# Propositional directed acyclic graph

A propositional directed acyclic graph (PDAG) is a data structure that is used to represent a Boolean function. A Boolean function can be represented as a rooted, directed acyclic graph of the following form:

• Leaves are labeled with ${\displaystyle \top }$ (true), ${\displaystyle \bot }$ (false), or a Boolean variable.
• Non-leaves are ${\displaystyle \bigtriangleup }$ (logical and), ${\displaystyle \bigtriangledown }$ (logical or) and ${\displaystyle \Diamond }$ (logical not).
• ${\displaystyle \bigtriangleup }$- and ${\displaystyle \bigtriangledown }$-nodes have at least one child.
• ${\displaystyle \Diamond }$-nodes have exactly one child.

Leaves labeled with ${\displaystyle \top }$ (${\displaystyle \bot }$) represent the constant Boolean function which always evaluates to 1 (0). A leaf labeled with a Boolean variable ${\displaystyle x}$ is interpreted as the assignment ${\displaystyle x=1}$, i.e. it represents the Boolean function which evaluates to 1 if and only if ${\displaystyle x=1}$. The Boolean function represented by a ${\displaystyle \bigtriangleup }$-node is the one that evaluates to 1, if and only if the Boolean function of all its children evaluate to 1. Similarly, a ${\displaystyle \bigtriangledown }$-node represents the Boolean function that evaluates to 1, if and only if the Boolean function of at least one child evaluates to 1. Finally, a ${\displaystyle \Diamond }$-node represents the complementary Boolean function its child, i.e. the one that evaluates to 1, if and only if the Boolean function of its child evaluates to 0.

## PDAG, BDD, and NNF

Every binary decision diagram (BDD) and every negation normal form (NNF) are also a PDAG with some particular properties. The following pictures represent the Boolean function ${\displaystyle f(x1,x2,x3)=-x1*-x2*-x3+x1*x2+x2*x3}$:

 BDD for the function f PDAG for the function f obtained from the BDD PDAG for the function f