Boolean satisfiability is prototypical $NP$ -complete problem.

There is no algorithm deciding $SAT$ in worst case in polynomial time unless $P=NP$ .

We always assume that $P\neq NP$ .

For $SAT$ problem, SOTA algorithm uses time $2^{(1-\frac{1}{O(\log(m/n))})n}m^{O(1)}$ .

For special case of $kSAT$ , there are faster algorithms, but all take time $2^{(1-\frac{c}{k})n}$ for some constant $0\lt c\lt k$ .

Schöning’s kSAT Algorithm

该算法在上学期随机算法课最后一节课马尔可夫链和随机游走中讲过，故此处略过。

分析过程很恶心。

The Sparsification Lemma

🎯Lemma 22. For all $\epsilon\gt0$ and integers $k$ , there exists a constant $C=C(\epsilon,k)$ and an algorithm that given as input a $kCNF$ $\Phi$ in $n$ variables runs in time $tn^{O(1)}$ , where $t\le2^{\epsilon n}$ , and give as output a list of $kCNF$ formulas $\Psi_1,\cdots,\Psi_t$ each with at most $Cn$ clauses such that

\Phi(x)=\bigvee_{i=1}^t\Psi_i(x),

for all $x\in\{0,1\}^n$ .

Reduce to Hitting Set Problem

Recall $HittingSet$ problem. Consider a universe $U$ of elements and a collection $\mathcal S$ of subsets of $\mathcal U$ . We say that $T\subseteq\mathcal U$ is a hitting set for $\mathcal S$ if $S\cap T\neq\empty$ for all $S\in\mathcal S$ . We denote by $\sigma(\mathcal S)$ the collection of all hitting sets of $\mathcal S$ .

$kHittingSet$ problem:

Instance: Collection $\mathcal S$ of subsets of a universe $\mathcal U=2^U$ .
Output: Hitting set $T$ for $\mathcal S$ minimizing $|T|$ .

We denote by $kHittingSet$ the restriction to collections of subsets of size at most $k$ .

We can transform $kSAT$ into $kHittingSet$ problem:

$\mathcal C$ is the subsets of $\mathcal U$ corresponding to the clauses of $\Phi$ .
An assignment to the variables of $\Phi$ can be viewed as a subset of $\mathcal U$ .
$\mathcal A$ is the collection of the $n$ sets $\{x_i,\overline{x_i}\}$ for $i=1,\cdots,n$ . Note that assignments to the variables of $\Phi$ are in one-to-one correspondence to hitting sets of $\mathcal A$ .
Defining $\mathcal S_\Phi=\mathcal C\cup\mathcal A$

Observation: There is a one-to-one correspondence between the set of satisfying assignments of $\Phi$ and hitting sets of $\mathcal S_\Phi$ of size $n$ .

Any collection of subsets $\mathcal T$ over $\mathcal U$ can be viewed as a CNF formula by letting each subset define a clause.

📖Definition 57. Let $\mathcal T$ and $\mathcal S$ be collections of subsets. We say that $\mathcal T$ is a restriction of $\mathcal S$ if for every $S\in\mathcal S$ there exists $T\in\mathcal T$ s.t. $T\subseteq S$ .

也就是集合的集合 $\mathcal S$ 中的每一个元素（是一个集合）是集合的集合 $\mathcal T$ 中对应有一个元素（是一个集合）的子集。

~~很绕，不是吗？~~

Clearly, if $\mathcal T$ is a restriction of $\mathcal S$ , then $\sigma(\mathcal T)\subseteq\sigma(\mathcal S)$ . Any satisfying assignment of CNF formula given by $\mathcal T$ is also a satisfying assignment of the CNF formula given by $\mathcal S$ .

🤔Lemma 23 (Hitting set correspondence). For all $\epsilon\gt0$ and integers $k$ , there exists a constant $C=C(\epsilon,k)$ and an algorithm that given as input collection $\mathcal S$ of subsets of size at most $k$ of a universe $\mathcal U$ of size $n$ runs in time $tn^{O(1)}$ , where $t\le 2^{\epsilon n}$ , and give as output a list of restrictions $\mathcal T_1,\cdots,\mathcal T_t$ each with at most $Cn$ subsets of $\mathcal U$ which are all of size at most $k$ s.t.

\sigma(\mathcal S)=\bigcup_{i=1}^t\sigma(\mathcal T_i).

We call a subset of $\mathcal U$ of size $j$ : $j$ -set.

The algorithm is a branching algorithm where each leaf of the recursion tree will give a restriction of $\mathcal S$ .

To branch, the algorithm looks for a large collection of subsets of the same size that has a non-empty intersection, and branches on whether or not the common intersection will be hit by the hitting set. We define such collections of sets as flowers.

📖Definition 58. Let $S_1,\cdots,S_c$ be $j$ -sets.

We say that these sets are a flower if $H=\bigcap_{i=1}^cS_i\neq\empty$ .
We call $H$ the heart of the flower and sets $(S_1\diagdown H),\cdots,(S_c\diagdown H)$ the petals.
Note that all petals have the same size $j-|H|$ , which we denote as petal-size.

定义一朵花：一组大小为 $j$ 的集合，它们的交集为空。

花心：这组集合的交集。对应定义花心的大小。

花瓣：每个集合去除花心。对应定义花瓣的大小。

The algorithm will be controlled by integer parameters $2=\theta_0\lt\theta_1\lt\cdots\lt\theta_{k-1}$ . We make $\theta_i$ increase rapidly as a parameter of $i$ .

📖Definition 59. Let $S_1,\cdots,S_c$ be a flower of $j$ -sets and petal-size $i$ .

The flower is large if $c\ge\theta_i$ .
The flower is good if
- it is a large flower chosen s.t. $j$ is chosen minimally
- and if more than one large flower of $j$ -sets exists then chosen s.t. $i$ is chosen minimally.

如果一朵花含有集合的个数足够大，则称这朵花是“大”的。

如果一朵花是“大”的，并且其中包含的集合大小、花瓣大小都是最小的，则称这朵花是“好”的。

~~很形象的定义，但还是很绕~~

The algorithm will eliminate sets of a collection $\mathcal T$ that contain other sets of the collection. If $S,T\in\mathcal T$ are such that $S\subsetneq T$ , we say that $T$ is eliminated by $S$ .

Let $\pi(\mathcal T)$ denote the collection of minimal sets of $\mathcal T$ , i.e., the sets of $\mathcal T$ that are not eliminated by sets of $\mathcal T$ .

Pseudo code of Reduce Algorithm

Reduce $(\mathcal S)$

Input: Collection $\mathcal S$ of subsets of size at most $k$ of a universe $\mathcal U$ .

Output: Restrictions $\mathcal T_1,\cdots,\mathcal T_t$ of s.t. $\sigma(\mathcal S)=\bigcup_{i=1}^t\sigma(\mathcal T_i)$ .

$\mathcal S\gets\pi(\mathcal S)$ .
If $\mathcal S$ has no large flower, return $\{\mathcal S\}$
Pick good flower $S_1,\cdots,S_c$ with heart $H=\bigcap_{i=1}^cS_i$ .
return Reduce $(S\cup\{H\})\bigcup$ Reduce $(\mathcal S\cup\{S_1\diagdown H,\cdots,S_c\diagdown H\})$ .

Correctness of Reduce

🤔Lemma 24. If Reduce $(\mathcal S)=\{\mathcal T_1,\cdots,\mathcal T_t\}$ then $\sigma(\mathcal S)=\bigcup_{i=1}^t\sigma(\mathcal T_i)$ .

Each $\mathcal T_i$ is a restriction of $\mathcal S$ . Thus $\sigma(\mathcal T_i)\subseteq\sigma(\mathcal S)\forall i$ .
Note that $\sigma(\pi(\mathcal T))=\sigma(\mathcal T)$ for any collection $\mathcal T$ . It follows that eliminating sets from $\mathcal S$ does not eliminate any hitting sets of $\mathcal S$ .
Suppose that $T$ is a hitting set of $\mathcal S$ and let $S_1,\cdots, S_c$ be a flower of $\mathcal S$ with heart $H$ . If $T\cap H\neq\empty$ , then $T$ is a hitting set of $S\cup\{H\}$ . If $T\cap H=\empty$ then $T$ must be a hitting set of $\mathcal S\cup\{S_1\diagdown H,\cdots,S_c\diagdown H\}$ . Thus any hitting set of $\mathcal S$ is preserved in at least one recursive branch.

It’s easy to see that Reduce always terminates.

It remains to establish qualitative properties of Reduce: Every restriction output by Reduce is sparse, i.e., contains at most $Cn$ sets, for a constant $C=C(k,\epsilon)$ .

🤔Lemma 25. Let $C=\sum_{i=1}^k(\theta_{i-1}-1)$ . Every restriction $\mathcal T$ output by Reduce contains at most $(\theta_{i-1}-1)n$ many $i$ -sets, and therefore at most $Cn$ sets in total.

Prove by contradiction. Use pigeonhole principle.

The number of restriction output by Reduce should be bounded by the number of leaves $t$ in the recursion tree.

🤔Lemma 26. Let $0\lt m\lt n$ be integers. It holds that

\sum_{i=1}^m{n\choose i}\le2^{H(\frac{m}{n})\cdot n}.\\ H(p)=p\log_2(1/p)+(1-p)\log_2(1/(1-p))

🤔Corollary 14. A binary tree of depth $h$ in which every root-to-leaf path follows the right child at most $\gamma h$ times has at most $2^{H(\gamma)h}$ leafs.

Proof by Lemma 26.

When $\gamma\le1/2$ we have $H(\gamma)\le2\gamma\log_2(1/\gamma)$ , thus we use the simpler upper bound $2^{(2\gamma\log_2(1/\gamma))h}$ on the number of leaves.

🤔Lemma 27. If Reduce $(\mathcal T)$ recursively calls Reduce $(\mathcal T')$ by adding an $i$ -set to $\mathcal T$ , then $\mathcal I_i(\mathcal T')$ holds. Furthermore in all recursive calls Reduce $(\mathcal T'')$ that follows, the invariant $\mathcal I_i(\mathcal T'')$ holds.

~~没咋看懂这个引理~~

🤔Lemma 28. Any set $S$ can eliminate at most $2(\theta_{i-1}-1)$ of the added $i$ -sets.

🤔Lemma 29. Let $1\lt i\lt k$ and suppose $\beta_{i-1}$ is s.t. at most $\beta_{i-1}n$ sets of size at most $i-1$ that are added along a given path of the recursion tree of Reduce. Then the number of $i$ -sets added along that path is at most $(2\beta_{i-1}+1)(\theta_{i-1}-1)n$ .

🤔Corollary 15. At most $\beta_in$ sets of size at most $i$ are added along any path of the recursion tree of Reduce, for all $1\le i\lt k$ .

🤔Lemma 30. Along any path of the recursion tree of Reduce at most $\beta_in/\theta_i$ families are created by adding petals of size $i$ for every $1\le i\lt k$ .

We set $\theta_i$ in such a way that the ratio $\theta_i/\beta_i$ is equal to an integer constant $\alpha$ , to be fixed later, for $1\le i\le k$ . Thus

\beta_i=4\beta_{i-1}\theta_{i-1}=4\alpha(\beta_{i-1})^2\\ \Rarr \beta_i=(4\alpha)^{2^{i-1}-1},\theta_i=\alpha(4\alpha)^{2^{i-1}-1}\forall 1\le i\le k

🤔Corollary 16. Along any path of the recursion tree of Reduce at most $kn/\alpha$ families are created by adding petals.

~~让我缓缓~~

The Exponential Time Hypothesis

A stronger hypothesis than $P\neq NP$ : Exponential time is required to decide $SAT$ .

Exponential time hypothesis (aka ETH): $kSAT$ requires worst-case linear exponential time, for all $k\ge3$ .

A strengthening of this hypothesis, strong exponential time hypothesis: it is not possible to save an exponential factor $2^{\epsilon n}$ for a fixed $1\gt\epsilon\gt0$ independent of $k$

📖Definition 60. Define $s_k=\inf\{\delta|kSAT\text{ is solvable in time }O(2^{\delta n}\}$ . The ETH is the statement that $\forall k\ge3:s_k\gt0$ , and SETH is the statement that $s_{\infty}=\lim_{k\to\infty}s_k=1$ .

🤔Proposition 20. For any $\epsilon\gt0$ and integer $k$ there is $C\gt0$ s.t. if there is an algorithm $A$ for solving $3SAT$ in time $2^{\delta n}n^{O(1)}$ there is an algorithm $B$ for $kSAT$ running in time $O(2^{(\epsilon+\delta kC)n}n^{O(1)})$ .

Input: $kCNF:\Phi$ on $n$ variables. the algorithm $B$ first invokes the algorithm of the Lemma 22 to compute $kCNF$ formulas $\Psi_1,\cdots,\Psi_t$ in time $tn^{O(1)}$ , where $t\le2^{\epsilon n}$ , and each $\Psi_i$ has hat most $Cn$ clauses.
$\Phi$ is satisfiable iff at least one of the formula $\Psi_i$ are satisfiable.
$B$ may apply the standard reduction from $kSAT$ to $3SAT$ , obtaining $3CNF$ formula $\Psi_1',\cdots,\Psi_t'$ , each having at most $n+Cn(k-3)\le kCn$ variables.
Using algorithm $A$ each $\Psi_i'$ can be checked for satisfiability in time $2^{\delta(kCn)n^{O(1)}}$ . This together means that $B$ solves $kSAT$ in time as claimed.

🤔Corollary 17. The following holds:

ETH is equivalent to $s_3\gt0$ .
SETH implies ETH.

🤔Proposition 21. ETH is equivalent to the existence of $\delta\gt0$ s.t. no algorithm can solve $3SAT$ in time $2^{\delta(n+m)}n^{O(1)}$ , where $n$ is the number of variables and $m$ the number of clauses.

Consider the independent set problem:

Instance: Graph $G=(V,E)$ , integer $K$ .
Question: Does $G$ have an independent set of size at least $K$ ?

🎯Theorem 60. If ETH holds, then there exists $\delta\gt0$ s.t. no algorithm can solve the independent set problem in time $2^{\delta n}n^{O(1)}$ , where $n$ is the number of vertices of the given graph $G$ .