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De Morgan’s laws

A pair of tautologies that capture an important relationship between

where

\[\begin{align*}
  & \lnot (P \land Q) \iff \lnot P \lor \lnot Q \\
  \qand & \lnot (P \lor Q) \iff \lnot P \land \lnot Q.
\end{align*}
\]

• Proof