Cook-Levin Theorem

Theorem
The Boolean Satisfiability Problem is NP-Complete.

The Boolean Satisfiability Problem is NP
Given a Boolean Satisfiability Problem with a set of variables $X$ and  clauses $L$ and a possible solution to the problem, it is a trivial  matter evaluate all the clauses in $L$ to verify the solution in  polynomial time. Because a potential solution can be verified or rejected in polynomial time it is NP.

All NP-complete problems are polynomially reducible to the Boolean Satisfiability problem
The objective is, given a non-deterministic Turing Machine $M$ with $k$  internal states and an input $x$, to construct a set of variables and  clauses that has a solution iff $M$ accepts the input $x$.

From the definition of NP we know that $M$ either accepts  the input $x$ within $p(|x|)$ steps, or it does not accept the input at  all, where $p$ is some polynomial.

Because a Turing Machine cannot use more squares of memory  then the number of steps that it runs, we only need to concern  ourselves with the first $p(|x|)$ squares of memory. Let $\Sigma$ denote the finite alphabet that the machine recognizes.

Let the variables for the Boolean Satisfiability Problem be given by

And let the clauses for the Boolean Satisfiability Problem be given by

A production rule in a non-deterministic Turing machine can be written in the form
 * $(q_a, \alpha) \to (q_b , \beta , D_1) \lor (q_c , \gamma , D_2)$

meaning if the machine is in state $q_a$ and reading $\alpha$ on the tape  either replace $\alpha$ with $\beta$ move one square in direction $D_1$  (either left or right) and change the internal state to $q_b$ or replace  $\alpha$ with $\gamma$ move one square in the $D_2$ direction and go  to internal state $q_c$. If $D_1$ is left and $D_2$ is right then this rule would translate to
 * $(q_a \land T_{m \ \alpha \ n} \land H_{m \ n}) \implies ((q_b \land T_{m \ \beta \ n+1} \land H_{m-1 \ n+1})\lor (q_c \land T_{m \ \gamma \ n+1} \land H_{m+1 \ n+1}) $

If the rule in the machine allows for more then two choices then this rule can be modified by adding more triplets to the right hand side of the implication rule. The length of this clause is determined by the number of choices that $M$ gives for a given internal state and a given input. Because this number of choices is bounded for any given machine, the total space used for this group of clauses is $O(p(|x|)^2)$

In total, the size of the the Boolean Satisfiability problem is $O(p(|x|)^2)$, with the constant depending on $M$.

The conversion from a description of $M$ to the Boolean Satisfiability problem is straightforward and can be done in polynomial time.

The problem described has a solution iff $M$ accepts $x$ as its input. All NP problems are polynomially reducible to the Boolean Satisfiability problem. Therefore the Boolean Satisfiability is NP-hard.

The Boolean Satisfiability Problem is NP-complete.

The boolean satisfiability problem was thus the first problem known to be NP-complete.