Let $x,y$ be two propositions. In the semantics of propositional logic, the following laws hold for the conjunction "$\wedge$", disjunction "$\vee$" and negation "$\neg$
":
$$\begin{array}{c}\neg(x\vee y)=(\neg x)\wedge (\neg y),\\\neg(x\wedge y)=(\neg x)\vee (\neg y).\end{array}$$
This law is named after Augustus De Morgan (1806 - 1871).
Proofs: 1
Proofs: 1