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Fix SplitODEProblem regression for non-IMEX ESDIRK methods (Kvaerno3/4/5) #3653

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Merged
ChrisRackauckas merged 4 commits into from
May 19, 2026
+30 −6
Merged

Fix SplitODEProblem regression for non-IMEX ESDIRK methods (Kvaerno3/4/5) #3653

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9c88eb5
add issplit(alg) conditions
JoshuaLampert May 19, 2026
bab7ead
add issplit(alg) conditions
JoshuaLampert May 19, 2026
dc27125
add test
JoshuaLampert May 19, 2026
fed9831
Merge branch 'master' into fix-sdirk
JoshuaLampert May 19, 2026
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12 changes: 6 additions & 6 deletions lib/OrdinaryDiffEqSDIRK/src/generic_imex_perform_step.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -445,13 +445,13 @@ end
return quote
$setup
if tab.explicit_first_stage
if integrator.f isa SplitFunction && tab.fsal && !repeat_step && !integrator.last_stepfail
if integrator.f isa SplitFunction && issplit(alg) && tab.fsal && !repeat_step && !integrator.last_stepfail
f_impl(zs[1], integrator.uprev, p, integrator.t)
zs[1] .*= dt
else
@.. broadcast = false zs[1] = dt * integrator.fsalfirst
end
if integrator.f isa SplitFunction
if integrator.f isa SplitFunction && issplit(alg)
@.. broadcast = false ks[1] = dt * integrator.fsalfirst - zs[1]
end
$stages_efs_true
Expand Down Expand Up @@ -482,7 +482,7 @@ end

$adaptive

if integrator.f isa SplitFunction
if integrator.f isa SplitFunction && issplit(alg)
integrator.f(integrator.fsallast, u, p, t + dt)
elseif tab.explicit_fsallast
integrator.f(integrator.fsallast, u, p, t + tab.fsallast_c * dt)
Expand Down Expand Up @@ -690,12 +690,12 @@ end
return quote
$setup
if tab.explicit_first_stage
if integrator.f isa SplitFunction
if integrator.f isa SplitFunction && issplit(alg)
$z1 = dt * f_impl(uprev, p, t)
else
$z1 = dt * integrator.fsalfirst
end
if integrator.f isa SplitFunction
if integrator.f isa SplitFunction && issplit(alg)
$k1 = dt * integrator.fsalfirst - $z1
end
$stages_efs_true
Expand Down Expand Up @@ -726,7 +726,7 @@ end

$adaptive

if integrator.f isa SplitFunction
if integrator.f isa SplitFunction && issplit(alg)
integrator.k[1] = integrator.fsalfirst
integrator.fsallast = integrator.f(u, p, t + dt)
integrator.k[2] = integrator.fsallast
Expand Down
24 changes: 24 additions & 0 deletions lib/OrdinaryDiffEqSDIRK/test/sdirk_convergence_tests.jl
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Expand Up @@ -183,3 +183,27 @@ end
sim_iip = test_convergence(dts, prob_iip, ARS343())
@test sim_iip.𝒪est[:l∞] ≈ 3 atol = testTol
end

# Regression test: Kvaerno3/4/5 with SplitODEProblem must integrate the full RHS (f1+f2),
# not just f1. These are non-IMEX (issplit=false) methods, so f.f2 must flow through
# fsalfirst rather than being split off into the explicit ks arrays (which have Ae=be=0).
# f = f1 + f2 = -u + 2u = u => exact solution u(t) = exp(t) * u0
@testset "Kvaerno SplitODEProblem" begin
dts = 1 .// 2 .^ (8:-1:4)

f1_oop = (u, p, t) -> -u
f2_oop = (u, p, t) -> 2u
ff_oop = SplitFunction(f1_oop, f2_oop; analytic = (u0, p, t) -> exp(t) * u0)
prob_oop = SplitODEProblem(ff_oop, 1.0, (0.0, 1.0))

f1_iip! = (du, u, p, t) -> (du .= -u)
f2_iip! = (du, u, p, t) -> (du .= 2u)
ff_iip = SplitFunction(f1_iip!, f2_iip!; analytic = (u0, p, t) -> exp(t) .* u0)
prob_iip = SplitODEProblem(ff_iip, [1.0, 0.5], (0.0, 1.0))

sim_oop = test_convergence(dts, prob_oop, Kvaerno4())
@test sim_oop.𝒪est[:l∞] ≈ 4 atol = testTol

sim_iip = test_convergence(dts, prob_iip, Kvaerno4())
@test sim_iip.𝒪est[:l∞] ≈ 4 atol = testTol
end
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