Correct lyap definition for complex scalar arguments (#39901)

JuliaLang · Mar 16, 2021 · da96fef · da96fef
1 parent ae26fc6
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diff --git a/stdlib/LinearAlgebra/src/dense.jl b/stdlib/LinearAlgebra/src/dense.jl
@@ -1568,4 +1568,4 @@ function lyap(A::StridedMatrix{T}, C::StridedMatrix{T}) where {T<:BlasFloat}
  rmul!(Q*(Y * adjoint(Q)), inv(scale))
 end
 lyap(A::StridedMatrix{T}, C::StridedMatrix{T}) where {T<:Integer} = lyap(float(A), float(C))
-lyap(a::T, c::T) where {T<:Number} = -c/(2a)
+lyap(a::Union{Real,Complex}, c::Union{Real,Complex}) = -c/(2real(a))
diff --git a/stdlib/LinearAlgebra/test/dense.jl b/stdlib/LinearAlgebra/test/dense.jl
@@ -767,6 +767,10 @@ end
  @test diag(zeros(0,1),2) == []
 end
 
+@testset "issue #39857" begin
+ @test lyap(1.0+2.0im, 3.0+4.0im) == -1.5 - 2.0im
+end
+
 @testset "Matrix to real power" for elty in (Float64, ComplexF64)
 # Tests proposed at Higham, Deadman: Testing Matrix Function Algorithms Using Identities, March 2014
  #Aa : only positive real eigenvalues