  The observations of the photometric space telescopes CoRoT and Kepler show
numerous Blazhko RR Lyrae stars which have non-repetitive modulation cycles.
The phenomenon can be explained by multi-periodic, stochastic or chaotic
processes. From a mathematical point of view, almost periodic functions
describe all of them. We assumed band-limited almost periodic functions either
for the light curves of the main pulsation or for the modulation functions. The
resulting light curves can generally be described analytically and it can also
be examined by numerical simulations. This presentation is a part of our
systematic study on the modulation of pulsating stars (Benko et al. 2009, 2011,
2012).
