This is inspired by the recent question Verification of a maximal antichain

The celebrated duality between finite posets and finite distributive lattices has several nice formulations. One of them assigns to a poset $P$ the lattice $\mathscr D\!P$ of its *downdeals* (I like this word invented, I think, by Freyd). A downdeal of $P$ a subset $D\subseteq P$ satisfying $p\leqslant q\in D$ $\Rightarrow$ $p\in D$. This is a (bounded) distributive lattice with respect to union and intersection operations. Conversely to a finite distributive lattice $L$ one assigns the poset $\Pi\!L$ of its *primes*. An element $p\in L$ is prime if $x\land y=p$ implies $x=p$ or $y=p$, and primes are ordered by divisibility: $p\leqslant q$ iff $p$ divides $q$, denoted $p|q$ i.e. $\exists x\ q=p\land x$, or equivalently just $p\land q=q$. This seems like an overcomplication in that it reverses the order inherited from $L$, but is just a matter of convenience: you may always switch to all kinds of equivalent definitions, like reversing the order in $P$ or in $L$, replacing primes by join-primes, or passing to complements of downdeals, which are *updeals*, or both, etc., etc.

The duality says two things. First, that every $L$ can be identified with the lattice of downdeals of its primes, i. e. an element $x\in L$ is uniquely determined by its prime divisors, $D_x:=\{p\in\Pi\!L\mid\exists y\ x=p\land y\}$; in other words, every $x$ is the meet of its prime divisors. Moreover, every downdeal $D$ of $\Pi\!L$ is $D_x$ for a unique $x\in L$, namely, for $x=\bigwedge D$.

Second, the duality says that every poset $P$ can be identified with the poset of primes of $\mathscr D\!P$. Namely, $p\in P$ becomes identified with $\not\uparrow\!\!p:=\{q\in P\mid p\not\leqslant q\}$ and each prime of $\mathscr D\!P$ is $\not\uparrow p$ for a unique $p\in P$. Moreover $p\leqslant q$ iff $\not\uparrow\!\!p\subseteq\not\uparrow\!\!q$.

Now for a finite poset $P$, its downdeals are in one-to-one correspondence with its antichains: to a downdeal $D$ one assigns the antichain $\max\!D$ of its maximal elements, and to an antichain $\alpha\subseteq P$ the downdeal $\downarrow\!\alpha$ of elements below $\alpha$, $\{p\mid\exists\ q\in\alpha\ p\leqslant q\}$.

My question is: can one characterize abstractly, algebraically, without appealing to this duality, those elements of a finite distributive lattice $L$ which correspond to *maximal* antichains of its dual poset?

More explicitly (I hope I did not make any mistakes when translating it): is there a purely algebraic characterization, without mentioning primes, of those $a\in L$ with the property that for any prime $p\notin D_a$ there is a prime $p'\in\max D_a$ with $p'|p$?

For that inspiring question we actually only need to consider *free* finite distributive lattices, which means considering only the posets $P$ which are full powersets of some finite set, ordered by inclusion. Not much seems to be known about the cardinality of the set of all maximal antichains in a powerset. According to OEIS, the sequence of these starts like $1,2,3,7,29,376,31746,...$

The question Map on class of all finite posets coming from maximal sized antichains seems to be very closely related, but that one concerns antichains of largest possible size, while mine is about all maximal antichains, i. e. antichains not contained in any other antichain. Clearly such antichains may have various sizes in general, in particular in powersets. For example, both the two element antichain $\{\{1\},\{2\}\}$ and the one element antichain $\{\{1,2\}\}$ are maximal antichains in the powerset of $\{1,2\}$.

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