MathDB
2024 TCS Problem 2

Source:

April 23, 2024
Tcs

Problem Statement

There are two different versions of this problem, each with different solutions. You must find bounds for each of these problems.
[*] Alice and Bob are playing a collaborative game in which they agree upon an encoding/strategy. Alice shuffles a deck of 52 cards (numbered 1-52) (*) and takes the first nn cards off the top of the deck. Alice then chooses one card to be put to the side, and chooses an ordering of the other n1n-1 cards. Bob then walks into the room seeing the n1n-1 cards in the order Alice put them in. For example, if Alice was given 9-4-10-51-7-8 and chose to put 8 to the side, she could put 5 cards in order of 9-4-10-51-7 which Bob would see.
Find the lowest nn for which Bob could guarantee that he could use Alice's encoding to find the card placed to the side.
(*) (If you would prefer, you can write your solution in terms of the deck being a standard deck with 4 suits and 13 ranks, there's a way to move between them that we can handle.)
[*] Alice and Bob are playing a collaborative game in which they agree upon an encoding/strategy. Alice shuffles a deck of 52 cards (numbered 1-52) and takes the first nn cards off the top of the deck. Alice then chooses one card to be put to the side, and chooses an ordering of the other n1n-1 cards. Carol then comes in to the room and randomly discards one of the n1n-1 cards, and places the cards back in a way that preserves the order Alice had. Bob then walks into the room seeing the n2n-2 cards in the order Alice put them in. For example, if Alice was given 1-2-3-4-5-6 and chose to put 6 to the side, she would put 5 cards in order of 1-2-3-4-5, and Carol removed 4, Bob would see 1-2-3-5.
Find the lowest nn for which Bob could guarantee that he could use Alice's encoding to find the card placed to the side.
Proposed by Eric Oh