From b273aa9460d932bff474ee698402902b1ae1c565 Mon Sep 17 00:00:00 2001
From: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Date: Wed, 23 Sep 2026 08:50:11 -0400
Subject: [PATCH] DDEProblemLibrary: fix sign of k1*A*u2 in RADAR5 Oregonator
 u1 equation

RADAR5-V2.1/OREGONATOR/dr-oregon.f has F(1) = +kM1*A*Y(2) - ..., but
prob_dde_RADAR5_oregonator shared `a = -k1*A*u2 - k2*u1*v` between both
components, flipping the sign of the first term in du[1]. With the wrong
sign u1 decays to zero and the solution stays flat at u2 = 1e-5 instead of
oscillating. Fix the RHS and docstring, add a test against the RADAR5 FCN.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Agent-Harness: Claude Code 2.1.280
Agent-Model: claude-opus-5-5[1m]
Agent-Session: https://claude.ai/code/session_01Vrsu4PESdABpNaxYiTBVfd
Claude-Session: https://claude.ai/code/session_01Vrsu4PESdABpNaxYiTBVfd
---
 lib/DDEProblemLibrary/Project.toml     |  2 +-
 lib/DDEProblemLibrary/src/radar5.jl    |  9 +++++----
 lib/DDEProblemLibrary/test/radar5.jl   | 17 +++++++++++++++++
 lib/DDEProblemLibrary/test/runtests.jl |  1 +
 4 files changed, 24 insertions(+), 5 deletions(-)
 create mode 100644 lib/DDEProblemLibrary/test/radar5.jl

diff --git a/lib/DDEProblemLibrary/Project.toml b/lib/DDEProblemLibrary/Project.toml
index d83632f..c34849c 100644
--- a/lib/DDEProblemLibrary/Project.toml
+++ b/lib/DDEProblemLibrary/Project.toml
@@ -1,6 +1,6 @@
 name = "DDEProblemLibrary"
 uuid = "f42792ee-6ffc-4e2a-ae83-8ee2f22de800"
-version = "0.1.9"
+version = "0.1.10"
 
 [deps]
 DiffEqBase = "2b5f629d-d688-5b77-993f-72d75c75574e"
diff --git a/lib/DDEProblemLibrary/src/radar5.jl b/lib/DDEProblemLibrary/src/radar5.jl
index 985fff6..4c45c84 100644
--- a/lib/DDEProblemLibrary/src/radar5.jl
+++ b/lib/DDEProblemLibrary/src/radar5.jl
@@ -54,7 +54,7 @@ Delay differential equation model from chemical kinetics, given by
 
 ```math
 \\begin{align*}
-  u_1'(t) &= - k_1 A u_2(t) - k_2 u_1(t) u_2(t - τ) + k_3 B u_1(t) - 2 k_4 u_1(t)^2, \\\\
+  u_1'(t) &= k_1 A u_2(t) - k_2 u_1(t) u_2(t - τ) + k_3 B u_1(t) - 2 k_4 u_1(t)^2, \\\\
   u_2'(t) &= - k_1 A u_2(t) - k_2 u_1(t) u_2(t - τ) + f k_3 B u_1(t),
 \\end{align*}
 ```
@@ -84,11 +84,12 @@ const prob_dde_RADAR5_oregonator = let k₁ = 1.34, k₂ = 1.6e9, k₃ = 8_000,
         v = h(p, t - τ; idxs = 2)
 
         # precalculations
-        a = -k₁ * A * u[2] - k₂ * u[1] * v
+        a = k₁ * A * u[2]
+        c = k₂ * u[1] * v
         b = k₃ * B * u[1]
 
-        du[1] = a + b - 2 * k₄ * u[1]^2
-        du[2] = a + f * b
+        du[1] = a - c + b - 2 * k₄ * u[1]^2
+        du[2] = -a - c + f * b
 
         return nothing
     end
diff --git a/lib/DDEProblemLibrary/test/radar5.jl b/lib/DDEProblemLibrary/test/radar5.jl
new file mode 100644
index 0000000..3f8b8e3
--- /dev/null
+++ b/lib/DDEProblemLibrary/test/radar5.jl
@@ -0,0 +1,17 @@
+using DDEProblemLibrary, Test
+
+@testset "RADAR5 Oregonator right-hand side" begin
+    # F(1), F(2) of FCN in RADAR5-V2.1/OREGONATOR/dr-oregon.f
+    k₁, k₂, k₃, k₄, f, A, B = 1.34, 1.6e9, 8.0e3, 4.0e7, 1.0, 6.0e-2, 6.0e-2
+    y2_lag = 3.0e-6
+    h(p, t; idxs = nothing) = idxs == 2 ? y2_lag : [0.0, y2_lag]
+    u = [2.0e-8, 7.0e-6]
+    expected = [
+        k₁ * A * u[2] - k₂ * u[1] * y2_lag + k₃ * B * u[1] - 2 * k₄ * u[1]^2,
+        -k₁ * A * u[2] - k₂ * u[1] * y2_lag + f * k₃ * B * u[1],
+    ]
+    prob = DDEProblemLibrary.prob_dde_RADAR5_oregonator
+    du = zeros(2)
+    prob.f(du, u, h, prob.p, 1.0)
+    @test du ≈ expected rtol = 1.0e-12
+end
diff --git a/lib/DDEProblemLibrary/test/runtests.jl b/lib/DDEProblemLibrary/test/runtests.jl
index d57e012..47dad9e 100644
--- a/lib/DDEProblemLibrary/test/runtests.jl
+++ b/lib/DDEProblemLibrary/test/runtests.jl
@@ -15,6 +15,7 @@ if TEST_GROUP == "Core" || TEST_GROUP == "All"
     @time @testset "Load Tests" begin
         @test DDEProblemLibrary isa Module
     end
+    @time @safetestset "RADAR5" include("radar5.jl")
 end
 
 # Quality assurance: no undefined exports, stale dependencies, etc.