Chap 2 Combinatorial Methods Ghahramani 3rd edition

p2. Outline 2.1 Introduction 2.2 Counting principle 2.3 Permutations 2.4 Combinations 2.5 Stirling ’ s formula

p Introduction If the sample space is finite and furthermore sample points are all equally likely, then P(A)=N(A)/N(S) So we study combinatorial analysis here, which deals with methods of counting.

p Counting principle Ex 2.1 How many outcomes are there if we throw 5 dice? Ex 2.2 In tossing 4 fair dice, P(at least one 3 among these 4 dice)=? Ex 2.3 Virginia wants to give her son, Brian, 14 different baseball cards within a 7-day period. If Virginia gives Brian cards no more than once a day, in how many way can this be done? Ex 2.6 (Standard Birthday Problem) P(at least two among n people have the same Bday)=?

p5. Counting principle Thm 2.3 A set with n elements has 2 n subsets. Ex 2.9 Mark has $4. He decides to bet $1 on the flip of a fair coin 4 times. What is the probability that (a) he breaks even; (b) he wins money?(use tree diagram)

p Permutations Ex people, Brown, Smith, and Jones, must be scheduled for job interviews. In how many different orders can this be done? Ex anthropology, 4 computer science, 3 statistics, 3 biology, and 5 music books are put on a bookshelf with a random arrangement. What is the probability that the books of the same subject are together?

p7. Permutations Ex 2.12 If 5 boys and 5 girls sit in a row in a random order, P(no two children of the same sex sit together)=? Thm 2.4 The number of distinguishable permutations of n objects of k different types, where n 1 are alike, n 2 are alike, …, n k are alike and n=n 1 +n 2 +…+n k is

p8. Permutations Ex 2.13 How many different 10-letter codes can be made using 3 a’s, 4 b’s, and 3 c’s? Ex 2.14 In how many ways can we paint 11 offices so that 4 of them will be painted green, 3 yellow, 2 white, and the remaining 2 pink? Ex 2.15 A fair coin is flipped 10 times. P(exactly 3 heads)=?

p Combinations Ex 2.16 In how many ways can 2 math and 3 biology books be selected from 8 math and 6 biology books? Ex instructors were selected randomly to ask whether they are happy with their teaching loads. The response of 32 were negative. If Drs. Smith, Brown, and Jones were among those questioned. P(all 3 gave negative responses)=?

p10. Combinations Ex 2.18 In a small town, 11 of the 25 schoolteachers are against abortion, 8 are for abortion, and the rest are indifferent. A random sample of 5 schoolteachers is selected for an interview. (a)P(all 5 are for abortion)=? (b)P(all 5 have the same opinion)=? Ex 2.19 In Maryland’s lottery, player pick 6 integers between 1 and 49, order of selection being irrelevant. P(grand prize)=? P(2 nd prize)=? P(3 rd prize)=?

p11. Combinations Ex cards are drawn from 52 without replacement. P(at least one of the cards is a king)=? Ex cards are drawn from 52. P(full house)=? Ex 2.24 A professor wrote n letters and sealed them in envelopes. P(at least one letter was addressed correctly)=? Hint: Let E i be the event that i th letter is addressed correctly. Compute P(E 1 U…UE n ) by inclusion- exclusion principle.

p12. Combinations Thm 2.5 (Binomial expansion) Ex 2.25 What is the coefficient of x 2 y 3 in the expansion of (2x+3y) 5 ? Ex 2.26 Evaluate the sum

p13. Combinations Ex 2.27 Evaluate the sum Ex 2.28 Prove that

p14. Combinations Ex 2.29 Prove the inclusion-exclusion principle. Ex 2.30 Distribute n distinguishable balls into k distinguishable cells so that n 1 balls are distributed into the first cell, n 2 balls into the second cell, …, n k balls into the k th cell, where n 1 +n 2 +…+n k =n. How many possible ways? Sol:

p15. Combinations Thm 2.6 (Multinomial expansion).

p Stirling’s formula