Deviations from periodicity are treated by quantum mechanical perturbation theory. KY Cygni is a variable star with a large amplitude but no clear periodicity. These parameters involve the spatial periodicity of the data, specifically its direction. They migrate between the deep and the peripheral, circulation exhibiting unique diurnal periodicity. Gratings with such small periodicity are called subwavelength gratings and exhibit special optical properties. Conversely, attack frequency may be highly unpredictable, showing no periodicity at all. For resonance to occur, it is necessary to have also periodicity in the excitation force. As a result of the refractive index periodicity, these systems diffract light. The elliptic functions thus have a double periodicity. Quasiperiodicity is the property of a system that displays irregular periodicity. This is an algebraic form of Bott periodicity. The window width formula 2 determines the longest periodicity captured by SSA. The window length formula 41 determines the longest periodicity captured by SSA. These techniques were used in Raoul Bott's proof of his periodicity theorem. These are red giants or supergiants with little or no detectable periodicity.