compact-spectral

Lean 4 proof of the spectral theorem for compact self-adjoint operators: a compact self-adjoint operator on a separable Hilbert space admits a Hilbert basis of eigenvectors.

Build status: Verified on Lean v4.29.1 with Mathlib. Zero sorries.

-- Eigenspaces are dense: their supremum has trivial orthogonal complement
theorem CompactSelfAdjoint.iSup_eigenspace_orthogonal_eq_bot_of_isCompactOperator_of_isSelfAdjoint
    {𝕜 : Type*} [RCLike 𝕜] {E : Type*}
    [NormedAddCommGroup E] [InnerProductSpace 𝕜 E] [CompleteSpace E]
    (T : E →L[𝕜] E) (hT : IsSelfAdjoint T) (hTc : IsCompactOperator (T : E → E)) :
    (⨆ μ : 𝕜, (T : Module.End 𝕜 E).eigenspace μ)ᗮ = ⊥

-- A compact self-adjoint operator admits a Hilbert basis of eigenvectors
theorem CompactSelfAdjoint.exists_hilbertBasis_hasEigenvector_of_isCompactOperator_of_isSelfAdjoint
    {𝕜 : Type*} [RCLike 𝕜] {E : Type*}
    [NormedAddCommGroup E] [InnerProductSpace 𝕜 E] [CompleteSpace E]
    (T : E →L[𝕜] E) (hT : IsSelfAdjoint T) (hTc : IsCompactOperator (T : E → E)) :
    ∃ (ι : Type _) (μ : ι → 𝕜) (b : HilbertBasis ι 𝕜 E),
      ∀ i, (T : Module.End 𝕜 E).HasEigenvector (μ i) (b i)

• Topology/WeakHilbertCompact — weak compactness lemmas for Hilbert spaces
• Analysis/InnerProductSpace/RayleighCompact — Rayleigh quotient bounds for compact operators
• Analysis/InnerProductSpace/CompactOperatorOrthonormal — orthonormal sequences from compact operator images
• Analysis/InnerProductSpace/CompactSelfAdjoint — core definitions and large-eigenspace construction
• Analysis/InnerProductSpace/CompactSelfAdjoint/Basic — fundamental lemmas
• Analysis/InnerProductSpace/CompactSelfAdjoint/Approximation — finite-rank approximation and key estimate
• Analysis/InnerProductSpace/CompactSelfAdjoint/OpNormEigenvalue — eigenvalue operator norm bounds
• Analysis/InnerProductSpace/CompactSelfAdjoint/SpectralFiniteness — finiteness of point spectrum outside any neighbourhood of zero
• Analysis/InnerProductSpace/CompactSelfAdjoint/CutoffProjector — orthogonal projection onto large-eigenvalue subspace
• Analysis/InnerProductSpace/CompactSelfAdjoint/SpectralTheorem — main results

Lean 4 + Mathlib. No external dependencies.

For more detail on the internal organization of the development, see docs/overview.md.

The development separates reusable weak-compactness, Rayleigh quotient, compact-operator, and compact self-adjoint spectral lemmas from the final Hilbert-basis theorem.

• Reed & Simon, Methods of Modern Mathematical Physics I: Functional Analysis, Academic Press, 1980
• Brezis, Functional Analysis, Sobolev Spaces and Partial Differential Equations, Springer, 2011