From e767efbd9c9f36b33576eacc864dd334cdc5fbf8 Mon Sep 17 00:00:00 2001
From: Afloat16 <132809511+Afloat16@users.noreply.github.com>
Date: Fri, 2 Oct 2026 02:25:09 -0700
Subject: [PATCH] Fix complex-state correction in ItoMilstein with real noise

---
 diffrax/_solver/milstein.py |   4 +-
 test/test_milstein.py       | 123 ++++++++++++++++++++++++++++++++++++
 2 files changed, 126 insertions(+), 1 deletion(-)
 create mode 100644 test/test_milstein.py

diff --git a/diffrax/_solver/milstein.py b/diffrax/_solver/milstein.py
index 897bc9c1..0791363e 100644
--- a/diffrax/_solver/milstein.py
+++ b/diffrax/_solver/milstein.py
@@ -311,7 +311,9 @@ def _to_treemap(_Δw, _g0):
         def __dot(_v0, _ΔwΔw):
             # _v0 has structure (leaf(y0), leaf(Δw), leaf(Δw))
             # _ΔwΔw has structure (leaf(Δw), leaf(Δw))
-            _out = jnp.tensordot(jnp.conj(_v0), _ΔwΔw, axes=jnp.ndim(_ΔwΔw))
+            # This contracts a directional derivative against the iterated noise
+            # increments; it is not a Hermitian inner product.
+            _out = jnp.tensordot(_v0, _ΔwΔw, axes=jnp.ndim(_ΔwΔw))
             # _out has structure (leaf(y0),)
             return _out
 
diff --git a/test/test_milstein.py b/test/test_milstein.py
new file mode 100644
index 00000000..c9eaa483
--- /dev/null
+++ b/test/test_milstein.py
@@ -0,0 +1,123 @@
+import diffrax
+import jax
+import jax.numpy as jnp
+import lineax as lx
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _standard_dtype_promotion():
+    # These SDEs intentionally combine real Wiener increments with complex states.
+    with jax.numpy_dtype_promotion("standard"):
+        yield
+
+
+@pytest.mark.parametrize("dtype", (jnp.float64, jnp.complex128))
+@pytest.mark.parametrize("use_operator", (False, True))
+def test_ito_milstein_real_noise_realification(dtype, use_operator):
+    # Two independent scalar linear SDEs. Their complex and real-coordinate
+    # representations must give the same Milstein step.
+    y0 = jnp.array([0.7, -1.1], dtype=dtype)
+    drift_coeff = jnp.array([-0.1, 0.4], dtype=dtype)
+    diffusion_coeff = jnp.array([0.3, -0.2], dtype=dtype)
+    if jnp.issubdtype(dtype, jnp.complexfloating):
+        y0 = y0 + 1j * jnp.array([1.2, -0.5])
+        drift_coeff = drift_coeff + 1j * jnp.array([0.2, -0.1])
+        diffusion_coeff = diffusion_coeff + 1j * jnp.array([0.8, 0.5])
+    dt = 0.125
+    dw = jnp.array([0.2, -0.1])
+
+    def diffusion(t, y, args):
+        diagonal = diffusion_coeff * y
+        if use_operator:
+            return lx.DiagonalLinearOperator(diagonal)
+        else:
+            return jnp.diag(diagonal)
+
+    drift = lambda t, y, args: drift_coeff * y
+    control = lambda t0, t1: dw
+    terms = diffrax.MultiTerm(
+        diffrax.ODETerm(drift), diffrax.ControlTerm(diffusion, control)
+    )
+    solver = diffrax.ItoMilstein()
+    actual, _, _, _, result = solver.step(terms, 0.0, dt, y0, None, None, False)
+
+    # Respect the existing conjugation convention for array-valued ControlTerm;
+    # the operator branch is an ordinary, non-conjugating matrix-vector product.
+    if use_operator:
+        g0 = diffusion_coeff * y0
+        directional_derivative = diffusion_coeff**2 * y0
+    else:
+        g0 = jnp.conj(diffusion_coeff * y0)
+        directional_derivative = jnp.abs(diffusion_coeff) ** 2 * y0
+    expected = (
+        y0
+        + drift_coeff * y0 * dt
+        + g0 * dw
+        + 0.5 * directional_derivative * (dw**2 - dt)
+    )
+    assert result == diffrax.RESULTS.successful
+    assert jnp.allclose(actual, expected, rtol=1e-12, atol=1e-12)
+
+    def encode(y):
+        return jnp.concatenate([jnp.real(y), jnp.imag(y)])
+
+    def decode(y):
+        real, imag = jnp.split(y, 2)
+        return real + 1j * imag
+
+    real_drift = lambda t, y, args: encode(drift(t, decode(y), args))
+
+    def real_diffusion(t, y, args):
+        g = diffusion_coeff * decode(y)
+        if not use_operator:
+            g = jnp.conj(g)
+        return jnp.concatenate([jnp.diag(jnp.real(g)), jnp.diag(jnp.imag(g))])
+
+    real_terms = diffrax.MultiTerm(
+        diffrax.ODETerm(real_drift), diffrax.ControlTerm(real_diffusion, control)
+    )
+    real_actual, _, _, _, _ = solver.step(
+        real_terms, 0.0, dt, encode(y0), None, None, False
+    )
+    assert jnp.allclose(encode(actual), real_actual, rtol=1e-12, atol=1e-12)
+
+
+@pytest.mark.parametrize("use_operator", (False, True))
+def test_ito_milstein_complex_parameter_gradient(use_operator):
+    y0 = jnp.array([0.7 + 1.2j])
+    dt = 0.125
+    dw = jnp.array([0.2])
+
+    def solve(rate):
+        def diffusion(t, y, args):
+            diagonal = rate * (0.3 + 0.8j) * y
+            if use_operator:
+                return lx.DiagonalLinearOperator(diagonal)
+            else:
+                return jnp.diag(diagonal)
+
+        terms = diffrax.MultiTerm(
+            diffrax.ODETerm(lambda t, y, args: jnp.zeros_like(y)),
+            diffrax.ControlTerm(
+                diffusion,
+                lambda t0, t1: dw,
+            ),
+        )
+        y1, _, _, _, _ = diffrax.ItoMilstein().step(
+            terms, 0.0, dt, y0, None, None, False
+        )
+        return jnp.sum(jnp.abs(y1) ** 2)
+
+    def reference(rate):
+        b = rate * (0.3 + 0.8j)
+        if use_operator:
+            g0 = b * y0
+            directional_derivative = b**2 * y0
+        else:
+            g0 = jnp.conj(b * y0)
+            directional_derivative = jnp.abs(b) ** 2 * y0
+        y1 = y0 + g0 * dw + 0.5 * directional_derivative * (dw**2 - dt)
+        return jnp.sum(jnp.abs(y1) ** 2)
+
+    assert jnp.allclose(jax.grad(solve)(1.0), jax.grad(reference)(1.0), atol=1e-12)