Disallow exponential!() for sparse matrices

src/ExponentialUtilities.jl

@@ -27,6 +27,7 @@ include("exp.jl")
 include("exp_baseexp.jl")
 include("exp_noalloc.jl")
 include("exp_generic.jl")
+include("exp_sparse.jl")
 include("phi.jl")
 include("arnoldi.jl")
 include("krylov_phiv.jl")

src/exp.jl

@@ -33,6 +33,7 @@ ExpMethodDiagonalization() = ExpMethodDiagonalization(true);
 
 Computes the matrix exponential with the method specified in `method`. The contents of `A` are modified, allowing for fewer allocations. The `method` parameter specifies the implementation and implementation parameters, e.g. [`ExpMethodNative`](@ref), [`ExpMethodDiagonalization`](@ref), [`ExpMethodGeneric`](@ref), [`ExpMethodHigham2005`](@ref). Memory
 needed can be preallocated and provided in the parameter `cache` such that the memory can be recycled when calling `exponential!` several times. The preallocation is done with the command [`alloc_mem`](@ref): `cache=alloc_mem(A,method)`.
+`A` may not be sparse matrix type, since exp(A) is likely to be dense.
 
 Example
 

src/exp_sparse.jl

@@ -0,0 +1,9 @@
+
+for expmeth in [ExpMethodDiagonalization, ExpMethodGeneric, ExpMethodHigham2005, ExpMethodHigham2005Base, ExpMethodNative]    
+    @eval function exponential!(A::AbstractSparseArray, method::$expmeth, cache=nothing)
+        throw(ErrorException("exp(A) on a sparse matrix is generally dense. This operation is "*
+        "not allowed with exponential. If you wished to compute exp(At)*v, see expv. "*
+        "Otherwise to override this error, densify the matrix before calling, "*
+        "i.e. exponential!(Matrix(A))"))
+    end
+end

test/basictests.jl

@@ -99,7 +99,7 @@ end
 if VERSION >= v"1.9"
     @testset "exponential! sparse" begin
         A = sparse([1, 2, 1], [2, 1, 1], [1.0, 2.0, 3.0])
-        exponential!(copy(A), ExpMethodGeneric()) ≈ exp(Array(A))
+        @test_throws ErrorException exponential!(A)
     end
 end