There are different ways to define a groupoid such as:
- Algebraic definition: same definition as a group but with a partial function replacing the binary operation.
- Categorical definition: a category in which every morphism is invertible.
- As an oriented graph (if <x,y> is an edge then <y,x> is not).
Groupoids form the underlying structure in homotopy type-theory (HoTT) Discussed on the page here.
Groupoids and Equivalences
|Group Definition||Equivalence Definition|
|Identity||a = a•Id = Id •a||
|Reverse||a•inv(a) = inv(a)•a = Id||
if a~b then b~a
|Associatively||a•(b•c) = (a•b)•c||
if a~b and b~c then a~c
A groupoid is a setoid with natural coherence axioms. A setoid is a set with an equivalence relation.
Another special case is G-sets, sets with an action of a group G.
Example 1 - Fifteen Puzzle
There used to be a puzzle, sometimes called the 15 puzzle, consisting of 15 sliding plastic tiles in a 4 by 4 frame. There is only one space without a tile, so the only moves possible are to slide the adjacent tiles into that space.
The tiles have letters or numbers on them and the goal is to get them in a certain order.
So for the first move, in this starting position, there are only 2 moves possible:
Say we move tile 1:
Now there are 3 moves possible:
This example illustrates the different ways of defining the groupoid:
So we could specify the state of the game by:
- The starting position.
- Together with the sequence of positions of the holes.
Since all arrows are reversible we can always get back to a previous state by retracing a path back. However, if we start with a hole in a particular place and then follow a circular path (for the hole) so the hole ends up in the same place the state (position of tiles) may change so this is not equal to Id.
|Categorical Definition||Groupoid Definition|
|Identity||Arrow from all objects to themselves that do not change it.||Identity element exists|
|Reverse||Arrows not generally reversible.||
All arrows can be reversed.
f • f-1 = Id
|Associatively||Composition of morphisms||Associatively|
A groupoid is a category in which every morphism is invertible. So this means that, when objects are connected by arrows, they are Isomorphic. However, just because the objects are isomorphic, does not mean the structure is not complicated as we saw with the 15-puzzle.
If the category is based on an underlying set then we can think of the arrows as permutations.
In an ordinary group (represented a permutation group) the functors are endofunctors with only a single object.
|In a groupoid the permutations can go between objects. Because the mappings are reversible the objects are isomorphic, we can get back to Id, however we don't have to use the reverse so its possible to use the round trip to permute the elements.|
Its possible to have permutations such that we can only permute the elements if we go round the loop but not if we reverse the path.
Depending on the complexity of the groupoid there may be multiple paths round the loop which produce the same permutation so we have possible equivalences between paths. That is, higher order equivalences.
An ω-groupoid is an ω-category in which all higher morphisms are equivalences.
A ω-category (or ∞-category) is a higher order category.
- all composition operations are strictly associative.
- all composition operations strictly commute with all others (strict exchange laws).
- all identity higher morphisms are strict identities under all compositions.
Fundamental Groupoid - Topology
The fundamental groupoid of a space is a groupoid with:
- Objects - points of a space.
- Morphisms - paths in X, identified up to endpoint-preserving homotopy.
paths from x to y in the space.
Also see fundamental groupoid on page here.
Example 2 - Computational Path
Here we are looking at the relationship between two types of equality: definitionally equality and propositional equality. These are explained on this page.
In Nat consider 'n', '0+n' and 'n+0' they are equal terms in some way.
'n' and '0+n' are definitionally equal because they automatically normalise to the same. Definitionally equality is denoted with a 3-bar equals sign (≡) or just by using the same symbol.
But 'N+0' is propositionally (but not definitionally) equal to N. It is denoted with a normal equals sign (=) and shown as an arrow between nodes.
This can get more complicated when we have types involving containers within containers.
Groupoid in Type Theory
- Used in Homotopy Type Theory (HoTT) see page here.
Computer Model of Groupoid
A model of a groupoid might contain a hierarchy of types:
- base : Type
- path : base -> base
- path2 : path -> path
- path3 : path2 -> path2
So the easiest base type to think about might be, for instance, a finite set. In that case a path would be the possible equalities between the elements of the set. This is a reversible mapping which is a permutation.
What would path2 look like? That is a mapping between permutations.
In the 15-puzzle example there is only one mapping (path) at most, between any two nodes. So the higher path is just a trivial Id mapping, then its trivial mappings all the way up. So that doesn't give us any intuition about higher level paths. Perhaps if we added some ability to switch routes (like a railway switching game?) would that do it?
Implement in Idris
Is it possible to implement a groupoid using Idris?
I was thinking about something like this:
||| Groupoid where base type is a ||| finite set (implemented as a list) record Groupoid (level : Nat) where constructor MkGroupoid source : Nat dest : Nat Path : List (Groupoid (level + 1)) fifteenPuzzle : Groupoid 0 fifteenPuzzle = let c1:Groupoid 2 = MkGroupoid 1 2  c2:Groupoid 2 = MkGroupoid 2 1  swap = [c1,c2] b1_2:Groupoid 1 =MkGroupoid 1 2 swap b1_5:Groupoid 1 =MkGroupoid 1 5 swap b2_3:Groupoid 1 =MkGroupoid 2 3 swap b2_6:Groupoid 1 =MkGroupoid 2 6 swap b3_4:Groupoid 1 =MkGroupoid 3 4 swap b3_7:Groupoid 1 =MkGroupoid 3 7 swap b4_8:Groupoid 1 =MkGroupoid 4 8 swap b5_6:Groupoid 1 =MkGroupoid 5 6 swap b5_9:Groupoid 1 =MkGroupoid 5 9 swap b6_7:Groupoid 1 =MkGroupoid 6 7 swap b6_10:Groupoid 1 =MkGroupoid 6 10 swap b7_8:Groupoid 1 =MkGroupoid 7 8 swap b7_11:Groupoid 1 =MkGroupoid 7 11 swap b8_12:Groupoid 1 =MkGroupoid 8 12 swap b9_10:Groupoid 1 =MkGroupoid 9 10 swap b9_13:Groupoid 1 =MkGroupoid 9 13 swap b10_11:Groupoid 1 =MkGroupoid 10 11 swap b10_14:Groupoid 1 =MkGroupoid 10 14 swap b11_12:Groupoid 1 =MkGroupoid 11 12 swap b11_15:Groupoid 1 =MkGroupoid 11 15 swap b12_16:Groupoid 1 =MkGroupoid 12 16 swap b13_14:Groupoid 1 =MkGroupoid 13 14 swap b14_15:Groupoid 1 =MkGroupoid 14 15 swap b15_16:Groupoid 1 =MkGroupoid 15 16 swap in the (Groupoid 0) (MkGroupoid 0 0 [b1_2,b1_5,b2_3,b2_6,b3_4,b3_7,b4_8, b5_6,b5_9,b6_7,b6_10,b7_8,b7_11,b8_12, b9_10,b9_13,b10_11,b10_14,b11_12,b11_15,b12_16, b13_14,b14_15,b15_16])