# Tag Archives: QBF

## Pre-Partitioned Truth Assignment

The next game in the appendix is “Sift”, but the Schaefer paper we’ve been using has that reduction depend on two other problems, so we’ll do those first.   Schaefer calls this problem “G%free(CNF)”, I made up the name we’re using.

The problem: Pre-Partitioned Truth Assignment.  This problem is not in the appendix.

The description: Given a CNF formula A, where the variables in A are partitioned into two equal size sets, V1 and V2.   On player 1’s turn player 1 chooses a variable in V1 and chooses to make it true or false,  On player 2’s turn, player 2 chooses a variable in V2 and chooses to make it true or false.  Player 1 wins if the variable assignments made by both players make A true.  Does A have a forced win?

Example: Here’s the equation from last time: {(x1 ∨ x2 ∨ x3), (~x1 ∨~x2∨ ~x3), (~x4, x1, ~x2)}  Lets say that V1 = {x1, x2} and V2 = {x3. x4}.  Then player 1 can force a win by setting x2 to false on their first turn, making the second and third clauses true.  Since player 2 can only set x3 or x4 on their turn, they can’t stop player 1 from setting x1 to true on their next turn, making all clauses true.

Here’s another example where player 1 loses: {(x1  ∨ x3 ∨ x4), (~x1 ∨ x3 ∨ x4), (x2 ∨ ~x3 ∨ ~x4).  If we again have  V1 = {x1, x2} and V2 = {x3. x4}, then player 2 wins by setting both x3 and x4 to false because no matter what player 1 does, they have to set x1 somehow, which will make one of the first two clauses false.

Reduction: (This problem is Theorem 3.8 in Schaefer’s paper).  Just like in most of the reductions in the Schaefer paper, we start with an instance of QBF where we have variables x1 through xn (n is assumed to be even), and the formula alternates with “There exists” (starting with x1, on all of the odd-numbered variables up through xn-1) and “For all” (starting with x2, on all of the even-numbered variables up through xn) quantifiers on the variables, leading into a CNF formula A0.  We need to build up an instance of Pre-Partitioned Truth Assignment by creating a new formula A’ and the variable sets V1 and V2.  For each variable in the QBF formula xi, our formula has 3 variables: xi, yi, and zi.  V1 holds xi, yi, and zi+1 when i is odd, and V2 holds xi, yi, and zi-1 if  i is even.

The formula is pretty complicated but is built to force zi to be set by one player to the same value of either xi or yi by the other player (and so must be set after the corresponding xi or yi is chosen).  The optimal move will be to play zi immediately after the corresponding xi or yi, forcing the choice back on the other player.  So that player will play the other of xi or yi (rather than open up some other variable’s x or y, only to be responded with by the corresponding z).   Then it will be the other player’s turn to choose the next xi+1 or yi+1.

The values of xi correspond to the literals in the original QBF formula, so assuming players play “correctly”, player 1 takes the role of the “there exists” player in the QBF formula, and player 2 takes the role of the “for all” player in the formula.  And if there is a way to set the x variables in the new formula to make it satisfiable, then there is a way to make the original QBF formula satisfiable.

Difficulty: 9.  For this problem, even Schaefer admits the proof is hard to follow, so he spends a page explaining what the goal of the construction is before diving into the proof.  I do like the reappearance of the “triples” of moves where a meaningful decision by a player is followed by 2 forced moves, making the next meaningful decision be placed on the other player.

## Generalized Kayles

I hadn’t heard of this game before encountering this problem.  It sort of feels a lot like Nim to me.

The problem: Generalized Kayles.  This is problem GP3 in the appendix.

The description: Given a graph G=(V,E). Players take turns removing vertices and all vertices adjacent to it from the graph.  The first player to have no vertices left loses.

Example: Here’s a graph:

Suppose player 1 chooses vertex c.  Then we also remove s,b, d, and t.  All that is left are vertices a and e.  Whichever vertex player 2 chooses, player 1 can choose the other one, and win.

Reduction: This one is again by Schaefer, and again uses the same problem which is either Sequential Truth Assignment or QBF, depending on how you look at it.

Just like last time, we’re given a formula: (∃ x1) (∀ x2) (∃ x3) … (∃ xn) (A1 ∧ A2 ∧ … Am), where each Ai is a disjunction of literals.  We’ll further assume n is odd (so the last quantifier is ∃).  The graph is built as follows:

• Each clause k in the formula gets a vertex x0,k.  These vertices are all connected to each other.
• Each variable xi in the formula gets two vertices: xi and ~xi, that have an edge between them. We also get y vertices: yi,j for all j 0 <= j < i
• We add an edge between xi and x0,k if xi appears as a literal in clause k.  Similarly, we add an edge between ~xi and x0,k if ~xi appears as a literal in clause k.
• Each yi,j vertex connects to all xk vertices where k <= i.  Each yi,j vertex also connects to all ya,b vertices where a < i and b < a.

Here’s a picture from the paper of what we get:

One main point that comes out of this construction is that players need to take their first n moves playing the x (or ~x) vertices in order.  If you go out of order, the opponent can play a y vertex and remove everything.  If you play a y vertex, the opponent can play an x vertex (or an x0 vertex) and remove everything.

If the original formula was satisfiable, player 1 (who is the ∃ player) starts by choosing either x1 or ~x1.  No matter which of x2 or ~x2 is chosen by the opponent (the ∀ player), player3 will have a way to set x3 to keep the formula satisfiable.  This process continues for all variables.  Once player 1 plays the last variable (xn or ~xn), all vertices have been removed from the graph- most importantly, all of the x0 vertices have been removed, because there is an edge from each one to each variable whose setting would satisfy the clause.  Thus, player 2 has no place to play, and player 1 wins.

If the formula is not satisfiable, then after taking turns choosing xi or ~xi for all i, there is still some x0 vertex in the graph (corresponding to a clause not satisfied by the variable choices made).  Player 2 can select that vertex, removing all x0 vertices from the graph, and will win.

Difficulty: 7.  I can see what the “jobs” of each vertex are: The xi  set the truth values of a variable, the x0 ensure that each clause is satisfied by a variable setting, and the y vertices are there to force players to choose the xi vertices in order.  I don’t think I could have come up with this though.

## Generalized Geography

It’s spring break here- I was expecting to take the week off, but got this post done anyway.  This might mean I skip next week though.

The problem: Generalized Geography.  This is problem GP2 in the appendix.

The description: Given a directed graph G=(V,A) and a starting vertex vo.  Players alternate choosing edges that leave the current vertex (starting with v0.  The next current vertex is the one at the end of the edge leaving from vo.  You can’t choose an edge that is already chosen.  The first player to be unable to choose an edge loses.  Does Player 1 have a forced win?

Example: This is the “geography” game kids play in the car, where you have to think of a place that has as its first letter the last letter of the previous choice.  As a graph problem, vertices correspond to letters, and edges to locations:

Note that the vertices can have self-loops, and (at least in the actual game) could have multiple edges between pairs of vertices.  There is no requirement to have any edges leave a vertex either- I remember instituting the “Phoenix rule”  on car trips because nobody could think of a place that started with X.

Anyway, if we start at E, player 1 has a forced win- they have to take “Egypt” to T, and then player 2 has to take “Texas” to S, and if player 1 chooses “Singapore” we’re in vertex E, it’s player 2’s turn, and they have no place left to pick.  (The actual game has many more edges, of course)

Reduction:  Schaefer says he’s building off of the Sequential Truth Assignment problem from last time, but his instance looks more like an instance of QBF. (I think this is a result of his claim that they’re basically the same problem).  So we’re given a formula: (∃ x1) (∀ x2) (∃ x3) … (∃ xn) (A1 ∧ A2 ∧ … Am), where each Ai is a disjunction of literals.  We’ll further assume n is odd (so the last quantifier is ∃)

We then go on to build a graph.  Each positive and negative literal gets a vertex.  Each variable x1 gets 4 vertices in the graph:

• xi corresponding to a positive literal
• ~xi corresponding to a negative literal
• 2 vertices u and v that control the “entrance” and “exit” for the setting of one of the 2 literal values.

We have a vertex for each clause (yi), and an additional vertex un+1 so the last vi has someplace to go.

Each set of these 4 vertices are set up in a diamond, with edges from ui to both xi and ~xi, and then from both of those to vi. vi then connects to ui+1, making a chain of diamonds. The final un+1 connects to each y vertex, and each y vertex connects to the literals that are in its clause.

Here’s the example picture from the paper:

Player 1 starts on vertex u1. The player chooses whether to make variable x1 positive or negative by going to that vertex (simulating a “there exists”), and player 2 has no choice but to move on to the v1 vertex, and player 1 has no choice to move to the next u2 (simulating a “for all”). This alternating process continues until we hit vertex un+1.  Since there are an odd number of variables, it is now player 2’s turn.

If the setting of the variables has not satisfied some clause, player 2 can move to the y vertex of that unsatisfied clause.  Then player 1 has to go to one of the 3 literals that appear in that clause.  Since the clause was not satisfied, all of these variables have not been chosen, so the edge from that literal to the v vertex is available for player 2 to pick.  But after that, the vertex from the v vertex to the next u vertex has already been chosen, so player 1 loses.

If the setting of the variables has satisfied every clause, then player 2 has to pick a move to a y vertex that has a literal chosen by a player to satisfy it. When player 1 moves to that vertex, the only edge exiting that vertex has already been chosen, so player 2 loses.

Difficulty: 7. I think this is very slick and elegant, but I don’t see a student getting there without lots of help.

## Sequential Truth Assignment

A whole bunch of the proofs in this section are done in the same paper and mainly use this problem.  So we’ll go out of order here- but really this is a restatement of the QBF problem from a couple of weeks ago.

The problem: Sequential Truth Assignment.  This is problem GP4 in the appendix.

The description: Given a set of variables and a collection of clauses (like a Satisfiability instance), players take turns giving truth assignments to sequential variables.  Player 1 wins if and only if all clauses are satisfied.  Does player 1 have a forced win?

Example: (x1 ∨ x2 ∨ x3) ∧ (~x1 ∨ ~x2 ∨ ~x3) ∧ (x1 ∨ ~x2 ∨ x3).

Player 1 can win by choosing x1 = true, which satisfies both the first and third clauses.  Then layer 2 has to set x2, so should set it to true (otherwise clause 2 is automatically satisfied). but player 1 then set x3 to false satisfying the second clause.

Reduction: The paper by Shaefer that has many of the reductions we’ll be doing in this section refers to the paper by Stockmeyer and Meyer that showed how QBF was NP-Complete (or worse- you might want to check out the comments of the QBF problem for a discussion) defined a formula Bk to be a formula on k sets of variables, with sets alternating between “For All” and “There Exists”.  Schaefer points out that if you allow players to assign values to these sets of variables (instead of one at a time, like our problem describes), then player 1 is the “There Exists” player and player 2 is the “For All” player, and you have basically the same problem.

To get to our “one variable at a time” version of the problem, we need to build a problem instance with one variable per set.   We can start with an instance of 3SAT and since the variables are set sequentially, we set up the quantifiers to be ∃ x1 ∀ x2 ∃ x3, and so on.

Difficulty: 5. I think it depends on how hard you see the intermediate step of going from QBF to the alternating of setting groups of variables.  Knowing you have to break the reduction into 2 steps is a bit hard as well.

## Generalized Hex

This is the first of the problems in the “Games and Puzzles” section.  The way they express these problems as logical formulas are pretty neat.

The problem: Generalized Hex.  This is problem GP1 in the appendix.

The description: Given a graph G=(V,E) and two vertices s,t in V.  Players alternate coloring vertices (except for s and t) “blue” (by player 1) and “red” (by player 2).  Player 1 wins if a path of blue vertices from s to t ever exists, and player 2 wins if that becomes impossible.  Does Player 1 have a forced win?

G&J mark this problem as “not known to be in NP” (like many of these problems), and the paper by Even and Tarjan mentions that it’s likely that this problem is even harder than NP-Complete problems, since we may not even be able to solve it in polynomial time with a non-deterministic algorithm.

Example: Here’s a picture of the “standard” Hex game that I took from the Internet:

(from https://www.maths.ed.ac.uk/~csangwin/hex/index.html)

The hexagonal spaces are the vertices of the graph.  In the “standard” game, the vertices are only adjacent to their neighbors, but the generalized game allows for different kinds of connections.  In the “standard” game of hex, you can connect from any hex on one blue edge of the board to any hex on the other blue edge- we can add that in the generalized game by making an edge from every cell on one blue edge to s, and another set of edges from every cell on the other side to t.

The other big difference is that in the standard game, either blue has a path connecting the 2 blue sides or red has a path connecting the 2 red sides (so blue not being able to create a path means red has a path).  In the generalized game, there are no “red sides”, so all we care about is whether blue can create a path or not.

In the above image, blue does not have a win (because red has already won).  But if we look at a slightly different board:

If the hex marked green is blue instead of red, then blue has a forced win, no matter whose turn it is.  Either of the two cells next to the left blue edge will win the game for blue.

Reduction: As I alluded to last week, many of these problems reduce from QBF.  So the question is: How do we take an arbitrary formula and turn it into a game board that has a forced win for player 1 if and only if the formula is solvable?

To help us, Even and Tarjan create some subgraphs that have properties that will help us.  First is a tree where the root connects to s, and some leaves connect to t:

It turns out that if it’s blue’s turn, blue has a forced win if and only if all of the leaves connect directly to t: Your first move is the root (above s), and then each move after that you choose the child node that is the root of a subtree that red has not played in.

If there are leaves that are not connected to t, red can win by always picking a node in the subtree that is connected to t.  In the graph above, if blue picks the root, red would pick the left child, forcing blue to move right. Then red picks the right child of blue’s node, and then the left child of blue’s next move.

(If blue doesn’t pick the root on their first move, or if blue doesn’t pick a child of their last move, red can disconnect the whole tree.)

The way to think of this graph is that it is similar to “and”.

Here’s another tree structure:

This graph has one child of the root connecting directly to t, and otherwise sets pairs of grandchildren that may or may not connect directly to t- all of the a’s connect to t, and the b’s may or may not connect to t.  If it’s blue’s turn, they have a forced win if and only if there is at least one b vertex that connects to t.  (Blue picks the root, red has to pick the child that connects directly to t, then blue picks the child that has both a and b connect to t, and red can’t stop it).

The way to think of this graph is “or”.  (The caption in the figure in the paper says this represents a “conjunction” which I’m pretty sure is a typo).

The third graph is based on variables:

There are 2 copies of each variable and their negation.  If it’s blue’s move, they can pick either x1(1) or x1(2) or their negations.  Then red has to pick the negated vertex of the opposite number (if blue picks x1(1) then red has to pick ~x1(2)).  This will be used to force a variable to only have one truth value.  If it’s red’s move, they can decide whether to use the positive or negated version of x1 and blue’s moves are forced.  So, if it’s blue’s move when this ladder is reached, it works like a “there exists”- blue can choose any combination of literals they want.  If it’s red’s move, this works like a “for all”- red can force blue into whatever variable configuration that they want.

So we’re given a QBF formula. We’ll assume the expression part of the formula is in CNF.  We create a graph that has s,t, 4 vertices for each variable (xi(1), xi(2), ~xi(1) and ~xi(2) for each variable xi).  For each change in quantifier (between ∀ and ∃) we add a “quantifier” vertex qi .  We can assume the first quantifier in the formula is a ∀, so the first quantifier vertex is placed in such a way that blue needs to play on it (effectively wasting its turn and giving red the initiative to select the first part of the next set of vertices).  If the next quantifier is a ∃, we add the quantifier vertex such that blue will reclaim the initiative

For each clause, we add an “and” tree, where the “b” leaves of the tree connect to the corresponding literals of the clause.

Here is the paper’s example for a small formula:

Some things I noticed to help me understand what is happening:

• The formula starts with ∃, so blue can pick either x(1) or ~x(1).
• The formula then moves into a ∀, so blue will have to pick q1, meaning red gets to decide whether blue will pick y(1) or ~y(1)
• The formula then moves into a ∃, which means after blue’s choice, red has to pick q2, giving blue the choice about z(1) or ~z(1)
• The l vertices denote the subtrees.  They are built out of the trees discussed earlier, and the paths through the “b” vertices (the one next to the “a” vertices) connect to the literals that are in each clause.

There are a lot more details about how it all gets exactly connected, but I think this shows the main idea of how the game is set up.

Difficulty: 8.  This is a very cool transformation, but you need a lot of help to see the structures and forcing moves.  On top of that, there are a lot of fiddly details that I’ve glossed over that are important in an actual reduction.

## Quantified Boolean Formulas

The next section in the appendix is the “Games and Puzzles” section.  But many of the reductions refer to this problem, so we’ll do this first.

The problem: Quantified Boolean Formulas (QBF).  This is problem LO11 in the appendix.

The description: Given a set U {u1..un} of variables, a formula F that consists of a quantifier Qi for each ui that is either ∀ or ∃, and a boolean expression E bound to the quantifiers, is F true?

Example: ∀ u1 ∀ u2 ∃ u3  (u1 ∨ u2 ∨ u3).  This is true because for each value of u1 and u2, we can find a value for u3 (namely “true”) that makes the expression true.

Reduction: G&J send you to the paper by Stockmeyer and Meyer that we did last week, and calls it a “generic transformation”, which means it builds a Turing Machine, similar to how Cook’s Theorem does.

But I think you can do a regular reduction from 3SAT: Given a SAT instance, we add a ∃ quantifier for each variable and then use the SAT formula as our E expression in this problem.  Then we’re just asking whether there exist values for each variable that make the formula true.

Difficulty: 3, assuming I’m not missing something.  I always worry when I find a “short cut” like this.