diff --git a/Complexity.lean b/Complexity.lean
index f06264477..278e7efce 100644
--- a/Complexity.lean
+++ b/Complexity.lean
@@ -2863,6 +2863,7 @@ public import Complexity.Foundations.NumberTheory.AKS.Introspective
 public import Complexity.Foundations.NumberTheory.AKS.PerfectPower
 public import Complexity.Foundations.NumberTheory.AKS.SmallR
 public import Complexity.Foundations.NumberTheory.Harmonic
+public import Complexity.Foundations.NumberTheory.Padics.AdjoinRootReduction
 public import Complexity.Foundations.NumberTheory.Padics.AdjoinRootValuation
 public import Complexity.Foundations.NumberTheory.Padics.RatValuation
 public import Complexity.Foundations.NumberTheory.PrimeCounting
diff --git a/Complexity/Foundations/NumberTheory/Padics/AdjoinRootReduction.lean b/Complexity/Foundations/NumberTheory/Padics/AdjoinRootReduction.lean
new file mode 100644
index 000000000..dfbb263db
--- /dev/null
+++ b/Complexity/Foundations/NumberTheory/Padics/AdjoinRootReduction.lean
@@ -0,0 +1,210 @@
+/-
+Copyright (c) 2026 Samuel Schlesinger. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Samuel Schlesinger
+-/
+
+module
+-- upstream: candidate (Mathlib, `Mathlib/NumberTheory/Padics/`)
+
+public import Complexity.Foundations.NumberTheory.Padics.AdjoinRootValuation
+public import Complexity.Foundations.NumberTheory.Padics.RatValuation
+public import Complexity.Foundations.RingTheory.GaloisRing
+import Complexity.Foundations.RingTheory.AdjoinRoot
+
+/-!
+# Reduction of `ℤ_(p)[X] / (f)` modulo `p ^ n`
+
+Let `p` be a prime and `f` a monic integer polynomial that is irreducible modulo `p`, and let
+`O = ℤ_(p)[X] / (f)` be the valuation ring of `AdjoinRoot.padicValuation` in `K = ℚ[X] / (f)`.
+
+**`O` is `AdjoinRoot (f.map ℤ_(p))`.** Reducing modulo `f` commutes with the inclusion
+`ℤ_(p) → ℚ` because `f` is monic, so the map `ℤ_(p)[X] / (f) → ℚ[X] / (f)` is injective
+(`AdjoinRoot.integerMap_injective`). By `AdjoinRoot.mem_integer_padicValuation_iff`, its image is
+`O` (`AdjoinRoot.integerEquiv`).
+
+**Reduction modulo `p ^ n`** (`AdjoinRoot.reduction`) is the ring homomorphism
+`O → (ℤ / p ^ n)[X] / (f) = GaloisRing p n f` that reduces each coordinate in the basis
+`1, X, …, X ^ (deg f - 1)` modulo `p ^ n` (`Rat.padicIntegerToZModPow`). The reductions are
+compatible: reducing modulo `p ^ N` and then modulo `p ^ n` is reducing modulo `p ^ n`
+(`AdjoinRoot.castHom_comp_reduction`). On the image of `ℤ[X] / (f)` the reduction reduces the
+integer coefficients (`AdjoinRoot.reduction_toRat`), and on `ℤ_(p)` it is
+`Rat.padicIntegerToZModPow` (`AdjoinRoot.reduction_algebraMap`).
+
+For `p = 2` these maps let a computation in the Galois rings `(ℤ / 2 ^ n)[U] / (f)` follow the
+contraction recursion of [OpenAI2026ETH] over `O`.
+
+## Main definitions
+
+* `AdjoinRoot.integerEquiv`: `ℤ_(p)[X] / (f)` is the valuation ring `O`.
+* `AdjoinRoot.reduction`: reduction `O → GaloisRing p n f`.
+
+## Main results
+
+* `AdjoinRoot.castHom_comp_reduction`: the reductions are compatible.
+* `AdjoinRoot.reduction_toRat`: on `ℤ[X] / (f)` the reduction reduces the integer coefficients.
+* `AdjoinRoot.reduction_algebraMap`: on `ℤ_(p)` it is `Rat.padicIntegerToZModPow`.
+
+## References
+
+* [OpenAI, *The Exponential Time Hypothesis is False*][OpenAI2026ETH]
+-/
+
+public section
+
+open Polynomial
+
+namespace AdjoinRoot
+
+variable {p : ℕ} [Fact p.Prime] {f : ℤ[X]}
+
+variable (p f) in
+/-- `f` over `ℤ_(p)` maps to `f` over `ℚ`. -/
+theorem map_subtype_map_integer :
+    (f.map (Int.castRingHom (Rat.padicValuation p).integer)).map
+      (Rat.padicValuation p).integer.subtype = f.map (Int.castRingHom ℚ) := by
+  rw [Polynomial.map_map]
+  congr 1
+
+variable (p f) in
+/-- The map `ℤ_(p)[X] / (f) → ℚ[X] / (f)`. -/
+noncomputable def integerMap :
+    AdjoinRoot (f.map (Int.castRingHom (Rat.padicValuation p).integer)) →+*
+      AdjoinRoot (f.map (Int.castRingHom ℚ)) :=
+  map (Rat.padicValuation p).integer.subtype _ _ (map_subtype_map_integer p f).symm.dvd
+
+theorem integerMap_mk (P : (Rat.padicValuation p).integer[X]) :
+    integerMap p f (mk _ P) = mk _ (P.map (Rat.padicValuation p).integer.subtype) :=
+  map_mk _ _ _ P
+
+variable (hf : f.Monic) (hirr : Irreducible (f.map (Int.castRingHom (ZMod p))))
+
+include hf in
+/-- **The map `ℤ_(p)[X] / (f) → ℚ[X] / (f)` is injective**, as `f` is monic. -/
+theorem integerMap_injective : Function.Injective (integerMap p f) := by
+  refine (injective_iff_map_eq_zero _).mpr fun x hx => ?_
+  have hg := hf.map (Int.castRingHom (Rat.padicValuation p).integer)
+  rw [integerMap, map_eq_zero_iff _ hg _ (map_subtype_map_integer p f).symm] at hx
+  have h0 : modByMonicHom hg x = 0 := Polynomial.ext fun i => Subtype.ext (by simpa using hx i)
+  rw [← mk_leftInverse hg x, h0, map_zero]
+
+include hf in
+/-- The image of `ℤ_(p)[X] / (f)` lies in the valuation ring. -/
+theorem integerMap_mem (x : AdjoinRoot (f.map (Int.castRingHom (Rat.padicValuation p).integer))) :
+    integerMap p f x ∈ (padicValuation p hf hirr).integer := by
+  obtain ⟨P, rfl⟩ := mk_surjective x
+  rw [integerMap_mk, mem_integer_padicValuation_iff]
+  intro i
+  rw [modByMonicHom_mk, ← map_subtype_map_integer p f,
+    ← map_modByMonic _ (hf.map (Int.castRingHom (Rat.padicValuation p).integer)), coeff_map]
+  exact ((P %ₘ f.map (Int.castRingHom (Rat.padicValuation p).integer)).coeff i).2
+
+/-- Every element of the valuation ring comes from `ℤ_(p)[X] / (f)`. -/
+theorem exists_integerMap_eq {x : AdjoinRoot (f.map (Int.castRingHom ℚ))}
+    (hx : x ∈ (padicValuation p hf hirr).integer) :
+    ∃ y, integerMap p f y = x := by
+  rw [mem_integer_padicValuation_iff] at hx
+  have hl : modByMonicHom (hf.map (Int.castRingHom ℚ)) x ∈
+      lifts (Rat.padicValuation p).integer.subtype :=
+    (lifts_iff_coeff_lifts _).mpr fun i => ⟨⟨_, (Valuation.mem_integer_iff _ _).mpr (hx i)⟩, rfl⟩
+  obtain ⟨Q, hQ⟩ := (mem_lifts _).mp hl
+  refine ⟨mk _ Q, ?_⟩
+  rw [integerMap_mk, hQ]
+  exact mk_leftInverse (hf.map _) x
+
+/-- **`ℤ_(p)[X] / (f)` is the valuation ring** `O` of `AdjoinRoot.padicValuation`. -/
+noncomputable def integerEquiv :
+    AdjoinRoot (f.map (Int.castRingHom (Rat.padicValuation p).integer)) ≃+*
+      (padicValuation p hf hirr).integer :=
+  RingEquiv.ofBijective ((integerMap p f).codRestrict _ (integerMap_mem hf hirr))
+    (show Function.Bijective _ from
+      ⟨fun _ _ h => integerMap_injective hf (congrArg Subtype.val h), fun x => by
+        obtain ⟨y, hy⟩ := exists_integerMap_eq hf hirr x.2
+        exact ⟨y, Subtype.ext hy⟩⟩)
+
+theorem coe_integerEquiv (y : AdjoinRoot (f.map (Int.castRingHom (Rat.padicValuation p).integer))) :
+    (integerEquiv hf hirr y : AdjoinRoot (f.map (Int.castRingHom ℚ))) = integerMap p f y := by
+  rw [integerEquiv, RingEquiv.ofBijective_apply]
+  rfl
+
+variable (n : ℕ)
+
+variable (p f) in
+/-- `f` over `ℤ_(p)` reduces to `f` over `ℤ / p ^ n`. -/
+theorem map_padicIntegerToZModPow_map_integer :
+    (f.map (Int.castRingHom (Rat.padicValuation p).integer)).map (Rat.padicIntegerToZModPow p n) =
+      f.map (Int.castRingHom (ZMod (p ^ n))) := by
+  rw [Polynomial.map_map]
+  congr 1
+  exact RingHom.ext_int _ _
+
+variable (p f) in
+/-- Reduction `ℤ_(p)[X] / (f) → (ℤ / p ^ n)[X] / (f)` of the coefficients. -/
+noncomputable def integerReduction :
+    AdjoinRoot (f.map (Int.castRingHom (Rat.padicValuation p).integer)) →+* GaloisRing p n f :=
+  map (Rat.padicIntegerToZModPow p n) _ _ (map_padicIntegerToZModPow_map_integer p f n).symm.dvd
+
+/-- **Reduction modulo `p ^ n`** of the valuation ring `O = ℤ_(p)[X] / (f)`: reduce each
+coordinate in the basis `1, X, …, X ^ (deg f - 1)` modulo `p ^ n`. -/
+noncomputable def reduction : (padicValuation p hf hirr).integer →+* GaloisRing p n f :=
+  (integerReduction p f n).comp (integerEquiv hf hirr).symm.toRingHom
+
+theorem reduction_integerEquiv
+    (y : AdjoinRoot (f.map (Int.castRingHom (Rat.padicValuation p).integer))) :
+    reduction hf hirr n (integerEquiv hf hirr y) = integerReduction p f n y := by
+  simp [reduction]
+
+variable {n} in
+/-- **The reductions are compatible**: reducing modulo `p ^ N` and then modulo `p ^ n`, `n ≤ N`,
+is reducing modulo `p ^ n`. -/
+theorem castHom_comp_reduction {N : ℕ} (h : n ≤ N) :
+    (map (ZMod.castHom (pow_dvd_pow p h) (ZMod (p ^ n))) _ _
+      (GaloisRing.map_castHom_map f (pow_dvd_pow p h)).symm.dvd).comp (reduction hf hirr N) =
+        reduction hf hirr n := by
+  have key : (map (ZMod.castHom (pow_dvd_pow p h) (ZMod (p ^ n))) _ _
+      (GaloisRing.map_castHom_map f (pow_dvd_pow p h)).symm.dvd).comp (integerReduction p f N) =
+        integerReduction p f n := by
+    refine ringHom_ext (RingHom.ext fun a => ?_) ?_
+    · simp only [integerReduction, RingHom.comp_apply, map_of]
+      congr 1
+      exact RingHom.congr_fun (Rat.castHom_comp_padicIntegerToZModPow p h) a
+    · simp only [integerReduction, RingHom.comp_apply, map_root]
+  ext x
+  obtain ⟨y, rfl⟩ := (integerEquiv hf hirr).surjective x
+  rw [RingHom.comp_apply, reduction_integerEquiv, reduction_integerEquiv, ← key,
+    RingHom.comp_apply]
+
+/-- **On `ℤ[X] / (f)` the reduction reduces the integer coefficients.** -/
+theorem reduction_toRat (o : AdjoinRoot f) :
+    reduction hf hirr n ⟨map (Int.castRingHom ℚ) f _ dvd_rfl o,
+      (Valuation.mem_integer_iff _ _).mpr (padicValuation_map_le_one hf hirr o)⟩ =
+        map (Int.castRingHom (ZMod (p ^ n))) f _ dvd_rfl o := by
+  obtain ⟨P, rfl⟩ := mk_surjective o
+  have hcoe : (integerEquiv hf hirr
+      (mk _ (P.map (Int.castRingHom (Rat.padicValuation p).integer))) :
+      AdjoinRoot (f.map (Int.castRingHom ℚ))) = map (Int.castRingHom ℚ) f _ dvd_rfl (mk f P) := by
+    rw [coe_integerEquiv, integerMap_mk, map_mk, Polynomial.map_map]
+    congr 2
+  have hy : integerEquiv hf hirr (mk _ (P.map (Int.castRingHom (Rat.padicValuation p).integer))) =
+      ⟨map (Int.castRingHom ℚ) f _ dvd_rfl (mk f P),
+        (Valuation.mem_integer_iff _ _).mpr (padicValuation_map_le_one hf hirr _)⟩ :=
+    Subtype.ext hcoe
+  rw [← hy, reduction_integerEquiv, integerReduction, map_mk, map_mk,
+    Polynomial.map_map]
+  congr 2
+  exact RingHom.ext_int _ _
+
+/-- **On `ℤ_(p)` the reduction is `Rat.padicIntegerToZModPow`.** -/
+theorem reduction_algebraMap (q : (Rat.padicValuation p).integer)
+    (hq : algebraMap ℚ (AdjoinRoot (f.map (Int.castRingHom ℚ))) q ∈
+      (padicValuation p hf hirr).integer) :
+    reduction hf hirr n ⟨_, hq⟩ =
+      algebraMap (ZMod (p ^ n)) _ (Rat.padicIntegerToZModPow p n q) := by
+  have hy : integerEquiv hf hirr (of _ q) = ⟨_, hq⟩ := by
+    apply Subtype.ext
+    rw [coe_integerEquiv, integerMap, map_of]
+    rfl
+  rw [← hy, reduction_integerEquiv, integerReduction, map_of]
+  rfl
+
+end AdjoinRoot
diff --git a/Complexity/Foundations/NumberTheory/Padics/RatValuation.lean b/Complexity/Foundations/NumberTheory/Padics/RatValuation.lean
index 3c249ac98..9282e422b 100644
--- a/Complexity/Foundations/NumberTheory/Padics/RatValuation.lean
+++ b/Complexity/Foundations/NumberTheory/Padics/RatValuation.lean
@@ -40,6 +40,7 @@ second reduces its `ℤ_(2)` coefficients modulo the working precision `2 ^ p`.
 * `Rat.padicValuation_pow_le_of_abs_lt`: a nonzero integer below `p ^ (a + 1)` in absolute value
   has valuation at least `v (p ^ a)`.
 * `Rat.padicValuation_add_pow_of_abs_lt`: adding `p ^ (a + 1)` to it keeps its valuation.
+* `Rat.castHom_comp_padicIntegerToZModPow`: the reductions are compatible.
 * `Rat.padicIntegerToZModPow_mul_den`: the reduction of `x` times the denominator of `x` is the
   numerator of `x`.
 
@@ -111,6 +112,14 @@ theorem coe_padicIntegerToPadicInt (x : (Rat.padicValuation p).integer) :
 noncomputable def padicIntegerToZModPow (n : ℕ) : (Rat.padicValuation p).integer →+* ZMod (p ^ n) :=
   (PadicInt.toZModPow n).comp (padicIntegerToPadicInt p)
 
+/-- **The reductions are compatible**: reducing modulo `p ^ n` and then modulo `p ^ m`, `m ≤ n`,
+is reducing modulo `p ^ m`. -/
+theorem castHom_comp_padicIntegerToZModPow {m n : ℕ} (h : m ≤ n) :
+    (ZMod.castHom (pow_dvd_pow p h) (ZMod (p ^ m))).comp (padicIntegerToZModPow p n) =
+      padicIntegerToZModPow p m := by
+  rw [padicIntegerToZModPow, padicIntegerToZModPow, ← RingHom.comp_assoc,
+    PadicInt.zmod_cast_comp_toZModPow m n h]
+
 /-- **The reduction of `a / b` is `a * b⁻¹`**: multiplied by the denominator it gives the
 numerator. -/
 theorem padicIntegerToZModPow_mul_den (n : ℕ) (x : (Rat.padicValuation p).integer) :
diff --git a/ComplexityTests/Foundations/NumberTheory/Padics/AdjoinRootReduction.lean b/ComplexityTests/Foundations/NumberTheory/Padics/AdjoinRootReduction.lean
new file mode 100644
index 000000000..3eef8e814
--- /dev/null
+++ b/ComplexityTests/Foundations/NumberTheory/Padics/AdjoinRootReduction.lean
@@ -0,0 +1,45 @@
+/-
+Copyright (c) 2026 Samuel Schlesinger. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Samuel Schlesinger
+-/
+
+import Complexity.Foundations.NumberTheory.Padics.AdjoinRootReduction
+import ComplexityTests.Foundations.NumberTheory.Padics.AdjoinRootValuation
+
+/-! Tests for the reductions `O → (ℤ / 2 ^ n)[U] / (U ^ 2 + U + 1)` of the valuation ring of
+`K = ℚ[U] / (U ^ 2 + U + 1)`. -/
+
+namespace ComplexityTests.AdjoinRootReduction
+
+open Polynomial ComplexityTests.AdjoinRootValuation
+
+/-- The valuation ring `O` of `K`. -/
+noncomputable abbrev O : Subring K := v.integer
+
+/-- Reduction of `O` modulo `2 ^ n`. -/
+noncomputable abbrev π (n : ℕ) : O →+* GaloisRing 2 n f :=
+  AdjoinRoot.reduction f_monic f_irreducible n
+
+/-- `U ∈ O` reduces to `U`. -/
+example (n : ℕ) : π n ⟨ι (AdjoinRoot.root f), (Valuation.mem_integer_iff _ _).mpr
+    (AdjoinRoot.padicValuation_map_le_one f_monic f_irreducible _)⟩ = AdjoinRoot.root _ := by
+  rw [AdjoinRoot.reduction_toRat, AdjoinRoot.map_root]
+
+/-- `1 + U ∈ O` reduces to `1 + U`. -/
+example (n : ℕ) : π n ⟨ι (1 + AdjoinRoot.root f), (Valuation.mem_integer_iff _ _).mpr
+    (AdjoinRoot.padicValuation_map_le_one f_monic f_irreducible _)⟩ = 1 + AdjoinRoot.root _ := by
+  rw [AdjoinRoot.reduction_toRat, map_add, map_one, AdjoinRoot.map_root]
+
+/-- On `ℤ_(2)`, the reduction is reduction modulo `2 ^ n`. -/
+example (n : ℕ) (q : (Rat.padicValuation 2).integer) (hq : algebraMap ℚ K q ∈ O) :
+    π n ⟨_, hq⟩ = algebraMap (ZMod (2 ^ n)) _ (Rat.padicIntegerToZModPow 2 n q) :=
+  AdjoinRoot.reduction_algebraMap f_monic f_irreducible n q hq
+
+/-- The reductions modulo `8` and then `2` agree with the reduction modulo `2`. -/
+example : (AdjoinRoot.map (ZMod.castHom (pow_dvd_pow 2 (by norm_num : 1 ≤ 3)) (ZMod (2 ^ 1))) _ _
+    (GaloisRing.map_castHom_map f (pow_dvd_pow 2 (by norm_num : 1 ≤ 3))).symm.dvd).comp (π 3) =
+      π 1 :=
+  AdjoinRoot.castHom_comp_reduction f_monic f_irreducible (by norm_num : 1 ≤ 3)
+
+end ComplexityTests.AdjoinRootReduction