For a given finite set of objects $X$, the set $H(X)$ of all
(hard) partitions of $X$ can be partially ordered by defining a
partition $P$ to be finer than a partition $Q$, i.e.,
$P \le Q$, if each class of $P$ is contained in some class of
$Q$. With this partial order, $H(X)$ becomes a bounded
lattice, with intersection and union of two elements given by
their greatest lower bound (meet) and their least upper bound
(join), respectively. Specifically, the meet of two partitions computed by cl_meet is
the partition obtained by intersecting the classes of the partitions;
the classes of the join computed by cl_join are obtained by
joining all elements in the same class in at least one of the
partitions. Obviously, the least and greatest elements of the
partition lattice are the partitions where each object is in a single
class (sometimes referred to as the splitter partition) or in
the same class (the lumper partition), respectively. Meet
and join of an arbitrary number of partitions can be defined
recursively.
In addition to computing the meet and join, the comparison operations
corresponding to the above partial order as well as min,
max, and range are available at least for R objects
representing partitions inheriting from "cl_partition".
The summary methods give the meet and join of the given partitions
(for min and max), or a partition ensemble with the meet
and join (for range).
If the partitions specified by x and y are soft
partitions, the corresponding nearest hard partitions are used.
Future versions may optionally provide suitable soft (fuzzy)
extensions for computing meets and joins.
The set of all dendrograms on $X$ can be ordered using pointwise
inequality of the associated ultrametric dissimilarities: i.e., if
$D$ and $E$ are the dendrograms with ultrametrics $u$ and
$v$, respectively, then $D \le E$ if $u_{ij} \le v_{ij}$
for all pairs $(i, j)$ of objects. This again yields a lattice
(of dendrograms). The join of $D$ and $E$ is the dendrogram
with ultrametrics given by $\max(u_{ij}, v_{ij})$ (as this gives
an ultrametric); the meet is the dendrogram with the maximal
ultrametric dominated by $\min(u_{ij}, v_{ij})$, and can be
obtained by applying single linkage hierarchical clustering to the
minima.
The set of all hierarchies on $X$ can be ordered by set-wise
inclusion of the classes: i.e., if $H$ and $G$ are two
hierarchies, then $H \le G$ if all classes of $H$ are also
classes of $G$. This yields a meet semilattice, with meet given
by the classes contained in both hierarchies. The join only exists if
the union of the classes is a hierarchy.
In each case, a modular semilattice is obtained, which allows for a
natural metrization via least element (semi)lattice move distances,
see Barthélémy, Leclerc and Monjardet (1981). These latticial metrics
are given by the BA/C (partitions), Manhattan (dendrograms), and
symdiff (hierarchies) dissimilarities, respectively (see
cl_dissimilarity).