“…The most explored case is η = 1, λ k h(λ k z), λ k ∈ C, see [6,8,9,13,14,16,22]. The paper [13] contains several remarks 1 on the general case (3). Among other things, our motivation for studying (1) is based on that one needs to perform notably less arithmetic operations while computing (1), the amplitude and frequency sums with equal amplitudes, than in the case of (2) and (4), although all these sums have similar approximative properties with respect to the number of free parameters, as will be shown below.…”