Introduction
We consider that we have a finite set and we want to form an infinite set from it as well. The goal of this study is to calculate the number of these sets, and therefore we must know the nature of this set and the laws to which it is subject, for example:
If we want to form phone numbers, we use the following known numbers 0,1,2,3,4,5,6,7,8,9, we can also repeat the chosen number paying attention to the order, because the phone number 038298741 is different from the number 038298714, even though the two numbers are made up of the same numbers and with the same frequency.
If we want to form a comittee of a certain number of members, here we cannot repeat the member meaning that specific person cannot hold the position of two members at the same time. As for the question raisedabout taking into account the order, in order to answer this question, we must know the nature of the positions formed for this committee. If these positions are different, the order is taken into account but if they are not different it is not taken into account.
From here comes the idea of permutations and combinations.