I have a question, probably relatively simple one. Once you want to find all the unique combinations of number 5 you count 5! = 120. Once you want to find all the combinations including repetition of value of number 5 you count 5 power of 5 = 3125.
In my case I have the following situation. I have 9 characters you need to use to perform unique permutations on 4 columns/variables, meaning, for example, having 'a, b, c, d, e, f, g, h, i' I perform all the possible unique 4 combinations by using the 9 characters: 'a b c d'; 'a b e f'; 'd e f g' etc. In each column of each string of 4 characters (e.g. 'a, a, b, c' not allowed) the value of character can't get repeated. Now, how to count the number of such unique combinations obeying the constrains? Cheers