diff --git a/LeanEval/Geometry/MostowRigidity.lean b/LeanEval/Geometry/MostowRigidity.lean
new file mode 100644
index 0000000..b3166a4
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+++ b/LeanEval/Geometry/MostowRigidity.lean
@@ -0,0 +1,130 @@
+import Mathlib.Algebra.Star.Unitary
+import Mathlib.Data.Real.Star
+import Mathlib.LinearAlgebra.Matrix.ConjTranspose
+import Mathlib.MeasureTheory.Group.FundamentalDomain
+import Mathlib.MeasureTheory.Measure.Haar.Basic
+import Mathlib.Topology.Instances.Matrix
+import Mathlib.Topology.MetricSpace.Pseudo.Defs
+import EvalTools.Markers
+
+/-!
+# Mostow rigidity
+
+A common form of Mostow's rigidity theorem says that two complete, finite-volume hyperbolic
+manifolds of dimension at least three must be isometric if they have isomorphic fundamental
+groups. Since the group of isometries of the hyperbolic space ℍⁿ is isomorphic to the
+projective orthogonal group PO(n,1), this can be shown to be equivalent to the following
+algebraic statement: two lattices (subgroups of finite covolume) in PO(n,1) with n ≥ 3 are
+isomorphic iff they are conjugate in PO(n,1). In this file we define the group PO(n,1) and
+state this algebraic form of Mostow rigidity. In fact, the isomorphism should agree with
+conju
+
+References:
+https://en.wikipedia.org/wiki/Mostow_rigidity_theorem#Algebraic_form
+Riccardo Benedetti, Carlo Petronio. *Lectures on Hyperbolic Geometry*, Chapter C
+(proof of geometric form in compact case in 50 pages).
+-/
+
+namespace LeanEval.Geometry
+
+universe u
+
+variable (p q : Type u) (α : Type*)
+
+def MatrixSum : Type _ := Matrix (p ⊕ q) (p ⊕ q) α
+
+variable [Fintype p] [Fintype q] [DecidableEq p] [DecidableEq q] [Ring α]
+
+instance : Ring (MatrixSum p q α) := inferInstanceAs (Ring (Matrix ..))
+
+variable {p q α} in
+def MatrixSum.ofMatrix : Matrix (p ⊕ q) (p ⊕ q) α ≃+* MatrixSum p q α := .refl _
+
+variable {p q α} in
+def pmOneMat : Matrix (p ⊕ q) (p ⊕ q) α := .fromBlocks 1 0 0 (-1)
+
+theorem pmOneMat_mul_pmOneMat : pmOneMat * pmOneMat = (1 : Matrix (p ⊕ q) _ α) := by
+  simp [pmOneMat, Matrix.fromBlocks_multiply]
+
+open MatrixSum Matrix
+
+section Star
+
+variable [StarRing α]
+
+omit [Fintype p] [Fintype q] in
+@[simp] theorem conjTranspose_pmOneMat : (pmOneMat : Matrix (p ⊕ q) _ α)ᴴ = pmOneMat := by
+  simp_rw [pmOneMat, conjTranspose, fromBlocks_transpose, fromBlocks_map]
+  simp [Matrix.map_neg]
+
+instance : StarRing (MatrixSum p q α) where
+  star A := .ofMatrix (pmOneMat * (ofMatrix.symm A).conjTranspose * pmOneMat)
+  star_involutive A := by
+    simp [← mul_assoc pmOneMat, mul_assoc (ofMatrix.symm A), pmOneMat_mul_pmOneMat]
+  star_mul A B := by
+    conv_lhs => rw [map_mul _ A, conjTranspose_mul, ← mul_one _ᴴ, ← pmOneMat_mul_pmOneMat]
+    simp [mul_assoc]
+  star_add A B := by simp [add_mul, mul_add]
+
+end Star
+
+section Topology
+
+variable [TopologicalSpace α]
+
+instance : TopologicalSpace (MatrixSum p q α) := inferInstanceAs (TopologicalSpace (Matrix ..))
+
+instance [T1Space α] : T1Space (MatrixSum p q α) := inferInstanceAs (T1Space (_ → _))
+
+instance [LocallyCompactSpace α] : LocallyCompactSpace (MatrixSum p q α) :=
+  inferInstanceAs (LocallyCompactSpace (_ → _))
+
+@[fun_prop] theorem MatrixSum.continuous_ofMatrix : Continuous (@ofMatrix p q α _ _ _ _ _) :=
+  continuous_id
+
+@[fun_prop] theorem MatrixSum.continuous_ofMatrix_symm : Continuous (@ofMatrix p q α ..).symm :=
+  continuous_id
+
+variable [IsTopologicalRing α]
+
+instance [Star α] [ContinuousStar α] : IsTopologicalRing (MatrixSum p q α) :=
+  inferInstanceAs (IsTopologicalRing (Matrix ..))
+
+variable [StarRing α] [ContinuousStar α]
+
+instance : ContinuousStar (MatrixSum p q α) where
+  continuous_star := by rw [star]; fun_prop
+
+theorem MatrixSum.isClosed_unitary [T1Space α] :
+    IsClosed (SetLike.coe <| unitary (MatrixSum p q α)) :=
+  .inter (isClosed_singleton.preimage <| by fun_prop) (isClosed_singleton.preimage <| by fun_prop)
+
+instance [T1Space α] [LocallyCompactSpace α] :
+    LocallyCompactSpace (unitary (MatrixSum p q α)) :=
+  (MatrixSum.isClosed_unitary ..).isClosedEmbedding_subtypeVal.locallyCompactSpace
+
+instance : IsTopologicalGroup (unitary (MatrixSum p q α)) where
+  continuous_inv := by simp_rw [Inv.inv, star]; fun_prop
+
+end Topology
+
+variable (p q : ℕ)
+
+/-- `PO p q` is the projective indefinite orthogonal group PO(p,q) over the reals. -/
+abbrev PO : Type _ := unitary (MatrixSum (Fin p) (Fin q) ℝ) ⧸ Subgroup.center _
+
+instance : MeasurableSpace (PO p q) := borel _
+instance : BorelSpace (PO p q) := ⟨rfl⟩
+
+open MeasureTheory
+
+noncomputable instance : MeasureSpace (PO p q) := ⟨.haar⟩
+
+@[eval_problem]
+theorem mostow_rigidity (n : ℕ) (hn : 3 ≤ n) (Γ Λ : Subgroup (PO n 1))
+    (disc_Γ : IsDiscrete (SetLike.coe Γ)) (disc_Λ : IsDiscrete (SetLike.coe Λ))
+    (covol_Γ : covolume Γ (PO n 1) ≠ ⊤) (covol_Λ : covolume Λ (PO n 1) ≠ ⊤)
+    (f : Γ ≃* Λ) : ∃ g : PO n 1, Γ.map (MulAut.conj g) = Λ := by
+  sorry
+
+end LeanEval.Geometry
diff --git a/manifests/problems.toml b/manifests/problems.toml
index 0bc4664..7fa2fc5 100644
--- a/manifests/problems.toml
+++ b/manifests/problems.toml
@@ -645,3 +645,12 @@ module = "LeanEval.NumberTheory.NeukirchUchida"
 holes = ["neukirch_uchida"]
 submitter = "Junyan Xu"
 source = "Jürgen Neukirch, Alexander Schmidt, Kay Wingberg. *Cohomology of Number Fields*, Theorem 12.2.1."
+
+[[problem]]
+id = "mostow_rigidity"
+title = "Mostow rigidity"
+test = false
+module = "LeanEval.Geometry.MostowRigidity"
+holes = ["mostow_rigidity"]
+submitter = "Junyan Xu"
+source = "https://en.wikipedia.org/wiki/Mostow_rigidity_theorem#Algebraic_form"
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