According to one of Libratus' creators, Professor Tuomas Sandholm, Libratus does not have a fixed built-in strategy, but an algorithm that. Libratus, an artificial intelligence developed by Carnegie Mellon University, made history by defeating four of the world's best professional poker players in a. Libratus adjusted on the fly. The computations were carried out on the new 'Bridges' supercomputer at the Pittsburgh Supercomputing Center. It used another 4. <
Poker Mensch gegen Maschine: Libratus, der GangsterBrown and Sandholm built a poker-playing AI called Libratus that decisively beat four leading human professionals in the two-player variant of. Libratus ist ein Computerprogramm für künstliche Intelligenz, das speziell für das Pokerspiel entwickelt wurde. Die Entwickler von Libratus beabsichtigen, dass es auf andere, nicht Poker-spezifische Anwendungen verallgemeinerbar ist. Es wurde an. Libratus, an artificial intelligence developed by Carnegie Mellon University, made history by defeating four of the world's best professional poker players in a.
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This combined uncertainty in poker has historically been challenging for AI algorithms to deal with. That is, until Libratus came along. Libratus used a game-theoretic approach to deal with the unique combination of multiple agents and imperfect information, and it explicitly considers the fact that a poker game involves both parties trying to maximize their own interests.
The poker variant that Libratus can play, no-limit heads up Texas Hold'em poker, is an extensive-form imperfect-information zero-sum game.
We will first briefly introduce these concepts from game theory. For our purposes, we will start with the normal form definition of a game. The game concludes after a single turn.
These games are called normal form because they only involve a single action. An extensive form game , like poker, consists of multiple turns.
Before we delve into that, we need to first have a notion of a good strategy. Multi-agent systems are far more complex than single-agent games.
To account for this, mathematicians use the concept of the Nash equilibrium. A Nash equilibrium is a scenario where none of the game participants can improve their outcome by changing only their own strategy.
This is because a rational player will change their actions to maximize their own game outcome. When the strategies of the players are at a Nash equilibrium, none of them can improve by changing his own.
Thus this is an equilibrium. When allowing for mixed strategies where players can choose different moves with different probabilities , Nash proved that all normal form games with a finite number of actions have Nash equilibria, though these equilibria are not guaranteed to be unique or easy to find.
While the Nash equilibrium is an immensely important notion in game theory, it is not unique. Thus, is hard to say which one is the optimal.
Such games are called zero-sum. Importantly, the Nash equilibria of zero-sum games are computationally tractable and are guaranteed to have the same unique value.
We define the maxmin value for Player 1 to be the maximum payoff that Player 1 can guarantee regardless of what action Player 2 chooses:. The minmax theorem states that minmax and maxmin are equal for a zero-sum game allowing for mixed strategies and that Nash equilibria consist of both players playing maxmin strategies.
As an important corollary, the Nash equilibrium of a zero-sum game is the optimal strategy. Crucially, the minmax strategies can be obtained by solving a linear program in only polynomial time.
While many simple games are normal form games, more complex games like tic-tac-toe, poker, and chess are not. In normal form games, two players each take one action simultaneously.
In contrast, games like poker are usually studied as extensive form games , a more general formalism where multiple actions take place one after another.
See Figure 1 for an example. All the possible games states are specified in the game tree. The good news about extensive form games is that they reduce to normal form games mathematically.
Since poker is a zero-sum extensive form game, it satisfies the minmax theorem and can be solved in polynomial time.
However, as the tree illustrates, the state space grows quickly as the game goes on. Even worse, while zero-sum games can be solved efficiently, a naive approach to extensive games is polynomial in the number of pure strategies and this number grows exponentially with the size of game tree.
Thus, finding an efficient representation of an extensive form game is a big challenge for game-playing agents. AlphaGo  famously used neural networks to represent the outcome of a subtree of Go.
While Go and poker are both extensive form games, the key difference between the two is that Go is a perfect information game, while poker is an imperfect information game.
In poker however, the state of the game depends on how the cards are dealt, and only some of the relevant cards are observed by every player.
To illustrate the difference, we look at Figure 2, a simplified game tree for poker. Note that players do not have perfect information and cannot see what cards have been dealt to the other player.
Let's suppose that Player 1 decides to bet. Player 2 sees the bet but does not know what cards player 1 has. In the game tree, this is denoted by the information set , or the dashed line between the two states.
An information set is a collection of game states that a player cannot distinguish between when making decisions, so by definition a player must have the same strategy among states within each information set.
Thus, imperfect information makes a crucial difference in the decision-making process. To decide their next action, player 2 needs to evaluate the possibility of all possible underlying states which means all possible hands of player 1.
Because the player 1 is making decisions as well, if player 2 changes strategy, player 1 may change as well, and player 2 needs to update their beliefs about what player 1 would do.
Heads up means that there are only two players playing against each other, making the game a two-player zero sum game.
No-limit means that there are no restrictions on the bets you are allowed to make, meaning that the number of possible actions is enormous.
In contrast, limit poker forces players to bet in fixed increments and was solved in . Nevertheless, it is quite costly and wasteful to construct a new betting strategy for a single-dollar difference in the bet.
Libratus abstracts the game state by grouping the bets and other similar actions using an abstraction called a blueprint. In a blueprint, similar bets are be treated as the same and so are similar card combinations e.
Ace and 6 vs. Ace and 5. The blueprint is orders of magnitude smaller than the possible number of states in a game. Libratus' creators intend for it to be generalisable to other, non-Poker-specific applications.
It was developed at Carnegie Mellon University, Pittsburgh. While Libratus was written from scratch, it is the nominal successor of Claudico.
Like its predecessor, its name is a Latin expression and means 'balanced'. Libratus was built with more than 15 million core hours of computation as compared to million for Claudico.
The computations were carried out on the new 'Bridges' supercomputer at the Pittsburgh Supercomputing Center.
According to one of Libratus' creators, Professor Tuomas Sandholm, Libratus does not have a fixed built-in strategy, but an algorithm that computes the strategy.
Their new method gets rid of the prior de facto standard in Poker programming, called "action mapping". As Libratus plays only against one other human or computer player, the special 'heads up' rules for two-player Texas hold 'em are enforced.
To manage the extra volume, the duration of the tournament was increased from 13 to 20 days. The four players were grouped into two subteams of two players each.
One of the subteams was playing in the open, while the other subteam was located in a separate room nicknamed 'The Dungeon' where no mobile phones or other external communications were allowed.
The Dungeon subteam got the same sequence of cards as was being dealt in the open, except that the sides were switched: The Dungeon humans got the cards that the AI got in the open and vice versa.
This setup was intended to nullify the effect of card luck. As written in the tournament rules in advance, the AI itself did not receive prize money even though it won the tournament against the human team.
During the tournament, Libratus was competing against the players during the days. Overnight it was perfecting its strategy on its own by analysing the prior gameplay and results of the day, particularly its losses.
Therefore, it was able to continuously straighten out the imperfections that the human team had discovered in their extensive analysis, resulting in a permanent arms race between the humans and Libratus.
It used another 4 million core hours on the Bridges supercomputer for the competition's purposes. Libratus had been leading against the human players from day one of the tournament.