comment fixes

Author: Harsh Singh

lib/OrdinaryDiffEqRosenbrock/src/rosenbrock_perform_step.jl

@@ -130,12 +130,12 @@ end
                 integrator.EEst = max(EEst, integrator.EEst)
             end
         else
-            # No H matrix: compute Hermite interpolation coefficients
-            # k[1] = dt*f₀ - (u - uprev), k[2] = 2*(u - uprev) - dt*(f₀ + f₁)
+            # No H matrix: store raw derivatives f₀, f₁ for standard Hermite interpolation.
+            # _ode_interpolant(interp_order=-1) expects k[1]=f₀, k[2]=f₁.
             f1 = f(u, p, t + dt)
             OrdinaryDiffEqCore.increment_nf!(integrator.stats, 1)
-            integrator.k[1] = @.. dt * fsalfirst_cache - (u - uprev)
-            integrator.k[2] = @.. 2 * (u - uprev) - dt * (fsalfirst_cache + f1)
+            integrator.k[1] = fsalfirst_cache  # f₀ = f(uprev, p, t)
+            integrator.k[2] = f1               # f₁ = f(u, p, t+dt)
         end
     end
 
@@ -294,12 +294,12 @@ end
                 integrator.EEst = max(EEst, integrator.EEst)
             end
         else
-            # No H matrix: compute Hermite interpolation coefficients
-            # k[1] = dt*f₀ - (u - uprev), k[2] = 2*(u - uprev) - dt*(f₀ + f₁)
+            # No H matrix: store raw derivatives f₀, f₁ for standard Hermite interpolation.
+            # _ode_interpolant(interp_order=-1) expects k[1]=f₀, k[2]=f₁.
             f(du, u, p, t + dt)
             OrdinaryDiffEqCore.increment_nf!(integrator.stats, 1)
-            @.. integrator.k[1] = dt * cache.fsalfirst - (u - uprev)
-            @.. integrator.k[2] = 2 * (u - uprev) - dt * (cache.fsalfirst + du)
+            @.. integrator.k[1] = cache.fsalfirst  # f₀ = f(uprev, p, t)
+            @.. integrator.k[2] = du               # f₁ = f(u, p, t+dt)
         end
     end
     cache.linsolve = linres.cache