A Near-Field Basis in Radially Symmetric Interior Transmission Problem
  The spectrum of interior transmission problem is the zero set of certain entire functional determinant. It is classic that we deploy the series of exponential polynomials to approximate the distribution of the roots of the entire functions of exponential type. We construct an exponential system in the form of $\{e^{ik_jr}\}$ according to the set of interior transmission eigenvalues $\{k_{j}\}$. The eigenvalues are the zeros of a sine-type function. In particular, they are intersection points of two asymptotically periodic entire functions. The intersection set is asymptotically sine-like near the real axis, so we may manage to construct a basis according to the class of spectral objects. Due to the result of Paley-Wiener theorem, the zero set generates a natural duality in the form of Fourier transform associated with exponential polynomials. Whenever there is a sufficient quantity of transmission eigenvalues, we are given a series of exponential polynomials to saturate the functional density, which completes a $L^{2}$-Riesz basis in a suitable ball. 
Lung-Hui Chen
math.AP