Schiff loves symmetry arguments. A long, grueling problem about a particle in a 2D anisotropic harmonic oscillator will have a solution that says: “By rotational invariance in the limit of equal frequencies, the degeneracy is lifted. The perturbed energies are…” And then it just gives the final eigenvalues. No perturbation integrals. No sum over intermediate states. Just the result, floating in the white space like a Zen koan.
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Once you have studied a solution, close the file and re-derive it from scratch on a blank sheet of paper. If you cannot reproduce the solution without glancing back, you have not learned it. The goal is to internalize the method , not memorize the answer . No perturbation integrals