Definition: Combinatorics is a branch of mathematics about the selection and deployment of elements of a set based on some conditions
Selected (or available) group of elements called compounds
Permutations
Definition: Permutations of 
the different elements are called finite ordered set (that is, such sets that specify the order of elements) can be removed from a set containing elements (if all elements of a given set of different — get permutations without repetitions, but if in the given set elements may be repeated, then get the permutations with repetitions)
Formula for the number of permutations 
Without repetition
where 
(read "n factorial")
For 
(by definition)
Example. The number of different six-digit numbers can be formed from the digits 
without repeating the numbers in one number equal to 
With repetitions
where 
Example.The number of different six-digit numbers can be formed from three twos, two sevens and one five
(considered 
)