Basic relations for the period variation models of variable stars
  Models of period variations are basic tools for period analyzes of variable stars. We introduce phase function and instant period and formulate basic relations and equations among them. Some simple period models are also presented. 
Zden\v{e}k Mikul\'a\v{s}ek, Tom\'a\v{s} Gr\'af, Miloslav Zejda, Liying   Zhu, Shen-Bang Qian
astro-ph.SR