Logical NOR
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This article is about NOR in the logical sense. For the electronic NOR gates see NOR gate, for other uses of similar terms, see NOR (disambiguation).
The logical NOR or joint denial is a boolean logic operator which produces a result that is the inverse of logical or. That is, (not or), p NOR q is only true when both p and q are false.
The NOR operator is also known as Webb-operation or Peirce arrow, named after Charles Peirce who demonstrated that any logical operation can be expressed in terms of logical NOR. Thus, as with NAND operator, NOR can be used by itself, without any other logical operator, to constitute a logical formal system (making NOR functionally complete).
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[edit] Definition
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.
The truth table of p NOR q (also written as p ⊥ q or p ↓ q) is as follows:
| p | q | p ↓ q |
|---|---|---|
| F | F | T |
| F | T | F |
| T | F | F |
| T | T | F |
One way of expressing p NOR q is 
, where the symbol 
signifies OR and the bar over the expression signifies the negation of the expression under the bar.
[edit] Joint Denial
NOR has the interesting feature that all other logical operators can be expressed by various functions of NOR.
| "not p" is equivalent to "p NOR p" |  |
| "p and q" is equivalent to "(p NOR p) NOR (q NOR q)" |  |
| "p or q" is equivalent to "(p NOR q) NOR (p NOR q)" |  |
| "p implies q" is equivalent to "((p NOR q) NOR q) NOR ((p NOR q) NOR q)" |  |
The logical NAND operator also has this ability to express all logical operations.
The computer used in the spacecraft that first carried humans to the moon, the Apollo Guidance Computer, was constructed entirely using NOR gates with three inputs.
