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5-step lesson. Step 1: 'Setup: H = H₀ + λH' where H₀ is solvable (eigenstates |n⁰⟩, eigenvalues Eₙ⁰ known) and H' is a small perturbation (λ<<1). Expand energy and states in powers of λ: Eₙ = Eₙ⁰ + λEₙ⁽¹⁾ + λ²Eₙ⁽²⁾ + ..., |n⟩ = |n⁰⟩ + λ|n⁽¹⁾⟩ + ...' Step 2: 'First-order energy correction (NON-degene

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