Vol 66, No 3 (2020): Spectral Analysis

We consider one-dimensional Dirac operator L_P,U with Birkhoff regular boundary conditions and summable potential P(x) on [0, $\pi$]. We introduce strongly and weakly regular operators. In both cases, asymptotic formulas for eigenvalues are found. In these formulas, we obtain main asymptotic terms and estimates for the second term. We specify these estimates depending on the functional class of the potential: L_p[0,$\pi$] with p ∈ [1,2] and the Besov space $B_{p,p\text{'}}^{\theta }[0,\pi ]$ with p ∈ [1,2] and θ ∈ (0,1/p). Additionally, we prove that our estimates are uniform on balls ${||P||}_{p,\theta }\le R$ Then we get asymptotic formulas for normalized eigenfunctions in the strongly regular case with the same residue estimates in uniform metric on x ∈ [0,$\pi$]. In the weakly regular case, the eigenvalues λ_2n and λ_2n+1 are asymptotically close and we obtain similar estimates for two-dimensional Riesz projectors. Next, we prove the Riesz basis property in the space (L₂[0,$\pi$])²  for a system of eigenfunctions and associated functions of an arbitrary strongly regular operator L_P,U. In case of weak regularity, the Riesz basicity of two-dimensional subspaces is proved.

In parallel with the L_P,U operator, we consider the Sturm–Liouville operator L_q,U generated by the differential -y'' + q(x)y expressionwith distribution potential q of first-order singularity (i.e., we assume that the primitive u = q⁽−¹⁾ belongs to L₂[0, $\pi$]) and Birkhoff-regular boundary conditions. We reduce to this case $-({\tau }_{1}y\text{'})\text{'}+i\left(\sigma y\right)\text{'}+i\sigma y\text{'}+{\tau }_{0}y$, operators of more general form where $\tau {\text{'}}_1, \sigma ,{\tau }_0^{(-1)}\in L_2$and ${\tau }_1>0$. For operator L_q,U, we get the same results on the asymptotics of eigenvalues, eigenfunctions, and basicity as for operator L_P,U .

Then, for the Dirac operator L_P,U, we prove that the Riesz basis constant is uniform over the balls${||P||}_{p,\theta }\le R$ for p>1 or θ>0. The problem of conditional basicity is naturally generalized to the problem

of equiconvergence of spectral decompositions in various metrics. We prove the result on equiconvergence by varying three indices: $f \in L_{\mu }[0,\pi ]$ (decomposable function), $P\in L_{\aleph }[0,\pi ]$ (potential), and $||S_m-S_m^0||\to 0, m\to \infty$ in $L_{\nu }[0,\pi ]$  (equiconvergence of spectral decompositions in the corresponding norm). In conclusion, we prove theorems on conditional and unconditional basicity of the system of eigenfunctions and associated functions of operator L_P,U in the spaces $L_{\mu }[0,\pi ], \mu \ne 2$, and in various Besov spaces $B_{p,q}^{\theta }[0,\pi ]$.