Determinacy is a subfield of

set theory Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly conce ...

, a branch of mathematics, that examines the conditions under which one or the other player of a game has a winning strategy, and the consequences of the existence of such strategies. Alternatively and similarly, "determinacy" is the property of a game whereby such a strategy exists. Determinacy was introduced by Gale and Stewart in 1950, under the name "determinateness". The games studied in set theory are usually

Gale A gale is a strong wind; the word is typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as sustained surface winds moving at a speed of between 34 and 47 knots (, or ).perfect information in which the players make an infinite sequence of moves and there are no draws. The field of game theory studies more general kinds of games, including games with draws such as

tic-tac-toe Tic-tac-toe (American English), noughts and crosses (Commonwealth English), or Xs and Os (Canadian or Irish English) is a paper-and-pencil game for two players who take turns marking the spaces in a three-by-three grid with ''X'' or ''O''. ...

chess Chess is a board game for two players, called White and Black, each controlling an army of chess pieces in their color, with the objective to checkmate the opponent's king. It is sometimes called international chess or Western chess to dist ...

, or

infinite chess Infinite chess is any variation of the game of chess played on an unbounded chessboard. Versions of infinite chess have been introduced independently by multiple players, chess theorists, and mathematicians, both as a playable game and as a mo ...

, or games with imperfect information such as poker.

Basic notions

Games

The first sort of game we shall consider is the two-player game of perfect information of length ω, in which the players play

natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called ''cardinal ...

s. These games are often called Gale–Stewart games. In this sort of game there are two players, often named ''I'' and ''II'', who take turns playing natural numbers, with ''I'' going first. They play "forever"; that is, their plays are indexed by the natural numbers. When they're finished, a predetermined condition decides which player won. This condition need not be specified by any definable ''rule''; it may simply be an arbitrary (infinitely long) lookup table saying who has won given a particular sequence of plays. More formally, consider a subset ''A'' of

Baire space In mathematics, a topological space X is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior. According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are e ...

; recall that the latter consists of all ω-sequences of natural numbers. Then in the game G_''A'', ''I'' plays a natural number ''a''₀, then ''II'' plays ''a''₁, then ''I'' plays ''a''₂, and so on. Then ''I'' wins the game if and only if :

\langle a_0,a_1,a_2,\ldots\rangle\in A

and otherwise ''II'' wins. ''A'' is then called the ''payoff set'' of G_''A''. It is assumed that each player can see all moves preceding each of his moves, and also knows the winning condition.

Strategies

Informally, a ''strategy'' for a player is a way of playing in which his plays are entirely determined by the foregoing plays. Again, such a "way" does not have to be capable of being captured by any explicable "rule", but may simply be a lookup table. More formally, a strategy for player ''I'' (for a game in the sense of the preceding subsection) is a function that accepts as an argument any finite sequence of natural numbers, of even length, and returns a natural number. If ''σ'' is such a strategy and <a₀,...,a_2n-1> is a sequence of plays, then ''σ''(<a₀,...,a_2n-1>) is the next play ''I'' will make, if ''I'' is following the strategy ''σ''. Strategies for ''II'' are just the same, substituting "odd" for "even". Note that we have said nothing, as yet, about whether a strategy is in any way ''good''. A strategy might direct a player to make aggressively bad moves, and it would still be a strategy. In fact it is not necessary even to know the winning condition for a game, to know what strategies exist for the game.

Winning strategies

A strategy is ''winning'' if the player following it must necessarily win, no matter what his opponent plays. For example, if ''σ'' is a strategy for ''I'', then ''σ'' is a winning strategy for ''I'' in the game G_''A'' if, for any sequence of natural numbers to be played by ''II'', say <a₁,a₃,a₅,...>, the sequence of plays produced by ''σ'' when ''II'' plays thus, namely :

\langle\sigma(\langle\rangle),a_1,\sigma(\langle\sigma(\langle\rangle),a_1\rangle),a_3,\ldots\rangle

is an element of ''A''.

Determined games

A (class of) game(s) is ''determined'' if for all instances of the game there is a winning strategy for one of the players (not necessarily the same player for each instance). Note that there cannot be a winning strategy for ''both'' players for the same game, for if there were, the two strategies could be played against each other. The resulting outcome would then, by hypothesis, be a win for both players, which is impossible.https://www.math.uni-hamburg.de/Infinite Games, Yurii Khomskii (2010)
Infinite Games, Yurii Khomskii (2010)

Determinacy from elementary considerations

All finite games of perfect information in which draws do not occur are determined. Real-world games of perfect information, such as

, or

, are always finished in a finite number of moves (in chess-games this assumes the 50-move rule is applied). If such a game is modified so that a particular player wins under any condition where the game would have been called a draw, then it is always determined. The condition that the game is always over (i.e. all possible extensions of the finite position result in a win for the same player) in a finite number of moves corresponds to the topological condition that the set ''A'' giving the winning condition for G_''A'' is

clopen In topology, a clopen set (a portmanteau of closed-open set) in a topological space is a set which is both open and closed. That this is possible may seem counter-intuitive, as the common meanings of and are antonyms, but their mathematical de ...

in the

topology In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ...

. For example, modifying the rules of chess to make drawn games a win for Black makes chess a determined game. As it happens, chess has a finite number of positions and a draw-by-repetition rules, so with these modified rules, if play continues long enough without White having won, then Black can eventually force a win (due to the modification of draw = win for black). The proof that such games are determined is rather simple: Player ''I'' simply plays ''not to lose''; that is, player ''I'' plays to make sure that player ''II'' does not have a winning strategy ''after'' ''Is move. If player ''I'' ''cannot'' do this, then it means player ''II'' had a winning strategy from the beginning. On the other hand, if player ''I'' ''can'' play in this way, then ''I'' must win, because the game will be over after some finite number of moves, and player ''I'' can't have lost at that point. This proof does not actually require that the game ''always'' be over in a finite number of moves, only that it be over in a finite number of moves whenever ''II'' wins. That condition, topologically, is that the set ''A'' is closed. This fact—that all closed games are determined—is called the ''Gale–Stewart theorem''. Note that by symmetry, all open games are determined as well. (A game is ''open'' if ''I'' can win only by winning in a finite number of moves.)

Determinacy from ZFC

David Gale David (; , "beloved one") (traditional spelling), , ''Dāwūd''; grc-koi, Δαυΐδ, Dauíd; la, Davidus, David; gez , ዳዊት, ''Dawit''; xcl, Դաւիթ, ''Dawitʿ''; cu, Давíдъ, ''Davidŭ''; possibly meaning "beloved one". w ...

and F. M. Stewart proved the open and closed games are determined. Determinacy for second level of the

Borel hierarchy In mathematical logic, the Borel hierarchy is a stratification of the Borel algebra generated by the open subsets of a Polish space; elements of this algebra are called Borel sets. Each Borel set is assigned a unique countable ordinal number called ...

games was shown by Wolfe in 1955. Over the following 20 years, additional research using ever-more-complicated arguments established that third and fourth levels of the Borel hierarchy are determined. In 1975, Donald A. Martin proved that all Borel games are determined; that is, if ''A'' is a Borel subset of Baire space, then G_''A'' is determined. This result, known as

Borel determinacy In descriptive set theory, the Borel determinacy theorem states that any Gale–Stewart game whose payoff set is a Borel set is determined, meaning that one of the two players will have a winning strategy for the game. A Gale-Stewart game is a poss ...

, is the best possible determinacy result provable in ZFC, in the sense that the determinacy of the next higher Wadge class is not provable in ZFC. In 1971, before Martin obtained his proof,

Harvey Friedman __NOTOC__ Harvey Friedman (born 23 September 1948)Handbook of Philosophical Logic, , p. 38 is an American mathematical logician at Ohio State University in Columbus, Ohio. He has worked on reverse mathematics, a project intended to derive the axi ...

showed that any proof of Borel determinacy must use the

axiom of replacement In set theory, the axiom schema of replacement is a schema of axioms in Zermelo–Fraenkel set theory (ZF) that asserts that the image of any set under any definable mapping is also a set. It is necessary for the construction of certain infinite ...

in an essential way, in order to iterate the powerset axiom transfinitely often. Friedman's work gives a level-by-level result detailing how many iterations of the powerset axiom are necessary to guarantee determinacy at each level of the

. For every integer ''n'', ZFC\P proves determinacy in the ''n''th level of the difference hierarchy of

\mathbf^0_3

sets, but ZFC\P does not prove that for every integer ''n'' ''n''th level of the difference hierarchy of

\Pi^0_3

sets is determined. See

reverse mathematics Reverse mathematics is a program in mathematical logic that seeks to determine which axioms are required to prove theorems of mathematics. Its defining method can briefly be described as "going backwards from the theorems to the axioms", in cont ...

for other relations between determinacy and subsystems of

second-order arithmetic In mathematical logic, second-order arithmetic is a collection of axiomatic systems that formalize the natural numbers and their subsets. It is an alternative to axiomatic set theory as a foundation for much, but not all, of mathematics. A precur ...

Determinacy and large cardinals

There is an intimate relationship between determinacy and large cardinals. In general, stronger large cardinal axioms prove the determinacy of larger

pointclass In the mathematical field of descriptive set theory, a pointclass is a collection of sets of points, where a ''point'' is ordinarily understood to be an element of some perfect Polish space. In practice, a pointclass is usually characterized by ...

es, higher in the

Wadge hierarchy In descriptive set theory, within mathematics, Wadge degrees are levels of complexity for sets of reals. Sets are compared by continuous reductions. The Wadge hierarchy is the structure of Wadge degrees. These concepts are named after William W. Wad ...

, and the determinacy of such pointclasses, in turn, proves the existence of

inner model In set theory, a branch of mathematical logic, an inner model for a theory ''T'' is a substructure of a model ''M'' of a set theory that is both a model for ''T'' and contains all the ordinals of ''M''. Definition Let L = \langle \in \rangle be ...

s of slightly weaker large cardinal axioms than those used to prove the determinacy of the pointclass in the first place.

Measurable cardinals

It follows from the existence of a measurable cardinal that every analytic game (also called a Σ¹₁ game) is determined, or equivalently that every coanalytic (or Π¹₁ ) game is determined. (See

Projective hierarchy In the mathematical field of descriptive set theory, a subset A of a Polish space X is projective if it is \boldsymbol^1_n for some positive integer n. Here A is * \boldsymbol^1_1 if A is analytic * \boldsymbol^1_n if the complement of A, X\se ...

for definitions.) Actually a measurable cardinal is more than enough. A weaker principle — the existence of 0^# is sufficient to prove coanalytic determinacy, and a little bit more: The precise result is that the existence of 0^# is equivalent to the determinacy of all levels of the difference hierarchy below the ω² level, i.e. ω·n-Π¹₁ determinacy for every

n

. From a measurable cardinal we can improve this very slightly to ω²-Π¹₁ determinacy. From the existence of more measurable cardinals, one can prove the determinacy of more levels of the difference hierarchy over Π¹₁.

Proof of determinacy from sharps

For every real number ''r'',

\Sigma^1_1(r)

determinacy is equivalent to existence of ''r''^#. To illustrate how large cardinals lead to determinacy, here is a proof of

\Sigma^1_1(r)

determinacy given existence of ''r''^#. Let ''A'' be a

\Sigma^1_1(r)

subset of the Baire space. ''A'' = p 'T''for some tree ''T'' (constructible from ''r'') on (ω, ω). (That is x∈A iff from some ''y'',

((x_0,y_0),(x_1,y_1),...)

is a path through ''T''.) Given a partial play ''s'', let

T_s

be the subtree of ''T'' consistent with ''s'' subject to max(y₀,y₁,...,y_len(s)-1)<len(s). The additional condition ensures that

T_s

is finite. Consistency means that every path through

T_s

is of the form

((x_0,y_0),(x_1,y_1),...,(x_i,y_i))

where

(x_0,x_1,...,x_i)

is an initial segment of ''s''. To prove that A is determined, define auxiliary game as follows:
In addition to ordinary moves, player 2 must play a mapping of

T_s

into ordinals (below a sufficiently large ordinal ''κ'') such that * each new move extends the previous mapping and * the ordering of the ordinals agrees with the

Kleene–Brouwer order In descriptive set theory, the Kleene–Brouwer order or Lusin–Sierpiński order is a linear order on finite sequences over some linearly ordered set (X, <), that differs from the more commonly used

indiscernible In mathematical logic, indiscernibles are objects that cannot be distinguished by any property or relation defined by a formula. Usually only first-order formulas are considered. Examples If ''a'', ''b'', and ''c'' are distinct and is a set ...

ordinals. By indiscernibility, if ''κ'' and the ordinals in the auxiliary response are in ''I'', then the moves by player 1 do not depend on the auxiliary moves (or on ''κ''), and so the strategy can be converted into a strategy for the original game (since player 2 can hold out with indiscernibles for any finite number of steps). Suppose that player 1 loses in the original game. Then, the tree corresponding to a play is well-founded. Therefore, player 2 can win the auxiliary game by using auxiliary moves based on the indiscernibles (since the order type of indiscernibles exceeds the Kleene–Brouwer order of the tree), which contradicts player 1 winning the auxiliary game.

Woodin cardinals

If there is a Woodin cardinal with a measurable cardinal above it, then Π¹₂ determinacy holds. More generally, if there are ''n'' Woodin cardinals with a measurable cardinal above them all, then Π¹_n+1 determinacy holds. From Π¹_n+1 determinacy, it follows that there is a transitive

containing ''n'' Woodin cardinals.

\Delta^1_2

(lightface) determinacy is equiconsistent with a Woodin cardinal. If

\Delta^1_2

determinacy holds, then for a Turing cone of ''x'' (that is for every real ''x'' of sufficiently high

Turing degree In computer science and mathematical logic the Turing degree (named after Alan Turing) or degree of unsolvability of a set of natural numbers measures the level of algorithmic unsolvability of the set. Overview The concept of Turing degree is fund ...

), L 'x''satisfies OD-determinacy (that is determinacy of games on integers of length ω and ordinal-definable payoff), and in HOD^{L 'x''/sup> $\omega_2^$ is a Woodin cardinal.

Projective determinacy

If there are infinitely many Woodin cardinals, then projective determinacy holds; that is, every game whose winning condition is a projective set

In the mathematical field of descriptive set theory, a subset A of a Polish space X is projective if it is \boldsymbol^1_n for some positive integer n. Here A is
* \boldsymbol^1_1 if A is analytic
* \boldsymbol^1_n if the complement of A, X\se ...
is determined. From projective determinacy it follows that, for every natural number ''n'', there is a transitive inner model that satisfies that there are ''n'' Woodin cardinals.

Axiom of determinacy

The axiom of determinacy, or AD, asserts that ''every'' two-player game of perfect information of length ω, in which the players play naturals, is determined.

AD is provably false from ZFC; using the axiom of choice

In mathematics, the axiom of choice, or AC, is an axiom of set theory equivalent to the statement that ''a Cartesian product of a collection of non-empty sets is non-empty''. Informally put, the axiom of choice says that given any collection ...
one may prove the existence of a non-determined game. However, if there are infinitely many Woodin cardinals with a measurable above them all, then L(R)

In set theory, L(R) (pronounced L of R) is the smallest transitive inner model of ZF containing all the ordinals and all the reals.
Construction
It can be constructed in a manner analogous to the construction of L (that is, Gödel's constructi ...
is a model of ZF that satisfies AD.

Consequences of determinacy

Regularity properties for sets of reals

If ''A'' is a subset of Baire space such that the Banach–Mazur game
In general topology, set theory and game theory, a Banach– Mazur game is a topological game played by two players, trying to pin down elements in a set (space). The concept of a Banach–Mazur game is closely related to the concept of Baire s ...
for ''A'' is determined, then either ''II'' has a winning strategy, in which case ''A'' is meager, or ''I'' has a winning strategy, in which case ''A'' is comeager on some open neighborhood.

This does not quite imply that ''A'' has the property of Baire

A subset A of a topological space X has the property of Baire (Baire property, named after René-Louis Baire), or is called an almost open set, if it differs from an open set by a meager set; that is, if there is an open set U\subseteq X such th ...
, but it comes close: A simple modification of the argument shows that if Γ is an adequate pointclass such that every game in Γ is determined, then every set of reals in Γ has the property of Baire.

In fact this result is not optimal; by considering the unfolded Banach–Mazur game we can show that determinacy of Γ (for Γ with sufficient closure properties) implies that every set of reals that is the ''projection'' of a set in Γ has the property of Baire. So for example the existence of a measurable cardinal implies Π¹₁ determinacy, which in turn implies that every Σ¹₂ set of reals has the property of Baire.

By considering other games, we can show that Π¹_''n'' determinacy implies that every Σ¹_''n''+1 set of reals has the property of Baire, is Lebesgue measurable
In measure theory, a branch of mathematics, the Lebesgue measure, named after French mathematician Henri Lebesgue, is the standard way of assigning a measure to subsets of ''n''-dimensional Euclidean space. For ''n'' = 1, 2, or 3, it coincides wit ...
(in fact universally measurable) and has the perfect set property In descriptive set theory, a subset of a Polish space has the perfect set property if it is either countable or has a nonempty perfect subset (Kechris 1995, p. 150). Note that having the perfect set property is not the same as being a p ...
.

Periodicity theorems

* The first periodicity theorem implies that, for every natural number ''n'', if Δ¹_2''n''+1 determinacy holds, then Π¹_2''n''+1 and Σ¹_2''n''+2 have the prewellordering property

In set theory, a prewellordering on a set X is a preorder \leq on X (a transitive and strongly connected relation on X) that is wellfounded in the sense that the relation x \leq y \land y \nleq x is wellfounded. If \leq is a prewellordering on ...
(and that Σ¹_2''n''+1 and Π¹_2''n''+2 do ''not'' have the prewellordering property, but rather have the separation property).
* The second periodicity theorem implies that, for every natural number ''n'', if Δ¹_2''n''+1 determinacy holds, then Π¹_2''n''+1 and Σ¹_2''n'' have the scale property. In particular, if projective determinacy holds, then every projective relation has a projective uniformization.
* The third periodicity theorem gives a sufficient condition for a game to have a definable winning strategy.

Applications to decidability of certain second-order theories

In 1969, Michael O. Rabin

Michael Oser Rabin ( he, מִיכָאֵל עוזר רַבִּין; born September 1, 1931) is an Israeli mathematician and computer scientist and a recipient of the Turing Award.
Biography Early life and education
Rabin was born in 1931 in ...
proved that the monadic second-order theory of ''n'' successors ( S2S for ''n'' = 2) is decidable. A key component of the proof requires showing determinacy of parity game

A parity game is played on a colored directed graph, where each node has been colored by a priority – one of (usually) finitely many natural numbers. Two players, 0 and 1, move a (single, shared) token along the edges of the graph. The own ...
s, which lie in the third level of the Borel hierarchy In mathematical logic, the Borel hierarchy is a stratification of the Borel algebra generated by the open subsets of a Polish space; elements of this algebra are called Borel sets. Each Borel set is assigned a unique countable ordinal number called ...
.

Wadge determinacy

Wadge determinacy is the statement that for all pairs ''A'', ''B'' of subsets of Baire space

In mathematics, a topological space X is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior.
According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are e ...
, the Wadge game G(''A'' ,''B'') is determined. Similarly for a pointclass

In the mathematical field of descriptive set theory, a pointclass is a collection of sets of points, where a ''point'' is ordinarily understood to be an element of some perfect Polish space. In practice, a pointclass is usually characterized by ...
Γ, Γ Wadge determinacy is the statement that for all sets ''A'', ''B'' in Γ, the Wadge game G(''A'', ''B'') is determined.

Wadge determinacy implies the semilinear ordering principle for the Wadge order. Another consequence of Wadge determinacy is the perfect set property In descriptive set theory, a subset of a Polish space has the perfect set property if it is either countable or has a nonempty perfect subset (Kechris 1995, p. 150). Note that having the perfect set property is not the same as being a p ...
.

In general, Γ Wadge determinacy is a consequence of the determinacy of Boolean combinations of sets in Γ. In the projective hierarchy

In the mathematical field of descriptive set theory, a subset A of a Polish space X is projective if it is \boldsymbol^1_n for some positive integer n. Here A is
* \boldsymbol^1_1 if A is analytic
* \boldsymbol^1_n if the complement of A, X\se ...
, Π¹₁ Wadge determinacy is equivalent to Π¹₁ determinacy, as proved by Leo Harrington

Leo Anthony Harrington (born May 17, 1946) is a professor of mathematics at the University of California, Berkeley who works in
recursion theory, model theory, and set theory.
Having retired from being a Mathematician, Professor Leo Harrington is ...
. This result was extended by Hjorth to prove that Π¹₂ Wadge determinacy (and in fact the semilinear ordering principle for Π¹₂) already implies Π¹₂ determinacy.

More general games

Games in which the objects played are not natural numbers

Determinacy of games on ordinals with ordinal definable payoff and length ω implies that for every regular cardinal ''κ''>ω there are no ordinal definable disjoint stationary subsets of ''κ'' made of ordinals of cofinality ω. The consistency strength of the determinacy hypothesis is unknown but is expected to be very high.

Games played on
trees

In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are u ...

Long games

Existence of ω₁ Woodin cardinals implies that for every countable ordinal α, all games on integers of length α and projective payoff are determined. Roughly speaking, α Woodin cardinals corresponds to determinacy of games on reals of length α (with a simple payoff set). Assuming a limit of Woodin cardinals ''κ'' with o(''κ'')=''κ''⁺⁺ and ω Woodin cardinals above ''κ'', games of variable countable length where the game ends as soon as its length is admissible relative to the line of play and with projective payoff are determined. Assuming that a certain iterability conjecture is provable, existence of a measurable Woodin cardinal implies determinacy of open games of length ω₁ and projective payoff. (In these games, a winning condition for the first player is triggered at a countable stage, so the payoff can be coded as a set of reals.)

Relative to a Woodin limit of Woodin cardinals and a measurable above them, it is consistent that every game on integers of length ω₁ and ordinal definable payoff is determined. It is conjectured that the determinacy hypothesis is equiconsistent with a Woodin limit of Woodin cardinals. ω₁ is maximal in that there are undetermined games on integers of length ω₁+ω and ordinal definable payoff.

Games of imperfect information

In any interesting game with imperfect information

In economics, perfect information (sometimes referred to as "no hidden information") is a feature of perfect competition. With perfect information in a market, all consumers and producers have complete and instantaneous knowledge of all market pri ...
, a winning strategy will be a mixed strategy

In game theory, a player's strategy is any of the options which they choose in a setting where the outcome depends ''not only'' on their own actions ''but'' on the actions of others. The discipline mainly concerns the action of a player in a game ...
: that is, it will give some probability of differing responses to the same situation. If both players' optimal strategies are mixed strategies then the outcome of the game cannot be ''certainly'' determinant (as it can for pure strategies, since these are deterministic). But the probability distribution of outcomes to opposing mixed strategies can be calculated. A game that requires mixed strategies is defined as ''determined'' if a strategy exists that yields a minimum expected value (over possible counter-strategies) that exceeds a given value. Against this definition, all finite
Finite is the opposite of infinite. It may refer to:
* Finite number (disambiguation)
* Finite set, a set whose cardinality (number of elements) is some natural number
* Finite verb, a verb form that has a subject, usually being inflected or marke ...
two-player zero-sum games are clearly determined. However, the determinacy of ''infinite'' games of imperfect information (Blackwell games) is less clear.

In 1969 David Blackwell

David Harold Blackwell (April 24, 1919 – July 8, 2010) was an American statistician and mathematician who made significant contributions to game theory, probability theory, information theory, and statistics. He is one of the eponyms of th ...
proved that some "infinite games with imperfect information" (now called "Blackwell games") are determined, and in 1998 Donald A. Martin proved that ordinary (perfect-information game) determinacy for a boldface pointclass implies Blackwell determinacy for the pointclass. This, combined with the Borel determinacy theorem of Martin, implies that all Blackwell games with Borel payoff functions are determined.
Martin conjectured that ordinary determinacy and Blackwell determinacy for infinite games are equivalent in a strong sense (i.e. that Blackwell determinacy for a boldface pointclass in turn implies ordinary determinacy for that pointclass), but as of 2010, it has not been proven that Blackwell determinacy implies perfect-information-game determinacy.

Quasistrategies and quasideterminacy

See also

* ω-automaton
In automata theory, a branch of theoretical computer science, an ω-automaton (or stream automaton) is a variation of finite automata that runs on infinite, rather than finite, strings as input. Since ω-automata do not stop, they have a variety o ...

* Solved game
A solved game is a game whose outcome (win, lose or draw) can be correctly predicted from any position, assuming that both players play perfectly.
This concept is usually applied to abstract strategy games, and especially to games with full inform ...

* Strictly determined game
* Topological game
In mathematics, a topological game is an infinite game of perfect information played between two players on a topological space. Players choose objects with topological properties such as points, open sets, closed sets and open coverings. Time ...

Footnotes

# This assumes that ''I'' is trying to get the intersection of neighborhoods played to be a singleton whose unique element is an element of ''A''. Some authors make that the goal instead for player ''II''; that usage requires modifying the above remarks accordingly.

References

*
*
*
*
*
*
*
*
*
PDF

*

External links

"Large Cardinals and Determinacy"
at the Stanford Encyclopedia of Philosophy

{{Game theory, state=collapsed}

Basic notions

Games

Strategies

Winning strategies

Determined games

Determinacy from elementary considerations

Determinacy from ZFC

Determinacy and large cardinals

Measurable cardinals

Proof of determinacy from sharps

Woodin cardinals

Projective determinacy

Axiom of determinacy

Consequences of determinacy

Regularity properties for sets of reals

Periodicity theorems

Applications to decidability of certain second-order theories

Wadge determinacy

More general games

Games in which the objects played are not natural numbers

Games played on trees In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are u ...

Long games

Games of imperfect information

Quasistrategies and quasideterminacy

See also

Footnotes

References

External links

Games played on
trees In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are u ...