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Convergence of spectral decompositions of Hill operators with trigonometric polynomial potentials

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Abstract

We consider the Hill operator

$$Ly = - y^{\prime \prime} + v(x)y, \quad0 \leq x \leq \pi,$$

subject to periodic or antiperiodic boundary conditions, with potentials v which are trigonometric polynomials with nonzero coefficients, of the form

  1. (i)

    ae −2ixbe 2ix;

  2. (ii)

    ae −2ixBe 4ix;

  3. (iii)

    ae −2ixAe −4ix + be 2ixBe 4ix.

Then the system of eigenfunctions and (at most finitely many) associated functions is complete but it is not a basis in \({L^2 ([0,\pi], \mathbb{C})}\) if |a| ≠ |b| in the case (i), if |A| ≠ |B| and neither −b 2/4B nor −a 2/4A is an integer square in the case (iii), and it is never a basis in the case (ii) subject to periodic boundary conditions.

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Djakov, P., Mityagin, B. Convergence of spectral decompositions of Hill operators with trigonometric polynomial potentials. Math. Ann. 351, 509–540 (2011). https://doi.org/10.1007/s00208-010-0612-5

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