Logical NOR
In boolean logic, logical nor or joint denial is a truth-functional operator which produces a result that is the negation of logical or. That is, a sentence of the form (p NOR q) is true precisely when neither p nor q is true—i.e. when both of p and q are false. In grammar, nor is a coordinating conjunction.
The NOR operator is also known as Peirce's arrow — Charles Sanders Peirce introduced the symbol ↓ for it, and demonstrated that the logical NOR is completely expressible: by combining uses of the logical NOR it is possible to express any logical operation on two variables. Thus, as with its dual, the NAND operator (a.k.a. the Sheffer stroke — symbolized as either | or /), NOR can be used by itself, without any other logical operator, to constitute a logical formal system (making NOR functionally complete). It is also known as Quine's dagger (his symbol was †), the ampheck (from Greek αμφηκης, cutting both ways; compare amphi-) by Peirce, or "neither-nor".
One way of expressing p NOR q is
, where the symbol
signifies OR and the bar signifies the negation of the expression under it: in essence, simply
. Other ways of expressing p NOR q are Xpq, and
.