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Unformatted text preview: CS103A HO #54 Combinatorics III 3/7/08 1 Permutations and Combinations with Repetition How many strings of length r can we form from the uppercase letters of the English alphabet, if repetition is allowed? 26 r Permutations Combinations (ordered) (unordered) P(n, r) = n(n1)(n2)···(nr+1) = n! (nr)! C(n, r) = = P(n, r) r! n! r! (nr)! Without repetition With repetition n r Set of size n, selecting r items, 0 ≤ r ≤ n Combinations with Repetition Consider 7 kinds of bills: $1, $2, $5, $10, $20, $50, $100 Problem: Suppose I have a bag with lots of bills in it, and I pull out 5 bills. How many different combinations could there be? Combinations with Repetition Consider 7 kinds of bills: $1, $2, $5, $10, $20, $50, $100 Problem: Suppose I have a bag with lots of bills in it, and I pull out 5 bills. How many different combinations could there be? Equivalent problem: Suppose I have 5 blank bills. How many ways can I print denominations on them? Equivalent problem: Suppose I have 7 empty bins, labeled with the denominations. I'm going to place 5 blank bills in them, to be printed later. How many ways can I distribute the 5 bills? Combinations with Repetition Consider 7 kinds of bills: $1, $2, $5, $10, $20, $50, $100 Problem: Suppose I have a bag with lots of bills in it, and I pull out 5 bills. How many different combinations could there be? Equivalent problem: Suppose I have 5 blank bills. How many ways can I print denominations on them? Equivalent problem: Suppose I have 7 empty bins, labeled with the denominations. I'm going to place 5 blank bills in them, to be printed later. How many ways can I distribute the 5 bills? Equivalent problem: How many sets can I form by selecting 5 items, with repetition allowed, from a set of 7 items? Equivalent problem: How many integer solutions are there to the equation x 1 + x 2 + x 3 + x 4 + x 5 + x 6 + x 7 = 5 where 0 ≤ x i ≤ 5 ? Combinations with Repetition Equivalent problem: Suppose I have 7 empty bins, labeled with the denominations. I'm going to place 5 blank bills in them, to be printed later. How many ways can I distribute the 5 bills? $1 $2 $5 $10 $20 $50 $100 * * * * * CS103A HO #54 Combinatorics III 3/7/08 2 Combinations with Repetition Equivalent problem: Suppose I have 7 empty bins, labeled with the denominations. I'm going to place 5 blank bills in them, to be printed later. How many ways can I distribute the 5 bills? * * * * * *   * *   *   * Combinations with Repetition Equivalent problem: Suppose I have 7 empty bins, labeled with the denominations. I'm going to place 5 blank bills in them, to be printed later. How many ways can I distribute the 5 bills? * * * * * *   * *   *   *           How many ways can we put 5 stars in these 11 slots?...
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 Winter '07
 Plummer,R
 Computer Science, Finite set, Bijection, equivalent problem

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