15 research outputs found
Equilibrium Points of an AND-OR Tree: under Constraints on Probability
We study a probability distribution d on the truth assignments to a uniform
binary AND-OR tree. Liu and Tanaka [2007, Inform. Process. Lett.] showed the
following: If d achieves the equilibrium among independent distributions (ID)
then d is an independent identical distribution (IID). We show a stronger form
of the above result. Given a real number r such that 0 < r < 1, we consider a
constraint that the probability of the root node having the value 0 is r. Our
main result is the following: When we restrict ourselves to IDs satisfying this
constraint, the above result of Liu and Tanaka still holds. The proof employs
clever tricks of induction. In particular, we show two fundamental
relationships between expected cost and probability in an IID on an OR-AND
tree: (1) The ratio of the cost to the probability (of the root having the
value 0) is a decreasing function of the probability x of the leaf. (2) The
ratio of derivative of the cost to the derivative of the probability is a
decreasing function of x, too.Comment: 13 pages, 3 figure
Lookahead Pathology in Monte-Carlo Tree Search
Monte-Carlo Tree Search (MCTS) is an adversarial search paradigm that first
found prominence with its success in the domain of computer Go. Early
theoretical work established the game-theoretic soundness and convergence
bounds for Upper Confidence bounds applied to Trees (UCT), the most popular
instantiation of MCTS; however, there remain notable gaps in our understanding
of how UCT behaves in practice. In this work, we address one such gap by
considering the question of whether UCT can exhibit lookahead pathology -- a
paradoxical phenomenon first observed in Minimax search where greater search
effort leads to worse decision-making. We introduce a novel family of synthetic
games that offer rich modeling possibilities while remaining amenable to
mathematical analysis. Our theoretical and experimental results suggest that
UCT is indeed susceptible to pathological behavior in a range of games drawn
from this family
Weakly balanced multi-branching AND-OR trees: Reconstruction of the omitted part of Saks-Wigderson (1986) (Theory and Applications of Proof and Computation)
We investigate variants of the Nash equilibrium for query complexity of Boolean functions. We reconstruct some omitted proofs and definitions in the paper of Saks and Wigderson (1986). In particular, by extending observation by Arimoto (2020), we introduce concepts of “weakly balanced multi-branching tree” as modified versions of “nearly balanced tree” of Saks and Wigderson, and we show recurrence formulas of randomized complexity for weakly balanced multi-branching trees
The phenomenon of Decision Oscillation: a new consequence of pathology in Game Trees
Random minimaxing studies the consequences of using a random number for scoring
the leaf nodes of a full width game tree and then computing the best move using the
standard minimax procedure. Experiments in Chess showed that the strength of play
increases as the depth of the lookahead is increased. Previous research by the authors
provided a partial explanation of why random minimaxing can strengthen play by showing
that, when one move dominates another move, then the dominating move is more likely
to be chosen by minimax. This paper examines a special case of determining the move
probability when domination does not occur. Specifically, we show that, under a uniform
branching game tree model, whether the probability that one move is chosen rather than
another depends not only on the branching factors of the moves involved, but also on
whether the number of ply searched is odd or even. This is a new type of game tree
pathology, where the minimax procedure will change its mind as to which move is best,
independently of the true value of the game, and oscillate between moves as the depth of
lookahead alternates between odd and even