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I have a vague memory of an infinite game due to Ernst Specker with the following properties:

(1) It is a two-person perfect information game, where the players move alternately.

(2) The possible moves depend only on the current position.

(3) There is no winning strategy where each move is based only on the current position

(4) There is a winning strategy (for one of the players) if the strategy uses the current position and the history of previous moves.

Can someone provide a reference or explanation for this game?

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    $\begingroup$ Does the payoff depend on the sequence of positions, or just on the "limit" (in some sense) of the sequence of positions? $\endgroup$
    – bof
    Commented Aug 4, 2021 at 2:45
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    $\begingroup$ It's not the one you're looking for but Gabriel Debs gave an example of a topological space whose Banach–Mazur game has a winning "historical" strategy but no winning "positional" strategy ("tactic"). $\endgroup$
    – bof
    Commented Aug 4, 2021 at 2:51
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    $\begingroup$ @bof If the payoff depends on the history, then there are easy examples: let first player win if he plays his red piece on move 17 and only then, but otherwise he loses. If the position doesn't tell him the turn number, then he can't have a position-based winning strategy. $\endgroup$ Commented Aug 4, 2021 at 9:01

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I don't know about the game attributed to Specker, but here is a simple game with your desired features.

Let us call it the Chocolatier's game. There are two players, the Chocolatier and the Glutton. To begin play, the Chocolatier serves up finitely many unique and exquisite chocolate creations on a platter, and then the Glutton chooses one of them to eat. Play continues — at each stage the Chocolatier adds finitely many additional chocolates to the platter, and the Glutton consumes one of those available. The excess uneaten chocolates accumulate on the platter as play progresses.

After infinite play, the Glutton wins if every single chocolate that was served was eventually consumed. Otherwise, the Chocolatier wins.

The Glutton can easily win simply by paying attention to the order in which the chocolates were added, and consuming them in that order. If only finitely many chocolates are added at each stage, the Glutton should simply make sure to consume them before moving on to the chocolates that were added at later stages.

Indeed, the Glutton can win even if the Chocolatier places countably many chocolates on each turn. At turn $n=2^k(2m+1)$, let the Glutton eat the $k$th chocolate added at stage $m$, if any, and otherwise eat arbitrarily.

But what about strategies that depend only on the current position, that is, the assortment of chocolates on the platter?

In one sense, it is easy to see that there can be no such winning strategy for the Glutton. If the Glutton will choose a particular chocolate from a given assortment, then let the Chocolatier simply replace it with an identical chocolate type on the next move, and again on all subsequent moves. If the strategy does not know the history, then the Glutton will choose it again every time, and the other chocolates will never be eaten.

Perhaps it makes a more interesting game, however, to say that the Chocolatier loses if chocolate types are ever repeated. And for this version of the game, there are some interesting things to say.

If there are only countably many chocolate types available in all, then again the Glutton has a winning strategy that depends only on the current assortment on offer. Namely, the Glutton should fix an enumeration of the possible chocolate types that might appear, and at each stage select the chocolate that appears earliest in this order — it is the tastiest-looking chocolate as defined by that priority. With this strategy he will succeed in eating all the chocolates, because at the limit, if any chocolate was left, there would have to be a tastiest-looking one (earliest in the enumeration), and this would have been eaten once the finitely many tastier chocolates had been consumed.

If the Chocolatier were uncountably creative, however, and able to serve up uncountably many different chocolate types, then this argument breaks down. Indeed, in this case I claim there is no winning strategy for the Glutton that depends only on the chocolate assortment on offer. And indeed, there is no such strategy that works even if we should insist that the Chocolatier present an assortment only of size two at each stage.

To see this, suppose the Glutton will follow a fixed strategy that selects a particular chocolate to eat from amongst any two chocolates.

For any given chocolate $c$, if there are infinitely many others $d_n$ that would be preferred to it by the strategy, if presented as a pair $\{c,d_n\}$, then the Chocolatier can present these pairs $\{c,d_0\}$, $\{c,d_1\}$, $\{c,d_2\}$, in turn. At each stage, $d_n$ would be consumed and the Chocolatier can present $\{c,d_{n+1}\}$. In the end, the inferior chocolate $c$ would never have been selected and so the Chocolatier will win.

So if this is a winning strategy for the Glutton, then we may assume that every chocolate is preferred to all but finitely many of the others. But this is simply impossible with an uncountable set. To see this, take any countably infinite set of chocolates and close under the finite witnesses of strictly preferred chocolates. One thereby constructs a countably infinite set $D$ of chocolates so that any chocolate in $D$ is preferred to any chocolate not in $D$. Since there were uncountably many chocolate types, there is some $c\notin D$. This contradicts our assumption that there are only finitely many chocolates preferred to a given chocolate.

Let me remark that in the version of the game where we do not allow the Chocolatier to repeat chocolate types (as opposed to saying that this is allowed, but causes the Chocolatier to lose), then we should really include the list of already-consumed chocolates as part of the position, and this complicates the arguments above. (I asked a followup question about this at The Chocolatier's game.) I suspect but do not yet know that the Glutton can have no winning strategy that depends only on these more general kinds of positions in the uncountable case.

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    $\begingroup$ Since the Glutton has a winning strategy that uses the history, the Chocolatier cannot have any kind of winning strategy, especially not a special strategy that depends only on the current position. $\endgroup$ Commented Aug 6, 2021 at 11:27
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    $\begingroup$ Another isomorphic version: the attacker adds finitely many more time bombs (to explode at infinity), but the defuser can only defuse one bomb each turn. Defuser wins if every bomb is defused. $\endgroup$ Commented Aug 6, 2021 at 12:23
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    $\begingroup$ @jcsahnwaldtReinstateMonica No, that is incorrect. If you are claiming to have a winning strategy, then I can know what it is, for the purpose of proving that it doesn't work. If there is any logically possible play (including those built under the assumption that you would follow it) according to which your strategy doesn't work, then it isn't actually a winning strategy. $\endgroup$ Commented Aug 6, 2021 at 16:41
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    $\begingroup$ In particular, I don't believe that there are any paradoxes at all in this vein. If you have a truly winning strategy, then it shouldn't matter whether anyone knows how you will play. Otherwise, it wasn't actually winning, if there is a play that defeats it. $\endgroup$ Commented Aug 6, 2021 at 16:50
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    $\begingroup$ In other words, part of what we mean by a winning strategy is that if someone truly has a winning strategy, then they can announce to the world that they intend to play according to it, and they will still win every time. (Specifically, a strategy is a function telling the player what move to make next, given the previous play, and a winning strategy is one with the property that every play of the game that accords with it is winning for that player.) $\endgroup$ Commented Aug 6, 2021 at 17:01

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