Intersection theory: Schubert calculus

experimental/IntersectionTheory/docs/doc.main

@@ -5,6 +5,7 @@
       "AbstractBundles.md",
       "AbstractVarietyMaps.md",
       "BlowUps.md",
+      "schubert.md",
       "BottFormulas.md",
       "examples.md"
    ],

experimental/IntersectionTheory/docs/src/examples.md

@@ -5,8 +5,20 @@ DocTestSetup = Oscar.doctestsetup()
 
 # Illustrating Examples From Enumerative Geometry
 
+#### How Many Lines in $\mathbb P^3$ Meet Four General Lines in $\mathbb P^3$?
 
-#### How Many Conics in $\mathbb P^3$ Meet Eight General Lines
+```jldoctest
+julia> G = abstract_grassmannian(2,4)
+AbstractVariety of dim 4
+
+julia> s1 = schubert_class(G, 1)
+-c[1]
+
+julia> integral(s1^4)
+2
+
+```
+#### How Many Conics in $\mathbb P^3$ Meet Eight General Lines in $\mathbb P^3$?
 
 ```jldoctest
 julia> G = abstract_grassmannian(3, 4)
@@ -69,7 +81,7 @@ julia> integral((6*HBl-2*e)^5)
 
 ```
 
-#### Number of Twisted Cubics on the General Quintic Hypersurface
+#### How Many Twisted Cubics lie on the General Quintic Hypersurface
 
 ```jldoctest
 julia> G = abstract_grassmannian(3, 5)

experimental/IntersectionTheory/docs/src/schubert.md

@@ -0,0 +1,58 @@
+```@meta
+CurrentModule = Oscar
+```
+
+# Schubert Calculus
+
+To recall the definition of Schubert cycles on a Grassmannian $\mathrm{G}(k,n)$, we think of $\mathrm{G}(k, n)$ as the
+Grassmannian $\mathrm{G}(k, W)$ of $k$-dimensional subspaces of an $n$-dimensional vector space $W$.
+
+A *flag* in $W$ is a strictly increasing sequence of linear subspaces
+
+$\{0\} \subset W_1 \subset \dots \subset W_{n-1} \subset W_n = W, \; \text{ with }\; \dim(W_i) = i.$
+
+Let such a flag  $\mathcal{W}$ be given. For any sequence $a = (a_1, \ldots, a_k)$
+of integers with $n-k \geq a_1 \geq \ldots \geq a_k \geq 0$, we define
+the *Schubert cycle* $\Sigma_a(\mathcal{W})$ by setting
+
+$\Sigma_a(\mathcal{W}):= \{ V \in \mathrm{G}(k, W)\mid \dim(W_{n-k+i-a_i} \cap V) \geq i, i = 1, \ldots, k \} \,.$
+
+Then we have:
+- The Schubert cycle $\Sigma_a(\mathcal{W})$ is an irreducible subvariety of $\mathrm{G}(k, W)$ of codimension $|a| = \sum a_i$.
+- Its cycle class $[\Sigma_a(\mathcal{W})]$ does not depend on the choice of the flag.
+
+We define the *Schubert class* of $a = (a_1, \ldots, a_k)$ to be the cycle class
+
+$\sigma_a := [\Sigma_a(\mathcal{W})] \,.$
+
+The number of Schubert classes on the Grassmannian $\mathrm{G}(k, n)$ is equal to $\binom{n}{k}$.
+
+Instead of $\sigma_a$, we write $\sigma_{a_1,\ldots,a_s}$  whenever
+$a = (a_1, \ldots, a_s, 0, \ldots, 0)$, and $\sigma_{p^i}$ whenever
+$a = (p, \ldots, p, 0, \ldots, 0) \in \mathbb Z^i \times \{0\}^{k-i}$.
+
+We discuss how the cycle classes $\sigma_{1^i}$, $i = 1, \ldots, k$, and  $\sigma_i$, $i = 1, \ldots, n-k$, are related to the Chern classes of the tautological vector bundles on $\mathrm{G}(k, W)$. Recall:
+
+- The *tautological subbundle* on $\mathrm{G}(k, W)$ is the vector bundle of rank $k$ whose fiber at $V \in \mathrm{G}(k, W)$ is the subspace $V \subset W$.
+- The *tautological quotient bundle* on $\mathrm{G}(k, W)$ is the vector bundle of rank $(n-k)$ whose fiber at $V \in \mathrm{G}(k, W)$ is the quotient vector space $W/V$.
+
+We denote these vector bundles by $S$ and $Q$, respectively.
+
+The Chern classes of $S$ and $Q$ are
+
+$c_i(S) = (-1)^i \sigma_{1^i} \; \text{ for } \;  i = 1, \ldots, k$
+
+and
+
+$c_i(Q) = \sigma_i \; \text{ for }\; i = 1, \ldots, n-k,$
+
+respectively.
+
+```@docs
+schubert_class(G::AbstractVariety, λ::Int...)
+```
+
+```@docs
+schubert_classes(G::AbstractVariety, m::Int)
+```
+

experimental/IntersectionTheory/src/IntersectionTheory.jl

@@ -75,8 +75,9 @@ include("Types.jl")
 include("Misc.jl")
 
 include("Bott.jl")   # integration using Bott's formula
-include("Main.jl")   # basic constructions for Schubert calculus
+include("Main.jl")   # basic constructors and functionality
 include("blowup.jl") # blowup
+include("schubert.jl") # Schubert calculus
 # include("Moduli.jl") # moduli of matrices, twisted cubics
 # include("Weyl.jl")   # weyl groups
 

experimental/IntersectionTheory/src/Main.jl

@@ -1971,7 +1971,7 @@ end
 @doc raw"""
     abstract_grassmannian(k::Int, n::Int; base::Ring = QQ, symbol::String = "c")
 
-Return the abstract Grassmannian $\mathrm{Gr}(k, n)$ of `k`-dimensional subspaces of an 
+Return the abstract Grassmannian $\mathrm{G}(k, n)$ of `k`-dimensional subspaces of an 
 `n`-dimensional vector space.
 
 # Examples
@@ -1986,9 +1986,23 @@ Quotient
     c[2] -> [2]
   by ideal (-c[1]^3 + 2*c[1]*c[2], c[1]^4 - 3*c[1]^2*c[2] + c[2]^2)
 
+julia> S = tautological_bundles(G)[1]
+AbstractBundle of rank 2 on AbstractVariety of dim 4
+
+julia> V = [chern_class(S, i) for i = 1:2]
+2-element Vector{MPolyQuoRingElem{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}}:
+ c[1]
+ c[2]
+
 julia> is_regular_sequence(gens(modulus(CR)))
 true
 
+julia> Q = tautological_bundles(G)[2]
+AbstractBundle of rank 2 on AbstractVariety of dim 4
+
+julia> tangent_bundle(G) == dual(S)*Q
+true
+
 ```
 """
 function abstract_grassmannian(k::Int, n::Int; base::Ring = QQ, symbol::String = "c")
@@ -2164,34 +2178,3 @@ function abstract_flag_variety(F::AbstractBundle, dims::Vector{Int}; symbol::Str
   if l == 2 set_attribute!(Fl, :grassmannian => :relative) end
   return Fl
 end
-
-@doc raw"""
-    schubert_class(G::AbstractVariety, λ::Int...)
-    schubert_class(G::AbstractVariety, λ::Vector{Int})
-    schubert_class(G::AbstractVariety, λ::Partition)
-
-Return the Schubert class $\sigma_\lambda$ on a (relative) Grassmannian $G$.
-"""
-function schubert_class(G::AbstractVariety, λ::Int...) schubert_class(G, collect(λ)) end
-function schubert_class(G::AbstractVariety, λ::Partition) schubert_class(G, Vector(λ)) end
-function schubert_class(G::AbstractVariety, λ::Vector{Int})
-  get_attribute(G, :grassmannian) === nothing && error("the abstract_variety is not a Grassmannian")
-  (length(λ) > rank(G.bundles[1]) || sort(λ, rev=true) != λ) && error("the Schubert input is not well-formed")
-  giambelli(G.bundles[2], λ)
-end
-
-@doc raw"""
-    schubert_classes(m::Int, G::AbstractVariety)
-
-Return all the Schubert classes in codimension $m$ on a (relative) Grassmannian $G$.
-"""
-function schubert_classes(G::AbstractVariety, m::Int)
-  get_attribute(G, :grassmannian) === nothing && error("the abstract_variety is not a Grassmannian")
-  S, Q = G.bundles
-  res = elem_type(G.ring)[]
-  for i in 0:rank(S)
-    append!(res, [schubert_class(G, l) for l in partitions(m, i, 1, rank(Q))])
-  end
-  return res
-end
-

experimental/IntersectionTheory/src/schubert.jl

@@ -0,0 +1,97 @@
+@doc raw"""
+    schubert_class(G::AbstractVariety, λ::Int...)
+    schubert_class(G::AbstractVariety, λ::Vector{Int})
+    schubert_class(G::AbstractVariety, λ::Partition)
+
+Return the Schubert class $\sigma_\lambda$ on a (relative) Grassmannian `G`.
+
+# Examples
+
+```jldoctest
+julia> G = abstract_grassmannian(2,4)
+AbstractVariety of dim 4
+
+julia> s0 = schubert_class(G, 0)
+1
+
+julia> s1 = schubert_class(G, 1)
+-c[1]
+
+julia> s2 = schubert_class(G, 2)
+c[1]^2 - c[2]
+
+julia> s11 = schubert_class(G, [1, 1])
+c[2]
+
+julia> s21 = schubert_class(G, [2, 1])
+-c[1]*c[2]
+
+julia> s22 = schubert_class(G, [2, 2])
+c[2]^2
+
+julia> s1*s1 == s2+s11
+true
+
+julia> s1*s2 == s1*s11 == s21
+true
+
+julia> s1*s21 == s2*s2 == s22
+true
+
+```
+
+```jldoctest
+julia> G = abstract_grassmannian(2,5)
+AbstractVariety of dim 6
+
+julia> s3 = schubert_class(G, 5-2)
+-c[1]^3 + 2*c[1]*c[2]
+
+julia> s3^2 == point_class(G)
+true
+
+julia> Q = tautological_bundles(G)[2]
+AbstractBundle of rank 3 on AbstractVariety of dim 6
+
+julia> chern_class(Q, 3)
+-c[1]^3 + 2*c[1]*c[2]
+
+```
+
+```jldoctest
+julia> G = abstract_grassmannian(2,4)
+AbstractVariety of dim 4
+
+julia> s1 = schubert_class(G, 1)
+-c[1]
+
+julia> s1 == schubert_class(G, [1, 0])
+true
+
+julia> integral(s1^4)
+2
+
+```
+"""
+function schubert_class(G::AbstractVariety, λ::Int...) schubert_class(G, collect(λ)) end
+function schubert_class(G::AbstractVariety, λ::Partition) schubert_class(G, Vector(λ)) end
+function schubert_class(G::AbstractVariety, λ::Vector{Int})
+  get_attribute(G, :grassmannian) === nothing && error("the abstract_variety is not a Grassmannian")
+  (length(λ) > rank(G.bundles[1]) || sort(λ, rev=true) != λ) && error("the Schubert input is not well-formed")
+  giambelli(G.bundles[2], λ)
+end
+
+@doc raw"""
+    schubert_classes(G::AbstractVariety, m::Int)
+
+Return all the Schubert classes in codimension `m` on a (relative) Grassmannian `G`.
+"""
+function schubert_classes(G::AbstractVariety, m::Int)
+  get_attribute(G, :grassmannian) === nothing && error("the abstract_variety is not a Grassmannian")
+  S, Q = G.bundles
+  res = elem_type(G.ring)[]
+  for i in 0:rank(S)
+    append!(res, [schubert_class(G, l) for l in partitions(m, i, 1, rank(Q))])
+  end
+  return res
+end