Thing I Learned Today: The 3SAT Problem

by Brian Shourd on October 30, 2012

Tags: math

Today’s post is short: I learned about the 3SAT Problem. The term “3SAT” stands for “Boolean Satisfyability Problem where each clause contains 3 variables”. Here is the problem:

Given an expression of the form $$(y_{i_1} \vee y_{i_2} \vee y_{i_3}) \wedge (y_{i_4} \vee y_{i_5} \vee y_{i_6}) \wedge \ldots \wedge (y_{i_{n-2}} \vee y_{i_{n-1}} \vee y_{i_n})$$ where each $$y_{i_k}$$ is one of the variables $$x_1, x_2, \ldots, x_m$$ or $$\neg x_1, \neg x_2, \ldots, \neg x_m$$ (where $$\neg$$ is logical negation), is there a way to assign TRUE or FALSE to each $$x_i$$ so that the entire expression evaluates to TRUE?

This family of problems is NP-Complete, which informally means that they are really hard to solve in general. That is, all known algorithms for solving all 3SAT problems increase in the time they take to work quickly when more and more variables are added. A specific 3SAT problem might be really easy to solve, or even entire classes of 3SAT problems. But it is definitely not easy to solve all 3SAT problems with a single, generic, algorithm.