# Positional determinacy for parity games, a backward approach by Muller and Schupp

This post revisits Muller and Schupp's backward approach to prove the positionality of parity games. The argument is simple and beautiful; credits go to Thomas Colcombet for this presentation.

We discussed the technical details of the construction informally together, and Thomas gave me a draft he wrote. Unhappy that I didn't write the draft myself, I wrote down my own version of the construction. It turned out to be technically identical. This post features additional explanations and unhelpful drawings.

The crux in a few sentences: assume we have a winning strategy for parity, we want to construct a positional one. The difficulty is that the strategy we start from may perform stupid moves at various times during the game. We define a preference order on plays, which induces the notion of the worst possible history when reaching a vertex. The positional strategy plays assuming this worst case; if the preference order is well chosen, this forces the strategy to play good moves, ensuring the parity condition.

We fix $\G$ a game with finite outdegree, $v_0$ a starting vertex and $\sigma$ a winning strategy for Eve, i.e. ensuring $\Parity$. The crucial assumption we make is that $\G$ is chronological: all paths from $v_0$ to a vertex $v$ have the same length.

#### Ordering histories

A history is a finite or infinite word over $\priority$, we let be the set of histories. It is equipped with the prefix distance, inducing a notion of limit for sequences of histories.

The parity condition is satisfied by infinite histories such that the **maximal** priority appearing infinitely
often is even.

For a history $p$, define $\tr_c(p)$ the suffix of $p$ that starts after the last occurrence of $c$. The infix from position $n$ to position $m$ is written $p[n,\ldots,m]$. For a history $p$, define $\xi_c(p)$ the set of positions for which $p$ has value $c$.

We will consider sets of natural numbers, ordered by $\lex$ defined as follows: for two sets $X,Y \subseteq \N$, we say that $X \lexstrict Y$ if there exists $n \in \N$ such that $n \in Y \setminus X$ and $X \cap \set{0,\ldots,n-1} = Y \cap \set{0,\ldots,n-1}$.

We define the order $\preceq$ over for finite histories; intuitively speaking, $p \preceq q$ reads: $p$ is worse than $q$ with respect to $\Parity$. We first define $\preceq_d$ by induction on the maximal priority $d$:

- If $d = 0$, then $p \preceq_d\ q$ if $|p| \le |q|$.
- If $d$ is odd, then $p \preceq_d\ q$ if
- If $d$ is even and different from $0$, then $p \preceq_d\ q$ if

Now define $\preceq$ as $\preceq_d$.

Fact:If $p \preceq q$, then for all $r$ we have $p \cdot r \preceq q \cdot r$.

#### Defining a positional strategy for Eve

A play is a finite or infinite word over $V$, which naturally induces a history. We assume that plays start in $v_0$. We identify a play and its induced history: for instance, for $\pi_1,\pi_2$ two finite plays, we say that $\pi_1 \preceq \pi_2$ if $c(\pi_1) \preceq c(\pi_2)$ holds, where $c(\pi_1)$ and $c(\pi_2)$ are the histories induced by $\pi_1$ and $\pi_2$.

In the following lemma, we make use of the finite-degree assumption.

Lemma:Let $(p_n)\nN$ be a sequence of finite plays. Then it contains a converging subsequence.

We define a positional strategy $\sigma’$ on all vertices reachable by $\sigma$. The definition is by induction on the rank, which is the length of the path from $v_0$. For the definition to make sense we need to ensure the following property: for all vertices $v$ reachable by $\sigma’$, there exists a play ending in $v$ consistent with both $\sigma$ and $\sigma’$.

Assume that $\sigma’$ has been defined on all vertices of rank $k$, then for all vertices $v \in \VE$ of rank $k+1$ reachable by $\sigma’$, define $\sigma’(v) = \sigma(\pi_v)$, where $\pi_v$ is a finite play ending in $v$ and consistent with both $\sigma$ and $\sigma’$, minimal with respect to $\preceq$ among all such plays. The property is clearly preserved by this construction.

Theorem:The strategy $\sigma’$ ensures $\Parity$.

**Proof:**
Consider a play $\pi = v_0 v_1 \ldots$ consistent with $\sigma’$.

For the sake of readability, write $\pi_n = \pi_{v_n}$. We first observe that $\pi_{n+1} \preceq \pi_n \cdot v_{n+1}$ for all $n$. Indeed, by definition of $\sigma’$, the play $\pi_n \cdot v_{n+1}$ is consistent with both $\sigma$ and $\sigma’$. Since the play $\pi_{n+1}$ is the worst play ending in $v_{n+1}$ consistent with both $\sigma$ and $\sigma’$, it is smaller than $\pi_n \cdot v_{n+1}$ with respect to $\preceq$.

Using this inequality, an easy induction shows that $\pi_m \preceq \pi_n \cdot \pi[n+1,\ldots,m]$ for all $n < m$, and also $\pi_n \preceq \pi[0,\ldots,n]$. Here we rely on the fact above.

Thanks to the lemma above, the sequence $(\pi_n)\nN$ contains a subsequence indexed by $I \subseteq \N$ which converges to an infinite play, written $\pi_\infty$. Observe that since for all $n \in I$, the finite play $\pi_n$ is consistent with $\sigma$, the limit $\pi_\infty$ is consistent with $\sigma$ hence satisfies $\Parity$.

We conclude using the following lemma.

Lemma:Let $\pi = v_0 v_1 \ldots$ be an infinite play and $(\pi_n)\nN$ be a sequence of finite plays, such that:

- $(\pi_n)\nN$ converges to an infinite play $\pi_\infty$,
- $\pi_\infty$ satisfies $\Parity$,
- for all $n$, $\pi_{n+1} \preceq \pi_n \cdot \pi[|\pi_n+1|,\ldots,|\pi_{n+1}|]$.

Then $\pi$ satisfies $\Parity$.

**Proof:**
We proceed by induction on the maximal priority $d$.
The case $d = 0$ is easy.

*First case:* $d$ is even and different from $0$.
There are two cases:

- Either $d$ appears infinitely many times in $\pi_\infty$. We show the following property:

For every $k$ such that $c(\pi_\infty)(k) = d$, there exists $k’ \ge k$ such that $c(\pi)(k’) = d$.

Consider such a $k$ and $\pi_n$ that coincides with $\pi_\infty$ on the first $k$ positions. Since $\pi_n \preceq \pi[0,\ldots,n]$, by definition $\max(\xi_d(\pi_n)) \le \max(\xi_d(\pi[0,\ldots,n]))$. Observe that $\max(\xi_d(\pi_n)) \ge k$ since $k \in \xi_d(\pi_n)$, so $\max(\xi_d(\pi[0,\ldots,n])) \ge k$, and the conclusion follows.

It follows that $d$ appears infinitely many times in $\pi$, so $\pi$ satisfies $\Parity$.

- Or $d$ appears finitely many times in $\pi_\infty$. We conclude by induction hypothesis, considering the sequence $(\tr_d(\pi_n))\nN$.

*Second case:* $d$ is odd.
We show the following property:

For every $n$, for every $k \ge n$ such that $c(\pi(k)) = d$, there exists $k’ \ge n$ such that $c(\pi_\infty(k’)) = d$.

Consider such $k$ and $k$ and $n_0,n_1$ such that $\pi_{n_0}$ coincides with $\pi_\infty$ on the first $n$ positions, and $\pi_{n_1}$ coincides with $\pi_\infty$ on the first $k$ positions. Without loss of generality, $n < n_0 < k < n_1$. We have $\pi_{n_1} \preceq \pi_{n_0} \cdot \pi[n_0+1,\ldots,n_1]$, so

Since $c(\pi(k)) = d$, there exists $k’ \le k$ such that $c(\pi_{n_1}(k’)) = d$. Since $\pi_{n_0}$ and $\pi_{n_1}$ both coincide with $\pi_\infty$ on the first $n$ positions, there are equal up to this position, so $k’ > n$. Since $\pi_{n_1}$ coincide with $\pi_\infty$ on the first $k$ positions, it follows that $c(\pi_\infty(k’)) = d$.

Since $d$ appears finitely many times in $\pi_\infty$, it implies that $d$ appears finitely many times in $\pi$. As above, we conclude by induction hypothesis, considering the sequence $(\tr_d(\pi_n))\nN$.