SimplicialSets.jl

This packages provides types and functions to work with simplicial sets. Various kinds of simplicial sets are supported, including symbolic simplices, products, bar constructions and Kan loop groups. The Eilenberg–Zilber maps and interval cut operations are also implemented.

The package uses LinearCombinations.jl to represent formal linear combinations of simplices. By default, coefficients are of type Int.

The package comes with an extensive documentation.

Examples

Basic operations and symbolic simplices

julia> using LinearCombinations, SimplicialSets

julia> x = SymbolicSimplex(:x, 4)
x[0,1,2,3,4]

julia> dim(x)
4

julia> using SimplicialSets: d, s

julia> d(x, 2)
x[0,1,3,4]

julia> s(x, 2)
x[0,1,2,2,3,4]

julia> isdegenerate(x), isdegenerate(s(x, 2))
(false, true)

julia> using LinearCombinations: diff

julia> diff(x)
x[0,1,2,3]-x[0,2,3,4]+x[1,2,3,4]+x[0,1,3,4]-x[0,1,2,4]

Loop groups

julia> x, y = SymbolicSimplex(:x, 3), SymbolicSimplex(:y, 3)
(x[0,1,2,3], y[0,1,2,3])

julia> u, v = LoopGroupSimplex(x), LoopGroupSimplex(y)
(⟨x[0,1,2,3]⟩, ⟨y[0,1,2,3]⟩)

julia> w = u*v
⟨x[0,1,2,3],y[0,1,2,3]⟩

julia> inv(w)
⟨y[0,1,2,3]⁻¹,x[0,1,2,3]⁻¹⟩

julia> diff(w)
-⟨x[0,1,3],y[0,1,3]⟩+⟨x[0,1,2],y[0,1,2]⟩
+⟨x[1,2,3]⁻¹,x[0,2,3],y[1,2,3]⁻¹,y[0,2,3]⟩

Eilenberg–Zilber maps

julia> x, y = SymbolicSimplex(:x, 2), SymbolicSimplex(:y, 2)
(x[0,1,2], y[0,1,2])

julia> z = ProductSimplex(x, y)
(x[0,1,2],y[0,1,2])

julia> ez(x, y)   # shuffle map
-(x[0,0,1,1,2],y[0,1,1,2,2])+(x[0,1,2,2,2],y[0,0,0,1,2])
+(x[0,0,1,2,2],y[0,1,1,1,2])+(x[0,0,0,1,2],y[0,1,2,2,2])
+(x[0,1,1,1,2],y[0,0,1,2,2])-(x[0,1,1,2,2],y[0,0,1,1,2])

julia> aw(z)   # Alexander–Whitney map
x[0,1]⊗y[1,2]+x[0]⊗y[0,1,2]+x[0,1,2]⊗y[2]

julia> shih(z)    # Eilenberg–MacLane homotopy
-(x[0,1,1,2],y[0,1,2,2])+(x[0,0,1,1],y[0,1,1,2])
-(x[0,0,0,1],y[0,1,2,2])+(x[0,0,1,2],y[0,2,2,2])

Let's check that shih is indeed a homotopy from the identity to ez∘aw:

julia> diff(shih(z)) + shih(diff(z)) == ez(aw(z)) - z
true

Let's verify the "side conditions" for the Eilenberg–Zilber maps:

julia> shih(ez(x, y)), aw(shih(z)), shih(shih(z))
(0, 0, 0)

Let's check that the shuffle map is commutative:

julia> x, y = SymbolicSimplex(:x, 1), SymbolicSimplex(:y, 3)
(x[0,1], y[0,1,2,3])

julia> t = tensor(x, y)
x[0,1]⊗y[0,1,2,3]

julia> ez(t)
-(x[0,0,1,1,1],y[0,1,1,2,3])+(x[0,0,0,1,1],y[0,1,2,2,3])
+(x[0,1,1,1,1],y[0,0,1,2,3])-(x[0,0,0,0,1],y[0,1,2,3,3])

julia> a = swap(ez(t))
-(y[0,1,2,3,3],x[0,0,0,0,1])-(y[0,1,1,2,3],x[0,0,1,1,1])
+(y[0,0,1,2,3],x[0,1,1,1,1])+(y[0,1,2,2,3],x[0,0,0,1,1])

julia> swap(t)
-y[0,1,2,3]⊗x[0,1]

julia> b = ez(swap(t))
-(y[0,1,2,3,3],x[0,0,0,0,1])-(y[0,1,1,2,3],x[0,0,1,1,1])
+(y[0,0,1,2,3],x[0,1,1,1,1])+(y[0,1,2,2,3],x[0,0,0,1,1])

julia> a == b
true

Interval cut operations

julia> sj = Surjection([1,3,2,1])
Surjection{3}([1, 3, 2, 1])

julia> x = SymbolicSimplex(:x, 2)
x[0,1,2]

julia> sj(x)
-x[0,1,2]⊗x[1]⊗x[0,1]-x[0,2]⊗x[1,2]⊗x[0,1]-x[0,1,2]⊗x[0,1]⊗x[0]
-x[0,1,2]⊗x[2]⊗x[1,2]+x[0,2]⊗x[0,1,2]⊗x[0]+x[0,2]⊗x[2]⊗x[0,1,2]
-x[0,1,2]⊗x[1,2]⊗x[1]

julia> diff(sj)
Surjection{3}([2, 3, 1])-Surjection{3}([1, 2, 3])

julia> diff(sj(x)) == diff(sj)(x) + (-1)^deg(sj) * sj(diff(x))
true