about project cyclic model analysis on sequential patterns

Question

It is a shopping project. In this project we are calculating the time difference in periodical item sets (if an item a is brought & after 3 days item b is brought then we calculate the all time difference of such periodicity). In my project paper there are 4 algorithms are there Trend Distribution function, Generalized periodicity detection, trend modeling & cyclic model analysis. In paper there is a formula f(x)=(24-x)/18*(sin(x)+1.5).(x,f(x)) are coordinates of graph. No much more is given about x&f(x) can you help me what is x & f(x)?