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Difference between revisions of "Involution"
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== Properties ==
 
== Properties ==
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* Function <math>f:X \rightarrow X</math> is an involution <math>\iff</math> <math>\forall x \in X</math> <math>f(x) = y \land f(y) = x</math>. This induces that both <math>(x,y)</math> and <math>(y,x)</math> are in f. By the definition of the inverse of a function, <math>\{ (y,x) | (x,y) \in f \}</math> is the inverse of the function f. Therefore, the function f must contain <math>f^{-1}</math>. From this, it is obtained that <math>f^{-1} \subseteq f</math>. Simmilalry, we can show that <math>f \subseteq f^{-1}</math>. Thus, <math>f = f^{-1}</math>.
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Rewriting the first line we have: function <math>f:X \rightarrow X</math> is an involution <math>\iff</math> <math>f = f^{-1}</math>.
 +
 +
 
* A function is an involution [[iff]] it is symmetric about the line <math>f(x)=x</math> in the coordinate plane.
 
* A function is an involution [[iff]] it is symmetric about the line <math>f(x)=x</math> in the coordinate plane.
  
 
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Revision as of 16:35, 8 September 2021

An involution is a function whose inverse is itself. From the perspective of set theory and functions, if a relation is a function and is symmetric, then it is an involution.


Examples

  • The function $y(x)=x$ has the inverse $x(y)=y$, which is the same function, and thus $f(x)=x$ is an involution.
  • The logical NOT is an involution because $\neg { \neg p} \equiv p$.
  • The additive negation is an involution because $--x=x$.
  • The identity function $I_x$ is an involution because $I_x:X \rightarrow X = \{(x,x) | x \in X\}$ therefore, $\forall (x,x) \in I_x$ $f(x) = x$ and $f(f(x)) = x$. Hence, it is an involution.
  • The multiplicative inverse is an involution because $\frac{1}{\frac{1}{x}}=x$. In fact, for any $n \neq 0, f(x)=\frac{n}{x}$ is an involution.

Properties

  • Function $f:X \rightarrow X$ is an involution $\iff$ $\forall x \in X$ $f(x) = y \land f(y) = x$. This induces that both $(x,y)$ and $(y,x)$ are in f. By the definition of the inverse of a function, $\{ (y,x) | (x,y) \in f \}$ is the inverse of the function f. Therefore, the function f must contain $f^{-1}$. From this, it is obtained that $f^{-1} \subseteq f$. Simmilalry, we can show that $f \subseteq f^{-1}$. Thus, $f = f^{-1}$.

Rewriting the first line we have: function $f:X \rightarrow X$ is an involution $\iff$ $f = f^{-1}$.


  • A function is an involution iff it is symmetric about the line $f(x)=x$ in the coordinate plane.

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