Logical NOR

The NOR operation is a logical operation on two logical values, typically the values of two propositions, that produces a value of true if and only if both operands are false. In other words, it produces a value of false if and only if at least one operand is true.

Truth table

The truth table of ${\displaystyle P\downarrow Q}$ is as follows:

${\displaystyle P}$	${\displaystyle Q}$	${\displaystyle P\downarrow Q}$
True	True	False
True	False	False
False	True	False
False	False	True

Logical equivalences

The logical NOR ${\displaystyle \downarrow }$ is the negation of the disjunction:

${\displaystyle P\downarrow Q}$	${\displaystyle \Leftrightarrow }$	${\displaystyle \neg (P\lor Q)}$
	${\displaystyle \Leftrightarrow }$	${\displaystyle \neg }$

Peirce is the first to show the functional completeness of non-disjunction while he doesn't publish his result.[2][3] Peirce used ${\displaystyle {\overline {\curlywedge }}}$ for non-conjunction and ${\displaystyle \curlywedge }$ for non-disjunction (in fact, what Peirce himself used is ${\displaystyle \curlywedge }$ and he didn't introduce ${\displaystyle {\overline {\curlywedge }}}$ while Peirce's editors made such disambiguated use).[3] Peirce called ${\displaystyle \curlywedge }$ as ampheck (from Ancient Greek ἀμφήκης, amphēkēs, "cutting both ways").[3]

In 1911, Stamm was the first to publish a description of both non-conjunction (using ${\displaystyle \sim }$, the Stamm hook), and non-disjunction (using ${\displaystyle *}$, the Stamm star), and showed their functional completeness.[4]Zach, R. (2023-02-18). "Sheffer stroke before Sheffer: Edward Stamm". Retrieved 2023-07-02.</ref> Note that most uses in logical notation of ${\displaystyle \sim }$ use this for negation.

In 1913, Sheffer described non-disjunction and showed its functional completeness. Sheffer used ${\displaystyle \mid }$ for non-conjunction, and ${\displaystyle \wedge }$ for non-disjunction.

In 1935, Webb described non-disjunction for ${\displaystyle n}$-valued logic, and use ${\displaystyle \mid }$ for the operator. So some people call it Webb operator,[5] Webb operation[6] or Webb function.[7]

In 1940, Quine also described non-disjunction and use ${\displaystyle \downarrow }$ for the operator.[8] So some people call the operator Peirce arrow or Quine dagger.

In 1944, Church also described non-disjunction and use ${\displaystyle {\overline {\vee }}}$ for the operator.[9]

In 1954, Bocheński used ${\displaystyle X}$ in ${\displaystyle Xpq}$ for non-disjunction in Polish notation.[10]

Logical NOR does not possess any of the five qualities (truth-preserving, false-preserving, linear, monotonic, self-dual) required to be absent from at least one member of a set of functionally complete operators. Thus, the set containing only NOR suffices as a complete set.

NOR has the interesting feature that all other logical operators can be expressed by interlaced NOR operations. The logical NAND operator also has this ability.

Expressed in terms of NOR ${\displaystyle \downarrow }$, the usual operators of propositional logic are:

${\displaystyle \neg P}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle P\downarrow P}$ ${\displaystyle \neg }$     ${\displaystyle \Leftrightarrow }$		${\displaystyle P\rightarrow Q}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle {\Big (}(P\downarrow P)\downarrow Q{\Big )}}$ ${\displaystyle \downarrow }$ ${\displaystyle {\Big (}(P\downarrow P)\downarrow Q{\Big )}}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle \downarrow }$
${\displaystyle P\land Q}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle (P\downarrow P)}$ ${\displaystyle \downarrow }$ ${\displaystyle (Q\downarrow Q)}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle \downarrow }$		${\displaystyle P\lor Q}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle (P\downarrow Q)}$ ${\displaystyle \downarrow }$ ${\displaystyle (P\downarrow Q)}$     ${\displaystyle \Leftrightarrow }$     ${\displaystyle \downarrow }$

• Bitwise NOR
• Boolean algebra
• Boolean domain
• Boolean function
• Functional completeness
• NOR gate
• Propositional logic
• Sole sufficient operator
• Sheffer stroke as symbol for the logical NAND

• Media related to Logical NOR at Wikimedia Commons