Lowest nonnegative solution of the Yukawa Hamiltonian

Two recent papers [J. Math. Phys. 13, 1825 (1972); 14, 1140 (1973)] obtained rough limits 1.68 ≥ b0 ≥ 1.67 for the quantity b0 such that an energy eigenvalue of the Yukawa Hamiltonian [−ℏ2∇2/2m−U0 exp(−μrc/ℏ)/r] must be zero or positive if b≡(m/μ) (2U0/cℏ)≤b0. We point out that (along with other aspects of the Yukawa equation) the quantity b0 has long been known to a much higher precision.