📘 Posets

📘 Posets#

Elements#

A partially ordered set (or poset) is a set equipped with a partial order. A partial order is a binary relation that is reflexive, antisymmetric, and transitive.

In Python, we can represent elements of a poset using a simple class with comparison methods.

The following PartiallyComparable protocol of the galactic.algebras.poset.core module defines the necessary comparison methods for elements in a poset:

class PartiallyComparable(Protocol):
    def __lt__(self, other: object) -> bool: ...
    def __gt__(self, other: object) -> bool: ...
    def __le__(self, other: object) -> bool: ...
    def __ge__(self, other: object) -> bool: ...

The galactic.algebras.lattice.examples.arithmetic.core module define the PrimeFactors class that implements this protocol.

from galactic.algebras.lattice.examples.arithmetic.core import PrimeFactors
a = PrimeFactors(6)
b = PrimeFactors(30)
display(a < b, a <= b, a > b, a >= b)

True

True

False

False

Collections#

To work with collections of poset elements, we can use the MutablePartiallyOrderedSet class from the galactic.algebras.poset.core module. This class provides methods to add elements, check for membership, and perform various poset operations.

Creating a poset#

from galactic.algebras.poset.core import MutablePartiallyOrderedSet
poset = MutablePartiallyOrderedSet[PrimeFactors]()
poset.add(a)
poset.add(b)

display(poset, list(poset))
display(a in poset, PrimeFactors(5) in poset)

<galactic.algebras.poset.core.MutablePartiallyOrderedSet object at 0x7fca545e8880>

[PrimeFactors(6), PrimeFactors(30)]

True

False

Collections of elements in a poset#

A poset handle various collections of its elements, such as:

  • top: the greatest elements in the poset.

  • bottom: The least elements in the poset.

display(poset.top, list(poset.top), poset.bottom, list(poset.bottom))

<galactic...Collection object at 0x7fca545e2f00>

[PrimeFactors(30)]

<galactic...Collection object at 0x7fca545e33e0>

[PrimeFactors(6)]

Views of a poset#

A view on poset can be computed to using the following methods:

  • ideal(): returns the ideal generated by a given element.

  • filter(): returns the filter generated by a given element.

poset.add(PrimeFactors(2))
display(poset.ideal(a), list(poset.ideal(a)))
display(poset.filter(a), list(poset.filter(a)))
display(poset.filter(a).ideal(a), list(poset.filter(a).ideal(a)))

<galactic.algebras.poset.core.PartiallyOrderedSetView object at 0x7fca545d4660>

[PrimeFactors(6), PrimeFactors(2)]

<galactic.algebras.poset.core.PartiallyOrderedSetView object at 0x7fca545d43c0>

[PrimeFactors(6), PrimeFactors(30)]

<galactic.algebras.poset.core.PartiallyOrderedSetView object at 0x7fca545d5000>

[PrimeFactors(6)]

Underlying partial order#

The underlying partial order of a poset can be accessed using the order attribute (and the cover attribute for the cover relation):

display(poset.order, list(poset.order))
display(poset.cover, list(poset.cover))

<galactic.algebras.poset.core.PartialOrder object at 0x7fca545e3130>

[(PrimeFactors(6), PrimeFactors(6)),
 (PrimeFactors(6), PrimeFactors(30)),
 (PrimeFactors(2), PrimeFactors(6)),
 (PrimeFactors(2), PrimeFactors(2)),
 (PrimeFactors(2), PrimeFactors(30)),
 (PrimeFactors(30), PrimeFactors(30))]

<galactic...CoveringRelation object at 0x7fca545fcdc0>

[(PrimeFactors(6), PrimeFactors(30)), (PrimeFactors(2), PrimeFactors(6))]
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