Online Card Games

Abstract

Consider the following one player game. A deck containing m copies of n different card types is shuffled uniformly at random. Each round the player tries to guess the next card in the deck, and then the card is revealed and discarded. It was shown by Diaconis, Graham, He, and Spiro that if m is fixed, then the maximum expected number of correct guesses that the player can achieve is asymptotic to H m log n, where H m is the mth harmonic number.In this paper we consider an adversarial version of this game where a second player shuffles the deck according to some (possibly non-uniform) distribution. We prove that a certain greedy strategy for the shuffler is the unique optimal strategy in this game, and that the guesser can achieve at most log n expected correct guesses asymptotically for fixed m against this greedy strategy.

Guessing cards with complete feedback

“…The natural question is now, assuming no powers of extrasensory perception, how many correct guesses can be expected under this optimal strategy? This game has also been analyzed in connection with clinical trials [1,8] and is generally well studied: for other results and variations, we refer to [2,4,5,6,7,11,12,13,15,16,17,19]. 1.2. Results.…”