Direct mapping and inverse mapping: when they can be equal?
$begingroup$
I read the following phrase from a textbook
Bijection $ f : M rightarrow M$ known as symmetry if $ f^{-1} = f$.
I am curious, are there any examples of $ f^{-1} = f$ outside of identity relation - $ id_X $ ?
relations inverse-function
$endgroup$
add a comment |
$begingroup$
I read the following phrase from a textbook
Bijection $ f : M rightarrow M$ known as symmetry if $ f^{-1} = f$.
I am curious, are there any examples of $ f^{-1} = f$ outside of identity relation - $ id_X $ ?
relations inverse-function
$endgroup$
$begingroup$
any sort of transposition will do. For example, let f($x_1,x_2,x_3$) = ($x_3,x_2,x_1$).
$endgroup$
– Joel Pereira
Jan 2 at 23:42
1
$begingroup$
$-x$ on $mathbb R$, $frac 1 x$ on $(0,infty)$. How many millions do you want?
$endgroup$
– Kavi Rama Murthy
Jan 3 at 0:05
add a comment |
$begingroup$
I read the following phrase from a textbook
Bijection $ f : M rightarrow M$ known as symmetry if $ f^{-1} = f$.
I am curious, are there any examples of $ f^{-1} = f$ outside of identity relation - $ id_X $ ?
relations inverse-function
$endgroup$
I read the following phrase from a textbook
Bijection $ f : M rightarrow M$ known as symmetry if $ f^{-1} = f$.
I am curious, are there any examples of $ f^{-1} = f$ outside of identity relation - $ id_X $ ?
relations inverse-function
relations inverse-function
edited Jan 3 at 0:52
Asaf Karagila♦
305k33435766
305k33435766
asked Jan 2 at 23:40
DaddyMDaddyM
1515
1515
$begingroup$
any sort of transposition will do. For example, let f($x_1,x_2,x_3$) = ($x_3,x_2,x_1$).
$endgroup$
– Joel Pereira
Jan 2 at 23:42
1
$begingroup$
$-x$ on $mathbb R$, $frac 1 x$ on $(0,infty)$. How many millions do you want?
$endgroup$
– Kavi Rama Murthy
Jan 3 at 0:05
add a comment |
$begingroup$
any sort of transposition will do. For example, let f($x_1,x_2,x_3$) = ($x_3,x_2,x_1$).
$endgroup$
– Joel Pereira
Jan 2 at 23:42
1
$begingroup$
$-x$ on $mathbb R$, $frac 1 x$ on $(0,infty)$. How many millions do you want?
$endgroup$
– Kavi Rama Murthy
Jan 3 at 0:05
$begingroup$
any sort of transposition will do. For example, let f($x_1,x_2,x_3$) = ($x_3,x_2,x_1$).
$endgroup$
– Joel Pereira
Jan 2 at 23:42
$begingroup$
any sort of transposition will do. For example, let f($x_1,x_2,x_3$) = ($x_3,x_2,x_1$).
$endgroup$
– Joel Pereira
Jan 2 at 23:42
1
1
$begingroup$
$-x$ on $mathbb R$, $frac 1 x$ on $(0,infty)$. How many millions do you want?
$endgroup$
– Kavi Rama Murthy
Jan 3 at 0:05
$begingroup$
$-x$ on $mathbb R$, $frac 1 x$ on $(0,infty)$. How many millions do you want?
$endgroup$
– Kavi Rama Murthy
Jan 3 at 0:05
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
For a set $M$, a function $f:Mto M$ such that $fcirc f=text{id}_M$ is usually known as an involution on $M$. There exists a one-to-one correspondence between the set $mathcal{I}$ of all involutions on $M$ and the set $mathcal{P}$ of partitions of $M$ into subsets of size $1$ or $2$. Such a one-to-one correspondence is given by $phi:mathcal{P}to mathcal{I}$ sending $Pinmathcal{P}$ to
$$f_P(x)=begin{cases}x&text{if }{x}in P,,\y&text{if }{x,y}in Ptext{ with }xneq y,.end{cases}$$
The inverse of $phi$ is $psi:mathcal{I}to mathcal{P}$ sending $finmathcal{I}$ to
$$P_f:=Big{{x},Big|,xin Mtext{ and }f(x)=xBig}cupBig{big{x,f(x)big},Big|,xin Mtext{ and }f(x)neq xBig},.$$
If $M$ is a finite set with $m$ elements and $a_m$ denotes the number of involutions on $M$, then $a_0=1$, $a_1=1$, and $$a_m=a_{m-1}+(m-1)a_{m-2}text{ for }m=2,3,4,ldots,.$$
If $M$ is an infinite set, then the cardinality of $mathcal{I}$ is $2^{|M|}$.
$endgroup$
add a comment |
$begingroup$
Here's a really simple example:
Let $X$ be a finite set with $vert X vert$ even; then number the elements of $X$ from $1$ to $2n$, $n in Bbb N$:
$X = {x_1, x_2, ldots, x_n, x_{n + 1}, x_{n + 2}, ldots, x_{2n} }; tag 1$
define
$f:X to X tag 2$
by
$f(x_i) = x_{i + n}, ; 1 le i le n, tag 3$
$f(x_i) = x_{i - n}, ; n + 1 le i le 2n; tag 4$
then since $n ge 2$,
$f ne text{Id}_X, tag 5$
but
$f(f(x_i)) = f(x_{i + n}) = x_i, ; 1 le i le n, tag 6$
$f(f(x_i)) = f(x_{i -n}) = x_i, ; n + 1 le i le 2n, tag 7$
so we see that
$f^2 = text{Id}_X; tag 8$
therefore, for $x in X$,
$f(f(x)) = x, tag 9$
whence
$f^{-1}(x) = f^{-1}(f(f(x)) = f(x). tag{10}$
$endgroup$
1
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
add a comment |
Your Answer
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2 Answers
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2 Answers
2
active
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$begingroup$
For a set $M$, a function $f:Mto M$ such that $fcirc f=text{id}_M$ is usually known as an involution on $M$. There exists a one-to-one correspondence between the set $mathcal{I}$ of all involutions on $M$ and the set $mathcal{P}$ of partitions of $M$ into subsets of size $1$ or $2$. Such a one-to-one correspondence is given by $phi:mathcal{P}to mathcal{I}$ sending $Pinmathcal{P}$ to
$$f_P(x)=begin{cases}x&text{if }{x}in P,,\y&text{if }{x,y}in Ptext{ with }xneq y,.end{cases}$$
The inverse of $phi$ is $psi:mathcal{I}to mathcal{P}$ sending $finmathcal{I}$ to
$$P_f:=Big{{x},Big|,xin Mtext{ and }f(x)=xBig}cupBig{big{x,f(x)big},Big|,xin Mtext{ and }f(x)neq xBig},.$$
If $M$ is a finite set with $m$ elements and $a_m$ denotes the number of involutions on $M$, then $a_0=1$, $a_1=1$, and $$a_m=a_{m-1}+(m-1)a_{m-2}text{ for }m=2,3,4,ldots,.$$
If $M$ is an infinite set, then the cardinality of $mathcal{I}$ is $2^{|M|}$.
$endgroup$
add a comment |
$begingroup$
For a set $M$, a function $f:Mto M$ such that $fcirc f=text{id}_M$ is usually known as an involution on $M$. There exists a one-to-one correspondence between the set $mathcal{I}$ of all involutions on $M$ and the set $mathcal{P}$ of partitions of $M$ into subsets of size $1$ or $2$. Such a one-to-one correspondence is given by $phi:mathcal{P}to mathcal{I}$ sending $Pinmathcal{P}$ to
$$f_P(x)=begin{cases}x&text{if }{x}in P,,\y&text{if }{x,y}in Ptext{ with }xneq y,.end{cases}$$
The inverse of $phi$ is $psi:mathcal{I}to mathcal{P}$ sending $finmathcal{I}$ to
$$P_f:=Big{{x},Big|,xin Mtext{ and }f(x)=xBig}cupBig{big{x,f(x)big},Big|,xin Mtext{ and }f(x)neq xBig},.$$
If $M$ is a finite set with $m$ elements and $a_m$ denotes the number of involutions on $M$, then $a_0=1$, $a_1=1$, and $$a_m=a_{m-1}+(m-1)a_{m-2}text{ for }m=2,3,4,ldots,.$$
If $M$ is an infinite set, then the cardinality of $mathcal{I}$ is $2^{|M|}$.
$endgroup$
add a comment |
$begingroup$
For a set $M$, a function $f:Mto M$ such that $fcirc f=text{id}_M$ is usually known as an involution on $M$. There exists a one-to-one correspondence between the set $mathcal{I}$ of all involutions on $M$ and the set $mathcal{P}$ of partitions of $M$ into subsets of size $1$ or $2$. Such a one-to-one correspondence is given by $phi:mathcal{P}to mathcal{I}$ sending $Pinmathcal{P}$ to
$$f_P(x)=begin{cases}x&text{if }{x}in P,,\y&text{if }{x,y}in Ptext{ with }xneq y,.end{cases}$$
The inverse of $phi$ is $psi:mathcal{I}to mathcal{P}$ sending $finmathcal{I}$ to
$$P_f:=Big{{x},Big|,xin Mtext{ and }f(x)=xBig}cupBig{big{x,f(x)big},Big|,xin Mtext{ and }f(x)neq xBig},.$$
If $M$ is a finite set with $m$ elements and $a_m$ denotes the number of involutions on $M$, then $a_0=1$, $a_1=1$, and $$a_m=a_{m-1}+(m-1)a_{m-2}text{ for }m=2,3,4,ldots,.$$
If $M$ is an infinite set, then the cardinality of $mathcal{I}$ is $2^{|M|}$.
$endgroup$
For a set $M$, a function $f:Mto M$ such that $fcirc f=text{id}_M$ is usually known as an involution on $M$. There exists a one-to-one correspondence between the set $mathcal{I}$ of all involutions on $M$ and the set $mathcal{P}$ of partitions of $M$ into subsets of size $1$ or $2$. Such a one-to-one correspondence is given by $phi:mathcal{P}to mathcal{I}$ sending $Pinmathcal{P}$ to
$$f_P(x)=begin{cases}x&text{if }{x}in P,,\y&text{if }{x,y}in Ptext{ with }xneq y,.end{cases}$$
The inverse of $phi$ is $psi:mathcal{I}to mathcal{P}$ sending $finmathcal{I}$ to
$$P_f:=Big{{x},Big|,xin Mtext{ and }f(x)=xBig}cupBig{big{x,f(x)big},Big|,xin Mtext{ and }f(x)neq xBig},.$$
If $M$ is a finite set with $m$ elements and $a_m$ denotes the number of involutions on $M$, then $a_0=1$, $a_1=1$, and $$a_m=a_{m-1}+(m-1)a_{m-2}text{ for }m=2,3,4,ldots,.$$
If $M$ is an infinite set, then the cardinality of $mathcal{I}$ is $2^{|M|}$.
edited Jan 4 at 0:42
answered Jan 3 at 1:19
BatominovskiBatominovski
33.1k33293
33.1k33293
add a comment |
add a comment |
$begingroup$
Here's a really simple example:
Let $X$ be a finite set with $vert X vert$ even; then number the elements of $X$ from $1$ to $2n$, $n in Bbb N$:
$X = {x_1, x_2, ldots, x_n, x_{n + 1}, x_{n + 2}, ldots, x_{2n} }; tag 1$
define
$f:X to X tag 2$
by
$f(x_i) = x_{i + n}, ; 1 le i le n, tag 3$
$f(x_i) = x_{i - n}, ; n + 1 le i le 2n; tag 4$
then since $n ge 2$,
$f ne text{Id}_X, tag 5$
but
$f(f(x_i)) = f(x_{i + n}) = x_i, ; 1 le i le n, tag 6$
$f(f(x_i)) = f(x_{i -n}) = x_i, ; n + 1 le i le 2n, tag 7$
so we see that
$f^2 = text{Id}_X; tag 8$
therefore, for $x in X$,
$f(f(x)) = x, tag 9$
whence
$f^{-1}(x) = f^{-1}(f(f(x)) = f(x). tag{10}$
$endgroup$
1
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
add a comment |
$begingroup$
Here's a really simple example:
Let $X$ be a finite set with $vert X vert$ even; then number the elements of $X$ from $1$ to $2n$, $n in Bbb N$:
$X = {x_1, x_2, ldots, x_n, x_{n + 1}, x_{n + 2}, ldots, x_{2n} }; tag 1$
define
$f:X to X tag 2$
by
$f(x_i) = x_{i + n}, ; 1 le i le n, tag 3$
$f(x_i) = x_{i - n}, ; n + 1 le i le 2n; tag 4$
then since $n ge 2$,
$f ne text{Id}_X, tag 5$
but
$f(f(x_i)) = f(x_{i + n}) = x_i, ; 1 le i le n, tag 6$
$f(f(x_i)) = f(x_{i -n}) = x_i, ; n + 1 le i le 2n, tag 7$
so we see that
$f^2 = text{Id}_X; tag 8$
therefore, for $x in X$,
$f(f(x)) = x, tag 9$
whence
$f^{-1}(x) = f^{-1}(f(f(x)) = f(x). tag{10}$
$endgroup$
1
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
add a comment |
$begingroup$
Here's a really simple example:
Let $X$ be a finite set with $vert X vert$ even; then number the elements of $X$ from $1$ to $2n$, $n in Bbb N$:
$X = {x_1, x_2, ldots, x_n, x_{n + 1}, x_{n + 2}, ldots, x_{2n} }; tag 1$
define
$f:X to X tag 2$
by
$f(x_i) = x_{i + n}, ; 1 le i le n, tag 3$
$f(x_i) = x_{i - n}, ; n + 1 le i le 2n; tag 4$
then since $n ge 2$,
$f ne text{Id}_X, tag 5$
but
$f(f(x_i)) = f(x_{i + n}) = x_i, ; 1 le i le n, tag 6$
$f(f(x_i)) = f(x_{i -n}) = x_i, ; n + 1 le i le 2n, tag 7$
so we see that
$f^2 = text{Id}_X; tag 8$
therefore, for $x in X$,
$f(f(x)) = x, tag 9$
whence
$f^{-1}(x) = f^{-1}(f(f(x)) = f(x). tag{10}$
$endgroup$
Here's a really simple example:
Let $X$ be a finite set with $vert X vert$ even; then number the elements of $X$ from $1$ to $2n$, $n in Bbb N$:
$X = {x_1, x_2, ldots, x_n, x_{n + 1}, x_{n + 2}, ldots, x_{2n} }; tag 1$
define
$f:X to X tag 2$
by
$f(x_i) = x_{i + n}, ; 1 le i le n, tag 3$
$f(x_i) = x_{i - n}, ; n + 1 le i le 2n; tag 4$
then since $n ge 2$,
$f ne text{Id}_X, tag 5$
but
$f(f(x_i)) = f(x_{i + n}) = x_i, ; 1 le i le n, tag 6$
$f(f(x_i)) = f(x_{i -n}) = x_i, ; n + 1 le i le 2n, tag 7$
so we see that
$f^2 = text{Id}_X; tag 8$
therefore, for $x in X$,
$f(f(x)) = x, tag 9$
whence
$f^{-1}(x) = f^{-1}(f(f(x)) = f(x). tag{10}$
answered Jan 3 at 0:28
Robert LewisRobert Lewis
47.5k23067
47.5k23067
1
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
add a comment |
1
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
1
1
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
Or, you know, a set with at least two elements, and just switch two of them.
$endgroup$
– Asaf Karagila♦
Jan 3 at 0:53
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
$begingroup$
@AsafKaragila: Oh yeah, of course. I guess I went for the obvious generalization! :)
$endgroup$
– Robert Lewis
Jan 3 at 0:58
add a comment |
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$begingroup$
any sort of transposition will do. For example, let f($x_1,x_2,x_3$) = ($x_3,x_2,x_1$).
$endgroup$
– Joel Pereira
Jan 2 at 23:42
1
$begingroup$
$-x$ on $mathbb R$, $frac 1 x$ on $(0,infty)$. How many millions do you want?
$endgroup$
– Kavi Rama Murthy
Jan 3 at 0:05