The WKB Approximation
Although this perturbation technique (due to Wentzel, Kramers, and Brillouin) has practical value essentially only for 1-D problems (or for problems separable into one-dimensionalized problems), it is of considerable interest through its connection with classical physics and the “old” (pre-1925) quantum theory. It is not only of historical theoretical importance, however. It can be very useful for problems involving quantum-mechanical tunneling.
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