diff --git a/LeanEval/Analysis/C1ToAnalytic.lean b/LeanEval/Analysis/C1ToAnalytic.lean
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+import Mathlib
+import EvalTools.Markers
+
+/-!
+# From C¹-manifold to real analytic
+
+If 1 ≤ k ≤ n where k and n are integers, ∞ or ω, every Cᵏ-structure on a (paracompact, Hausdorff)
+manifold can be upgraded to a compatible Cⁿ structure, and uniquely so in the sense that any
+two Cⁿ-structures inducing the given Cᵏ-structure are connected by an isotopy through
+Cᵏ-diffeomorphisms.
+
+This essentially says the classification of Cᵏ-manifolds (k ≥ 1) is the same problem as
+the classification of smooth (or real analytic) manifolds.
+
+Counterexamples abound if the Hausdorff assumption is removed, but it is not clear whether the
+paracompact assumption is necessary. https://en.wikipedia.org/wiki/Long_line_(topology)#Properties
+claims that every smooth structure on the long line extends to infinitely many real analytic
+structures, but this appears to be an overclaim, see https://mathoverflow.net/questions/404692.
+
+## References
+
+* Koji Shiga. Some aspects of real-analytic manifolds and differentiable manifolds.
+  J. Math. Soc. Japan 16(2): 128-142 (April, 1964). DOI: 10.2969/jmsj/01620128
+
+* https://mathoverflow.net/questions/8789/can-every-manifold-be-given-an-analytic-structure/8799
+  mentions work of Morrey, Grauert and Whitney in the real analytic case.
+
+* https://en.wikipedia.org/wiki/Differential_structure#Existence_and_uniqueness_theorems
+-/
+
+variable (k n : WithTop ℕ∞)
+variable (M N V W : Type*) [TopologicalSpace M] [TopologicalSpace N]
+variable [NormedAddCommGroup V] [NormedSpace ℝ V]
+variable [NormedAddCommGroup W] [NormedSpace ℝ W]
+
+open scoped Manifold
+
+/-- A Cᵏ-structure on a manifold can be upgraded to a compatible Cⁿ-structure if `1 ≤ k ≤ n`
+(if `n ≤ k` this is trivial). We use two isomorphic model vector spaces V and W in the statement
+to avoid introducing two `ChartedSpace V M` instances. -/
+@[eval_problem]
+theorem exists_isManifold_of_le (hk : 1 ≤ k) [FiniteDimensional ℝ V] (e : V ≃ₗ[ℝ] W)
+    [ChartedSpace V M] [IsManifold 𝓘(ℝ,V) k M] [T2Space M] [ParacompactSpace M] :
+    ∃ (_ : ChartedSpace W M) (_ : IsManifold 𝓘(ℝ,W) n M)
+      (f : M ≃ₘ^k⟮𝓘(ℝ,V), 𝓘(ℝ,W)⟯ M), f.toHomeomorph = .refl _ := by
+  sorry
+
+/-- If `1 ≤ k ≤ n`, then any Cᵏ-diffeomorphism between two Cⁿ-manifolds is Cᵏ-isotopic to a
+Cⁿ-diffeomorphism via Cᵏ-diffeomorphisms. Again, this is probably trivially true fo `n ≤ k`,
+and the model spaces are automatically isomorphic given the diffeomorphism unless both spaces
+are empty. -/
+@[eval_problem]
+theorem exists_homotopy_of_diffeomorph (hk : 1 ≤ k) (hkn : k ≤ n) [FiniteDimensional ℝ V]
+    [ChartedSpace V M] [IsManifold 𝓘(ℝ,V) n M] [ChartedSpace W N] [IsManifold 𝓘(ℝ,W) n N]
+    [T2Space M] [ParacompactSpace M] (e : V ≃ₗ[ℝ] W) (f : M ≃ₘ^k⟮𝓘(ℝ,V), 𝓘(ℝ,W)⟯ N) :
+    ∃ H : ℝ × M → N, ContMDiff (𝓘(ℝ,ℝ).prod 𝓘(ℝ,V)) 𝓘(ℝ,W) k H ∧
+      (H ⟨0, ·⟩) = f ∧ ∃ g : M ≃ₘ^n⟮𝓘(ℝ,V), 𝓘(ℝ,W)⟯ N, (H ⟨1, ·⟩) = g := by
+  sorry
diff --git a/manifests/problems/c1_analytic.toml b/manifests/problems/c1_analytic.toml
new file mode 100644
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+++ b/manifests/problems/c1_analytic.toml
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+id = "c1_analytic"
+title = "From C¹-manifold to real analytic"
+group = "formalization-evaluation"
+status = "archived"
+visible = true
+statement_revision = 1
+tags = []
+module = "LeanEval.Analysis.C1ToAnalytic"
+holes = ["exists_isManifold_of_le", "exists_homotopy_of_diffeomorph"]
+submitter = "Junyan Xu"
+
+[[status_history]]
+status = "archived"
+effective_date = "2026-09-26"
+reason = "policy"