**7**

votes

**0**answers

116 views

### Fixed points and universal maps for posets

In a recent post about f.p.p. for poset products, @M.Mirabi brought back an old-standing problem about the fixed point property of the product of two arbitrary posets which already enjoy the fixed ...

**16**

votes

**6**answers

2k views

### The category of posets

I am trying to teach myself category theory and, as a begginer, I am looking for
examples that I have a hands-on experience with.
Almost every introductory text in category theory contains following ...

**27**

votes

**3**answers

813 views

### Is the fixed point property for posets preserved by products?

Recall that a partially ordered set (poset) $P$ has the fixed point property (FPP) if any order preserving function $f:P\longrightarrow P$ has a fixed point.
Theorem. Suppose $P$ and $Q$ are posets ...

**10**

votes

**1**answer

685 views

### What is the size of a largest antichain in this poset?

Let $[n]:=\lbrace 1, \dots, n \rbrace$. We define a partial ordering on the set of subsets of $[n]$ as follows. We say that $X \preceq Y$ if there is an injective map $f:X \to Y$ such that $x \leq ...

**7**

votes

**0**answers

410 views

### The name for a partial order

In a recent paper of mine, my co-authors found that a partial order that we were using was contained in a paper by Kundgen. In it, he called it "right-shifted partial order". I was curious, and found ...

**7**

votes

**2**answers

385 views

### A sequence of generic filters that does not come from an iteration

Fix a countable transitive model $M$ of ZFC.
In my answer to this question I indicated that there are forcing iterations
$((Q_\alpha:\alpha\leq\omega),(\dot P_\alpha:\alpha<\omega))$ in $M$ and ...

**4**

votes

**1**answer

158 views

### Principal Order Ideals in the Weak Bruhat Order

Let $\sigma\in S_n$ be a permutation on $n$ elements, and $\mathrm{Inv}(\sigma):=\{(i,j) : 1\leq i<j\leq n\text{ and }\sigma(i)>\sigma(j)\}$ be its set of inversions. In the weak order on ...

**2**

votes

**1**answer

92 views

### Are the connected components of a Priestley space closed?

Preliminaries A Priestley space is both a poset and a topological space. The topologically connected components of the space are trivially closed. (They are just the points of the underlying set.) But ...

**1**

vote

**2**answers

944 views

### On the number of antichains of a poset

I am not an expert in combinatorics, but I need to count (or to approximate) the number of antichains of a poset.
The idea relies on approximating this number by embedding my poset into another one, ...