# Construction of finite atomic and coatomic lattices from incidences#

This module provides the function `lattice_from_incidences()` for computing finite atomic and coatomic lattices in the sense of partially ordered sets where any two elements have meet and joint. For example, the face lattice of a polyhedron.

sage.geometry.hasse_diagram.lattice_from_incidences(atom_to_coatoms, coatom_to_atoms, face_constructor=None, required_atoms=None, key=None, **kwds)#

Compute an atomic and coatomic lattice from the incidence between atoms and coatoms.

INPUT:

• `atom_to_coatoms` – list, `atom_to_coatom[i]` should list all coatoms over the `i`-th atom;

• `coatom_to_atoms` – list, `coatom_to_atom[i]` should list all atoms under the `i`-th coatom;

• `face_constructor` – function or class taking as the first two arguments sorted `tuple` of integers and any keyword arguments. It will be called to construct a face over atoms passed as the first argument and under coatoms passed as the second argument. Default implementation will just return these two tuples as a tuple;

• `required_atoms` – list of atoms (default:None). Each non-empty “face” requires at least one of the specified atoms present. Used to ensure that each face has a vertex.

• `key` – any hashable value (default: None). It is passed down to `FinitePoset`.

• all other keyword arguments will be passed to `face_constructor` on each call.

OUTPUT:

Note

In addition to the specified partial order, finite posets in Sage have internal total linear order of elements which extends the partial one. This function will try to make this internal order to start with the bottom and atoms in the order corresponding to `atom_to_coatoms` and to finish with coatoms in the order corresponding to `coatom_to_atoms` and the top. This may not be possible if atoms and coatoms are the same, in which case the preference is given to the first list.

ALGORITHM:

The detailed description of the used algorithm is given in [KP2002].

The code of this function follows the pseudo-code description in the section 2.5 of the paper, although it is mostly based on frozen sets instead of sorted lists - this makes the implementation easier and should not cost a big performance penalty. (If one wants to make this function faster, it should be probably written in Cython.)

While the title of the paper mentions only polytopes, the algorithm (and the implementation provided here) is applicable to any atomic and coatomic lattice if both incidences are given, see Section 3.4.

In particular, this function can be used for strictly convex cones and complete fans.

REFERENCES: [KP2002]

AUTHORS:

• Andrey Novoseltsev (2010-05-13) with thanks to Marshall Hampton for the reference.

EXAMPLES:

Let us construct the lattice of subsets of {0, 1, 2}. Our atoms are {0}, {1}, and {2}, while our coatoms are {0,1}, {0,2}, and {1,2}. Then incidences are

```sage: atom_to_coatoms = [(0,1), (0,2), (1,2)]
sage: coatom_to_atoms = [(0,1), (0,2), (1,2)]
```

and we can compute the lattice as

```sage: from sage.geometry.cone import lattice_from_incidences
sage: L = lattice_from_incidences(atom_to_coatoms, coatom_to_atoms); L          # needs sage.graphs
Finite lattice containing 8 elements with distinguished linear extension
sage: for level in L.level_sets(): print(level)                                 # needs sage.graphs
[((), (0, 1, 2))]
[((0,), (0, 1)), ((1,), (0, 2)), ((2,), (1, 2))]
[((0, 1), (0,)), ((0, 2), (1,)), ((1, 2), (2,))]
[((0, 1, 2), ())]
```

For more involved examples see the source code of `sage.geometry.cone.ConvexRationalPolyhedralCone.face_lattice()` and `sage.geometry.fan.RationalPolyhedralFan._compute_cone_lattice()`.