Poker

Poker is a card game played with a normal deck of 52 cards. Sometimes, additional cards called "jokers" are also used. In straight or draw poker, each player is normally dealt a hand of five cards. Depending on the variant, players then discard and redraw cards, trying to improve their hands. Bets are placed at each discard step. The number of possible distinct five-card hands is equal to the number of possible ways of picking five cards out of a deck of 52, namely

where is a binomial coefficient.

There are special names for specific types of hands. A royal flush is an ace, king, queen, jack, and 10, all of one suit. A straight flush is five consecutive cards all of the same suit (but not a royal flush), where an ace may count as either high or low. A full house is three-of-a-kind and a pair. A flush is five cards of the same suit (but not a royal flush or straight flush). A straight is five consecutive cards (but not a royal flush or straight flush), where an ace may again count as either high or low.

The probabilities of being dealt five-card poker hands of a given type (before discarding and with no jokers) on the initial deal are given below (Packel 1981). The "computation" and "counts" columns give the numbers of possible five-card hands of each type, and how this number is computed. As usual, denotes a binomial coefficient. The odds of a hand with probability are .

hand	computation	counts	probability ()	odds
royal flush	4	4
straight flush		36
four of a kind		624
full house
flush
straight
three of a kind			0.0211
two pair			0.0475
one pair			0.423

Gadbois (1996) gives probabilities for hands if two jokers are included, and points out that it is impossible to rank hands in any single way which is consistent with the relative frequency of the hands.

Wolfram Web Resources