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CS 173: Discrete Mathematical Structures

CS 173: Discrete Mathematical Structures. Cinda Heeren heeren@cs.uiuc.edu Siebel Center, rm 2213 Office Hours: W 12:30-2:30. CS 173 Announcements. Homework #7 due 10/23, 8a. Exam #2, 11/3, 7-9p. No class 11/3. Why am I showing this to you?. Beautiful patterns Recursive defn

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CS 173: Discrete Mathematical Structures

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  1. CS 173:Discrete Mathematical Structures Cinda Heeren heeren@cs.uiuc.edu Siebel Center, rm 2213 Office Hours: W 12:30-2:30

  2. CS 173 Announcements • Homework #7 due 10/23, 8a. • Exam #2, 11/3, 7-9p. • No class 11/3. Cs173 - Spring 2004

  3. Why am I showing this to you? • Beautiful patterns • Recursive defn • New type of proof • Applications in more complex counting techniques CS 173 Binomial Coefficients (a + b)2 = a2 + 2ab + b2 (a + b)3 = a3 + 3ab2 + 3a2b + b3 (a + b)4 = a4 + 4ab3 + 6a2b2 + 4a3b + b4 Cs173 - Spring 2004

  4. What is coefficient of a9b3 in (a + b)12? • 36 • 220 • 15 • 6 • No clue CS 173 Binomial Coefficients (a + b)2 = a2 + 2ab + b2 (a + b)3 = a3 + 3a2b + 3ab2 + b3 (a + b)4 = a4 + 4a3b + 6a2b2 + 4ab3 + b4 Cs173 - Spring 2004

  5. = a4 + a3b + a2b2 + ab3 + b4 Binomial Theorem: Let x and y be variables, and let n be any nonnegative integer. Then CS 173 Binomial Coefficients (a + b)4 = (a + b)(a + b)(a + b)(a + b) Cs173 - Spring 2004

  6. Binomial Theorem: Let x and y be variables, and let n be any nonnegative integer. Then 17 9 3a 2b CS 173 Binomial Coefficients What is the coefficient of a8b9 in the expansion of (3a +2b)17? What is n? What is j? What is x? Cs173 - Spring 2004 What is y?

  7. = a4 + a3b + a2b2 + ab3 + b4 • 10C6 • 9C4 • 9C5 • 8C4 + 8C5 • No clue CS 173 Binomial Coefficients (a + b)4 = (a + b)(a + b)(a + b)(a + b) Cs173 - Spring 2004

  8. Powers of 2 2n nC0 nC1 nC2 CS 173 Binomial Coefficients Sum each row of Pascal’s Triangle: Two proofs that Suppose you have a set of size n. How many subsets does it have? How many subsets of size 0 does it have? How many subsets of size 1 does it have? How many subsets of size 2 does it have? Cs173 - Spring 2004 Count all subsets in this way, and we have the result!

  9. Powers of 2 Done CS 173 Binomial Coefficients Sum each row of Pascal’s Triangle: Two proofs that Let x=1 and y=1 in Binomial Theorem. Cs173 - Spring 2004

  10. n-1Cj-1 n-1Cj CS 173 Pascal’s Identity A relationship between the entries in Pascal’s . Suppose T is a set, |T|=n. Let a be an element in T, and let S = T - {a}. Let’s count the nCj subsets of size j. Note that some of these contain a, and some don’t. How many contain a? How many don’t? Cs173 - Spring 2004

  11. A m items B n items CS 173 Vandermonde’s Identity Let m, n, and r be nonnegative integers with r not exceeding either m or n. Then To choose r items, take some from A and some from B. All possible ways of doing this gives the result. Cs173 - Spring 2004

  12. CS 173 Combinations with repetition Suppose you want to buy 5 bags of chips from the 3 kinds you like at Meijer. In how many different ways can you stock up? Out of 7 items, we are choosing 2 to be bars. From that, and our understanding of the model, we can report the answer. Cs173 - Spring 2004

  13. Example: How many solutions are there to the equation When the variables are nonnegative integers? 13C3 CS 173 Combinations with repetition There are n+r-1Cr, r-sized combinations from a set of n elements when repetition is allowed. Cs173 - Spring 2004

  14. 6 3 CS 173 Permutations with indistinguishable objects How many different strings can be made from the letters in the word rat? How many different strings can be made from the letters in the word egg? Cs173 - Spring 2004

  15. 8C4, now 4 spots are left 4C2, now 2 spots are left 2C2, now 0 spots are left CS 173 Permutations with indistinguishable objects How many different strings can be made from the letters in the phrase nano-nano? Key thoughts: 8 positions, 3 kinds of letters to place. In how many ways can we place the ns? In how many ways can we place the as? In how many ways can we place the os? Cs173 - Spring 2004

  16. CS 173 Permutations with indistinguishable objects How many distinct permutations are there of the letters in the word APALACHICOLA? How many if the two Ls must appear together? How many if the first letter must be an A? Cs173 - Spring 2004

  17. CS 173 A little practice A turtle begins at the upper left corner of an n x m grid and meanders to the lower right corner. How many routes could she take if she only moves right and down? Cs173 - Spring 2004

  18. CS 173 A little practice In how many ways can 11 identical computer science books and 8 identical psychology books be distributed among 5 students? Hint: forget about the psychology books for the moment. Hint: how can you combine your soln for the CS books with your soln for the Psych books? Cs173 - Spring 2004

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