Upper bounds on eigenvalue spacing for decaying potentials

Abstract: We study decaying half-line Schrödinger operators and the local eigenvalue spacing of their Dirichlet restrictions. While absolutely continuous spectrum is strongly associated with bulk universality and clock behavior, singular spectral measures can correspond to varied local behaviors. In this work, the rate of decay of the potential is shown to give upper bounds for the spacing of Dirichlet eigenvalues on finite intervals.

• The paper identifies an upper bound separator (h) on eigenvalue spacing for Dirichlet Schrödinger operators in terms of the integrated, decaying potential.
• For decaying potential ranges in $L^{1}$, $ ace{placed}{V}$ governed by $L^2$, the ratio $h(x)= o(x^{-1/2))$ renders left-sorted.
• The paper does not provide mesures as such lower bound to clustering and establishes no sinusoidal revelation of typical upper bound rates.

Overview

This paper by Milivoje Lukić and Brian Simanek establishes upper bounds on the local spacing of Dirichlet eigenvalues for half-line Schrödinger operators with decaying potentials, dedicated to Barry Simon on his 80th birthday. The central object is the half-line operator $H = -\frac{d^2}{dx^2} + V$ on $(0,\infty)$, where $V$ is real-valued and locally integrable, together with its finite-volume restrictions $H_X = -\frac{d^2}{dx^2} + V$ on $L^2((0,X))$ with Dirichlet boundary conditions at $0$ and $X$. Each $H_X$ has purely discrete spectrum, and the paper's main theorem controls how densely these eigenvalues can cluster in terms of the growth of $\int_0^x |V(t)|\,dt$. This is the half-line Schrödinger analogue of Simanek's earlier result on zero spacings of paraorthogonal polynomials on the unit circle [Sim20], which itself refined work of Golinskii [Gol02].

The motivation comes from the qualitative principle that regular (clock-like) eigenvalue spacing of $H_X$ signals absolutely continuous spectrum of $(0,\infty)$0, while singular spectrum permits irregular clustering. For $(0,\infty)$1, the canonical spectral measure is purely absolutely continuous on $(0,\infty)$2 with positive continuous density; Maltsev's bulk universality result then yields uniform clock spacing at asymptotic distance $(0,\infty)$3, consistent with the paper's bound. The genuinely new territory is $(0,\infty)$4, where spectral transitions between $(0,\infty)$5 and $(0,\infty)$6 decay are governed by a substantial literature (Kiselev, Remling, Christ–Kiselev, Deift–Killip, Killip–Simon, and others), and where Breuer's and Breuer–Seelig's results show that the spacing–spectrum connection is only qualitative, not a strict rule.

Main result

The main theorem states: fix a locally integrable potential $(0,\infty)$7 and $(0,\infty)$8. There exists a function

$(0,\infty)$9

such that for any interval $V$0 with $V$1 and any $V$2, the operator $V$3 has at least one eigenvalue in $V$4. In other words, no gap of length exceeding $V$5 can occur among the eigenvalues of $V$6 above energy $V$7. The additive constant $V$8 inside the parentheses accommodates the free case $V$9.

The strength of this bound depends entirely on the growth rate of the integrated potential. When $H_X = -\frac{d^2}{dx^2} + V$0, one obtains $H_X = -\frac{d^2}{dx^2} + V$1, matching the clock spacing scale from universality theory. A useful remark connects the integral condition to standard decay hypotheses: for $H_X = -\frac{d^2}{dx^2} + V$2,

• if $H_X = -\frac{d^2}{dx^2} + V$3 or $H_X = -\frac{d^2}{dx^2} + V$4 (the amalgamated norm with $H_X = -\frac{d^2}{dx^2} + V$5), then $H_X = -\frac{d^2}{dx^2} + V$6;
• under the weak counterparts $H_X = -\frac{d^2}{dx^2} + V$7 or $H_X = -\frac{d^2}{dx^2} + V$8, one has the big-$H_X = -\frac{d^2}{dx^2} + V$9 version $L^2((0,X))$0.

Thus, for example, $L^2((0,X))$1 gives $L^2((0,X))$2, so the theorem applies across the entire range of decay conditions studied in the spectral transition literature, using only local integrability as the pointwise assumption — a notable advantage of amalgamated norms.

Method of proof

The proof rests on Prüfer variables, which parametrize the Dirichlet eigensolution $L^2((0,X))$3 at energy $L^2((0,X))$4 (with $L^2((0,X))$5, $L^2((0,X))$6) as

$L^2((0,X))$7

where the phase satisfies the first-order equation

$L^2((0,X))$8

Setting $L^2((0,X))$9, zeros of $0$0 occur exactly when $0$1, i.e., when $0$2 is an eigenvalue of $0$3. The key finite-length lemma shows that if

$0$4

then $0$5; since $0$6 is continuous in $0$7, some $0$8 lands in $0$9, producing an eigenvalue of $X$0 in $X$1. The estimate follows directly from integrating the phase equation and bounding $X$2. Theorem 1 then follows by taking $X$3 and restricting to $X$4.

The argument is short and transparent, but it is an upper-bound technique only: it guarantees that eigenvalues cannot be too sparse, saying nothing about lower bounds on spacing. A remark notes that adding a constant $X$5 to the potential shifts energies without changing spacings and could improve the criterion for a fixed interval, but would worsen the asymptotic growth of $X$6 for decaying potentials, so it is not exploited.

Limitations and open questions

The paper is explicit about the scope of its contribution. First, the bound is one-sided: it precludes large gaps in the Dirichlet spectrum but provides no control over minimal spacing, so it cannot by itself distinguish absolutely continuous from singular spectral behavior — indeed, prior work of Breuer and Breuer–Seelig demonstrates that singular measures can exhibit sine-kernel-type local statistics, confirming the connection is not deterministic. Second, the function $X$7 is constructed explicitly from the integrated potential and is not claimed to be sharp; whether the $X$8 rate can be improved, or matched by lower bounds on clustering for specific decaying potentials, remains open. Third, the result concerns Dirichlet restrictions on fixed intervals $X$9 away from the origin; behavior near zero energy, where the Weyl sequence structure differs, is outside the framework. Finally, the paper treats only the continuum half-line Schrödinger setting; transferring the corresponding refinements known for OPUC/paraorthogonal polynomials (e.g., connections to the spectral measure's local statistics as in Breuer–Seelig) to this setting is left unaddressed.

Conclusion

The paper delivers a clean, Prüfer-variable-based theorem relating the growth of $H_X$0 to guaranteed eigenvalue density for finite-interval Dirichlet Schrödinger operators, thereby extending the orthogonal-polynomial spacing program of Last–Simon and Simanek to the continuum setting. Its applicability through weak amalgamated norms makes it compatible with the sharp decay thresholds governing absolutely continuous spectrum, positioning the result as a quantitative upper-bound companion to the qualitative spacing–spectral-type correspondence.