We study bound-state solutions of the Klein-Gordon equationfor bounded vector potentials which in one spatial dimension have the form V (x) = v f (x), where f (x) ≤ 0 is the shape of a finite symmetric central potential that is monotone non-decreasing on [0, ∞) and vanishes as x → ∞. Two principal results are reported. First, it is shown that the eigenvalue problem in the coupling parameter v leads to spectral functions of the form v = G(E) which are concave, and at most uni-modal with a maximum near the lower limit E = −m of the eigenenergy E ∈ (−m, m). This formulation of the spectral problem immediately extends to central potentials in d > 1 spatial dimensions. Secondly, for each of the dimension cases, d = 1 and d ≥ 2, a comparison theorem is proven, to the effect that if two potential shapes are ordered f1(r) ≤ f2(r), then so are the corresponding pairs of spectral functions G1(E) ≤ G2(E) for each of the existing eigenvalues. These results remove the restriction to positive eigenvalues necessitated by earlier comparison theorems for the Klein-Gordon equation.Hall and Harb at a function v(E), whereas E(v) is a two-valued expression. In section 2, we establish the principal features of the spectral curves v(E) for the class of negative bounded potentials that vanish at infinity. In section 3 we solve the Klein-Gordon equation analytically for the square-well potential in d ≥ 1 dimensions. In section 4 we prove some comparison theorems: the principal results claim that for any discrete eigenvalue E ∈ (−m, m) and negative potential-shape functions f 1 (r) and f 2 (r) we haveIn section 5 we exhibit a complete recipe for spectral bounds for this class of potentials based on comparisons with the exactly soluble square-well problem.II. GENERAL FEATURES OF THE SPECTRAL CURVE G(E) = v(E).
A. One-dimensional caseThe Klein-Gordon equation in one dimension is given by: