diff --git a/test/test_continuouspolynomial.jl b/test/test_continuouspolynomial.jl
index 600c70b..b889eca 100644
--- a/test/test_continuouspolynomial.jl
+++ b/test/test_continuouspolynomial.jl
@@ -222,5 +222,7 @@ using ContinuumArrays: plan_grid_transform
 
         f = expand(C,exp)
         @test diff(diff(f))[0.1] ≈ exp(0.1)
+
+        @test diff(C)[0.1,1:5] ≈ [0,-1.5,1.5,0,0]
     end
 end
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diff --git a/test/test_dirichlet.jl b/test/test_dirichlet.jl
index 00b5f5d..ec9d1cb 100644
--- a/test/test_dirichlet.jl
+++ b/test/test_dirichlet.jl
@@ -79,4 +79,14 @@ using PiecewiseOrthogonalPolynomials: ArrowheadMatrix, plan_grid_transform
         @test sprint(show, C) == "ContinuousPolynomial{1}(-1:1)"
         @test sprint(show, P) == "ContinuousPolynomial{0}(-1:1)"
     end
+
+    @testset "diff" begin
+        Q = DirichletPolynomial(range(-1,1; length=4))
+        @test diff(Q)[0.1,1:5] ≈ [-1,0,1.6,0,-0.92]
+        u = expand(Q, x -> (1-x^2)*exp(x))
+        let x = 0.1
+            @test diff(u)[x] ≈ (1-2x-x^2)*exp(x)
+            @test diff(diff(u))[x] ≈ (-1-4x-x^2)*exp(x)
+        end
+    end
 end
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