dobdo99 wrote:What is the relationship between constants and the time series functions?

The time series function, which is found by GP algorithm, will consist of past values (like x[t-1], x[t-2]) and constants which you've specified. As you've already seen before, one of the solutions for one of the time series you had was x[t-1] + 2, where "2" is a constant selected from the list you gave.

dobdo99 wrote:why do we have these specific numbers as constants?

You can actually choose any numbers you like. Just keep in mind that you specify limited amount of constant, so you better choose those, which can be used to easily create other possible constants. For example, suppose you have a time series like x[t-1]+6. As you can see from the code, "6" is not in the list of constants. But GP algorithm can create it quite easily using "2*3", which are available. Of course there are some functions available, like ln, sin, cos, exp, etc., which can be also used in GP expressions.