# NotLink

A **NotLink** is a type of Link that can be used to invert the truth value of it's target.

During pattern matching, the AbsentLink can be used to indicate absence. Although AbsentLink is conceptually equivalent to (NotLink (PresentLink ...)), in practice, the NotLink cannot be used in this way, as it requires a negative confirmation, which is hard to do. There are some deep theoretical reasons for this; see in particular the Wikipedia article as well as the Stanford article on intuitionistic logic.

See also this page for some more information.

## PLN Semantics

In PLN **NotLink** corresponds to the negation of a concept or a predicate, formally

NotLink <TV> A

with

**Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://api.formulasearchengine.com/v1/":): {\displaystyle TV.s = \frac{\sum_x f_{\neg}(A(x).s) \times x.s}{\sum_x x.s}}**

Where **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://api.formulasearchengine.com/v1/":): {\displaystyle f_{\neg}}**
denotes the function associated with the fuzzy connector **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://api.formulasearchengine.com/v1/":): {\displaystyle \neg}**
, usually **Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://api.formulasearchengine.com/v1/":): {\displaystyle f_{\neg}(x)=1-x}**
.