complement of the image of the complement.

A conceptual way to think about the left and right adjoints of taking inverse image is that they are given by fiberwise existential vs. universal quantification. That is, if $f : X \to Y$ is a map of sets and $f^{\ast} : 2^Y \to 2^X$ the inverse image map, its left adjoint takes a subset $S \subseteq X$ to the set

$$T = \{ y \in Y : \exists x \in f^{-1}(y) : x \in S \}$$

while its right adjoint takes a subset $S \subseteq X$ to the set

$$T = \{ y \in Y : \forall x \in f^{-1}(y) : x \in S \}.$$

Following your thoughts, we look for a functor $F:\mathcal P(X)\to\mathcal P(Y)$ which satisfies

$$\hom_{\mathcal P(X)}(f^{-1}(V),\,U) \simeq \hom_{\mathcal P(Y)}(V,\,F(U))\,.$$

Since both categories in question are partial orders by inclusion, this means exactly that

$$f^{-1}(V)\subseteq U\ \iff\ V\subseteq F(U)$$

for all $U\subseteq X,\ V\subseteq Y$.

This suggests $F(U):=\bigcup\{V\,:\,f^{-1}(V)\subseteq U\}\ =\ \{y\in Y\,:\,f^{-1}(y)\subseteq U\}$. $\ $(See also the comments.)

Note that, when viewing $f:X\to Y$ as an $Y$-indexed collection of its fibers $\{f^{-1}(y)\}_{y\in Y}$, then $F(U)$ just picks the indices of those fibers which are *fully* contained in $U$.

- For a set of symmetric matrices $A_i$ of order p, show that if the sum of their ranks is p, $A_iA_j=0$
- Is it possible to practice mental math too often?
- Openness of linear mapping
- Where can I find SOLUTIONS to real analysis problems?
- Example where $f\circ g$ is bijective, but neither $f$ nor $g$ is bijective
- Intuition for $\omega^\omega$
- How can a normal vector and a vector on the plane give an equation of the plane?
- The norm and the spectrum of $B(L^p(X,\mu))$
- Mean curvature in terms of Christoffel symbols
- Absolute value of a real number
- How come the cross product of two planes is collinear with the direction vector of the line?
- Intuition in algebra?
- Modules over $k$
- Can anyone help me with a solution?
- Proof/intuition that any number can be expressed in binary form and every number will have a unique representation?