Combinatorics - Combinatorial Analysis
    Combinations
      Combinations
      Combinations with repetition
Combinations
Given a set of n different elements or objects. Select a subset of r elements out of n. Such selection is called the combination.
A combination is an unordered arrangement of r objects selected from n different objects taken r at a time.
The number of distinct combinations selecting r elements out of n is
   
Therefore, combinations must differ from each other at least in one element.
Example:  Find the number of combinations of size 3 that can be made from digits 1, 2, 3, 4, 5 and write them out.
Solution:  Since, n = 5 and r = 3 then
The combinations are,    1 2 3         2 3 4         3 4 5.
                                    1 2 4         2 3 5
                                    1 2 5         2 4 5
                                    1 3 4
                                    1 3 5
                                    1 4 5
Example:  Find the number of combinations of size 4 that can be made from letters A, B, C, D, E, F and write them out.
Solution:  Since, n = 6 and r = 4 then
The combinations are,    A B C D         B C D E         C D E F.
                                    A B C E         B C D F
                                    A B C F         B C E F
                                    A B D E         B D E F
                                    A B D F
                                    A B E F
                                    A C D E
                                    A C D F
                                    A C E F
                                    A D E F
Combinations with repetition
The number of ways to choose r objects from a set of n different objects, so that an object can be chosen more than once
   
Remember that combinations must differ from each other at least in one element.
Example:  Find the number of combinations of size 3 that can be made from digits 1, 2, 3, 4 if repetition is allowed, and write them out.
Solution:  Since, n = 4 and r = 3 then
The combinations are,     1 1 1          2 2 2          3 3 3          4 4 4          1 2 3
                                     1 1 2          2 2 1          3 3 1          4 4 1          1 2 4
                                     1 1 3          2 2 3          3 3 2          4 4 2          1 3 4
                                     1 1 4          2 2 4          3 3 4          4 4 3          2 3 4
 
Example:  Find the number of combinations of size 4 that can be made from letters A, B, C if repetition is allowed, and write them out.
Solution:  Since, n = 3 and r = 4 then
The combinations with repetition are,     A A A A          B B B B          C C C C          A A B C
                                                          A A A B          B B B C          C C C A          B B A C
                                                          A A B B          B B C C          C C A A          C C A B.
                                                          A B B B          B C C C          C A A A 
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