Computing Probabilities Sorting Lists Algorithms & True Statements Worksheet

Question Description

Here are two (simpli??ed) versions of Roulette. Compute the probabil- ities of losing in each.

(a) Las Vegas roulette. There are 38 (uniformly random) outcomes: two green numbers 0 and 00, and 36 red and black numbers 1 to36 (half red, half black). You bet on a red or black color (say red). You then win if you get a red number, and lose if you get a non-red number. What’s the probability that you lose?
(b) Monte-Carlo roulette. There are 37 (again, uniformly random) outcomes: 0 is green, and 36 red and black numbers 1 to 36(again half red, half black). You bet on a red or black color (say red). You then win if you get a red number, and lose if you get a black number. If you get the green 0, you get a special reroll, where if you get a red then you get your money back (so you don’t win but don’t lose either), but you lose if you get a green or black on the reroll. What is your probability of losing?

Then, comment on the values themselves.

Suppose you have 15 people coming together to form a committee of5 people. 6 out of 15 of the people are spies. What’s the probability that at least half of the committee are spies?
True or false? (Warning: as in all my problems, you need to explain your answers to get any credit.)(a) (Z/6Z)? has a primitive root. (b) (Z/13Z)? has a generator. (c) (Z/15Z)? has a generator.
Similar to Di??e-Hellman, here’s the ??Zhang key-exchange??:?? We pick a big prime p and a generator g.?? Alice has a secret a ? Z/(p ? 1)Z. 1

?? Bobhasasecretb?Z/(p?1)Z.
?? Alice sends ga to Bob. Bob sends gb to Alice.

(a) Show that a key is indeed exchanged; that is, Alice and Bob can both compute g?a?b (mod p).