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The authors study in the JWKB approximation the energy levels of the symmetric anharmonic oscillators V(x)=Ax 2 +Bx 4 for different signs and values of A and B. Comparisons are made with published results for specific cases and with numerical calculations. They given an additional example of exact value, to add to the very rare catalogue of known examples.
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Sánchez et al. (1986) studied this question.
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