# Boolean Algebra

As the name suggests, **Boolean algebra** is algebra of 0 and 1, or FALSE and TRUE.

## Basic Operations

▪ AND \((x\wedge y)\) is 1 if both \(x\) and \(y\) are 1, otherwise 0. It is also known as conjunction. (In more intuitive terms, A AND B is true only if both A and B are true.)

▪ OR \((x\vee y)\) is 0 if both \(x\) and \(y\) are 0, otherwise 1. (This is an "inclusive or". In other words, A OR B is true if one or the other **or both** are true.)

▪ NOT \((\sim x)\) is 0 if \(x\) is 1, and 1 if \(x\) is 0. (This simply flips from true to false, so NOT true is false, and NOT false is true.

## Truth Tables

A truth table is a table which gives input and output of an operator for different values of \(x\) and \(y.\)

Truth table for AND: \(\begin{array} {| c | c | c | c |} \hline x & y & x\wedge y \\ \hline 0 & 0 & 0\\ \hline 0 & 1 & 0\\ \hline 1 & 0 & 0\\ \hline 1 & 1 & 1\\ \hline \end{array}\)

Truth table for OR: \(\begin{array} {| c | c | c | c |} \hline x & y & x\vee y \\ \hline 0 & 0 & 0\\ \hline 0 & 1 & 1\\ \hline 1 & 0 & 1\\ \hline 1 & 1 & 1\\ \hline \end{array}\)

Truth table for NOT: \(\begin{array} {| c | c | c | c |} \hline x & ~x \\ \hline 0 & 1\\ \hline 1 & 0\\ \hline \end{array}\)

Draw the truth table for \(\sim (x\vee y)\).

\(\sim (x\vee y)\) implies NOT \(x\vee y\).

So we will first draw the table for \(x\vee y,\) and then reverse the final outputs in the end.

\(\begin{array} {| c | c | c | c |} \hline x & y & x\vee y & \sim (x\vee y) \\ \hline 0 & 0 & 0& 1\\ \hline 0 & 1 & 1& 0\\ \hline 1 & 0 & 1& 0\\ \hline 1 & 1 & 1& 0\\ \hline \end{array}\)

## Laws

Associativity

\(x\vee (y\vee z)=(x\vee y)\vee z\)

\(x\wedge (y\wedge z)=(x\wedge y)\wedge z\)

Commutativity

\(x\vee y=y\vee x\)

\(x\wedge y=y\wedge x\)

Distributivity

\(x\vee (y\wedge z)= (x\vee y)\wedge (x\vee z)\)

\(x\wedge (y\vee z)=(x\wedge y)\vee (x\wedge z)\)

Identity

\(x\vee 0=x\)

\(x\wedge 1=x\)

Annihilator

\(x\vee 1=1\)

\(x\wedge 0=0\)

Idempotence

\(x\vee x=x\)

\(x\wedge x=x\)

Absorption

\(x\vee (x\wedge y)=x\)

\(x\wedge (x\vee y)=x\)

Complementation

\(x\vee \sim x=1\)

\(x\wedge \sim x=0\)

Double negation

- \(\sim (\sim x)=x\)

De Morgan's Laws

\(\sim x\vee \sim y=\sim (x\wedge y)\)

\(\sim x\wedge \sim y=\sim (x\vee y)\)

## Secondary operators

\( \begin{array} {| c | c | c | c | c |} \hline x & y & x\to y & x \oplus y & x \equiv y \\ \hline 0 & 0 & 1 & 0& 1\\ \hline 1 & 0 & 0 & 1& 0\\ \hline 0 & 1 & 1 & 1& 0\\ \hline 1 & 1 & 1& 0 & 1\\ \hline \end{array}\)

Material implication \(x \to y\) is defined as \(\sim x\vee y\): that is, \(y\) when \(x\) is true and \(x\) when \(x\) is false.

Exclusive Or (XOR) \(x\oplus y\) is defined as \((x\vee y)\wedge \sim (x\wedge y)\).

Equivalence (Boolean equality) \(x\equiv y\) is defined as \(\sim (x\oplus y)\). It is true when \(x\) and \(y\) are equal and false when they are not.

## Applications

While boolean algebra is used often in coding, it has its most direct application in logic circuits. In a circuit a 0 can be considered a circuit that is OFF and a 1 is a circuit that is ON. AND, OR, and NOT gates each have their own symbol.

The inputs are on the left side of the gate and the outputs are on the right side.