commutation_matrix * vec method

src/additional_functions/commutation_matrix.jl

@@ -36,6 +36,13 @@ Base.size(A::CommutationMatrix, dim::Integer) =
 Base.length(A::CommutationMatrix) = A.n²^2
 Base.getindex(A::CommutationMatrix, i::Int, j::Int) = j == A.transpose_inds[i] ? 1 : 0
 
+function Base.:(*)(A::CommutationMatrix, B::AbstractVector)
+    size(A, 2) == size(B, 1) || throw(
+        DimensionMismatch("A has $(size(A, 2)) columns, but B has $(size(B, 1)) elements"),
+    )
+    return B[A.transpose_inds]
+end
+
 function Base.:(*)(A::CommutationMatrix, B::AbstractMatrix)
     size(A, 2) == size(B, 1) || throw(
         DimensionMismatch("A has $(size(A, 2)) columns, but B has $(size(B, 1)) rows"),

test/unit_tests/matrix_helpers.jl

@@ -21,8 +21,10 @@ m = 5
     # lmul!
     D = sprand(n^2, n^2, 0.1)
     E = copy(D)
+    F = Matrix(E)
     lmul!(K, D)
     @test D == K * E
+    @test D == K * F
 end
 
 @testset "Duplication / elimination matrix" begin