define nsymbol

Project.toml

@@ -1,7 +1,7 @@
 name = "SymmetrySectors"
 uuid = "f8a8ad64-adbc-4fce-92f7-ffe2bb36a86e"
 authors = ["ITensor developers <[email protected]> and contributors"]
-version = "0.1.8"
+version = "0.1.9"
 
 [deps]
 BlockArrays = "8e7c35d0-a365-5155-bbbb-fb81a777f24e"

src/abstractsector.jl

@@ -52,6 +52,13 @@ to_gradedrange(c::AbstractSector) = to_gradedrange(labelled(1, c))
 to_gradedrange(l::LabelledInteger) = gradedrange([l])
 to_gradedrange(g::AbstractUnitRange) = g
 
+function nsymbol(s1::AbstractSector, s2::AbstractSector, s3::AbstractSector)
+  full_space = to_gradedrange(s1 ⊗ s2)
+  i = findfirst(==(s3), blocklabels(full_space))
+  isnothing(i) && return 0
+  return unlabel(blocklengths(full_space)[i])
+end
+
 # ===============================  Fusion rule interface  ==================================
 function fusion_rule(c1::AbstractSector, c2::AbstractSector)
   return fusion_rule(combine_styles(SymmetryStyle(c1), SymmetryStyle(c2)), c1, c2)

test/test_fusion_rules.jl

@@ -9,6 +9,7 @@ using SymmetrySectors:
   U1,
   Z,
   block_dimensions,
+  nsymbol,
   quantum_dimension,
   trivial
 using TensorProducts: ⊗, tensor_product
@@ -29,6 +30,7 @@ using TestExtras: @constinferred
     @test (@constinferred q ⊗ q) == q
     @test (@constinferred q ⊗ z0) == z0
     @test (@constinferred z1 ⊗ q) == z1
+    @test nsymbol(q, q, q) == 1
 
     # test different input number
     @test tensor_product(z0) == z0
@@ -46,6 +48,8 @@ using TestExtras: @constinferred
     @test q1 ⊗ q2 == U1(3)
     @test q2 ⊗ q1 == U1(3)
     @test (@constinferred q1 ⊗ q2) == q3
+    @test nsymbol(q1, q2, q3) == 1
+    @test nsymbol(q1, q1, q3) == 0
   end
 
   @testset "O2 fusion rules" begin
@@ -262,6 +266,8 @@ end
     @test space_isequal(
       tensor_product(dual(g5), dual(g6)), gradedrange([s1 => 2, f3 => 1, c3 => 1, ad8 => 1])
     )
+
+    @test nsymbol(ad8, ad8, ad8) == 2
   end
 
   @testset "Mixed GradedUnitRange - Sector fusion rules" begin