Infinitely many discontinuities for a bijective function from $[0,1)$ to $(0,1)$
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Show that any bijection from $[0,1)$ to $(0,1)$ has infinitely many discontinuities.
I have thought about this question but I have no any idea. Any idea is valuable for me, thanks.
general-topology
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add a comment |
$begingroup$
Show that any bijection from $[0,1)$ to $(0,1)$ has infinitely many discontinuities.
I have thought about this question but I have no any idea. Any idea is valuable for me, thanks.
general-topology
$endgroup$
$begingroup$
@Ian such a bijection from $[0,a)$ to $(b,c)$ can be discontinuous at $0$, right?
$endgroup$
– Darío G
Sep 7 '16 at 17:08
$begingroup$
See my answer here (after the "edit"): math.stackexchange.com/questions/966299/…
$endgroup$
– Moishe Kohan
Sep 10 '16 at 3:46
add a comment |
$begingroup$
Show that any bijection from $[0,1)$ to $(0,1)$ has infinitely many discontinuities.
I have thought about this question but I have no any idea. Any idea is valuable for me, thanks.
general-topology
$endgroup$
Show that any bijection from $[0,1)$ to $(0,1)$ has infinitely many discontinuities.
I have thought about this question but I have no any idea. Any idea is valuable for me, thanks.
general-topology
general-topology
edited Sep 7 '16 at 16:58
Alexis Olson
4,5481030
4,5481030
asked Sep 7 '16 at 16:52
user365user365
747
747
$begingroup$
@Ian such a bijection from $[0,a)$ to $(b,c)$ can be discontinuous at $0$, right?
$endgroup$
– Darío G
Sep 7 '16 at 17:08
$begingroup$
See my answer here (after the "edit"): math.stackexchange.com/questions/966299/…
$endgroup$
– Moishe Kohan
Sep 10 '16 at 3:46
add a comment |
$begingroup$
@Ian such a bijection from $[0,a)$ to $(b,c)$ can be discontinuous at $0$, right?
$endgroup$
– Darío G
Sep 7 '16 at 17:08
$begingroup$
See my answer here (after the "edit"): math.stackexchange.com/questions/966299/…
$endgroup$
– Moishe Kohan
Sep 10 '16 at 3:46
$begingroup$
@Ian such a bijection from $[0,a)$ to $(b,c)$ can be discontinuous at $0$, right?
$endgroup$
– Darío G
Sep 7 '16 at 17:08
$begingroup$
@Ian such a bijection from $[0,a)$ to $(b,c)$ can be discontinuous at $0$, right?
$endgroup$
– Darío G
Sep 7 '16 at 17:08
$begingroup$
See my answer here (after the "edit"): math.stackexchange.com/questions/966299/…
$endgroup$
– Moishe Kohan
Sep 10 '16 at 3:46
$begingroup$
See my answer here (after the "edit"): math.stackexchange.com/questions/966299/…
$endgroup$
– Moishe Kohan
Sep 10 '16 at 3:46
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
HINT: Suppose that $f:[0,1)to(0,1)$ is a bijection with only finitely many discontinuities. Let $F$ be the set of points at which $f$ is not continuous. Then $[0,1)setminus F$ is a union of intervals, and $f$ is continuous on each of these intervals. Moreover, all of these intervals are open except possibly one of the form $[0,a)$, in the case in which $f$ is continuous at $0$. Let $mathscr{I}$ be the set of these intervals.
Use the continuity of $fupharpoonright I$ and the fact that $f$ is a bijection to show that $f[I]$ is an interval for each $Iinmathscr{I}$. Moreover, $f[I]$ is an open interval if $I$ is.
Show that if $0in F$, so that each $Iinmathscr{I}$ is an open interval, then on the one hand $|f[F]|=|mathscr{I}|-1$, since $(0,1)setminus f[F]$ is the union of $|mathscr{I}$ open intervals, but on the other hand $|F|=|mathscr{I}|$. Conclude that $0notin F$, so $f$ is continuous at $0$.
Let $F={a_1,ldots,a_n}$, where $0<a_1<ldots<a_n<1$. Let $I_0=[0,a_1)$, for $k=1,ldots,n-1$ let $I_k=(a_k,a_{k+1})$, and let $I_n=(a_n,1)$. Then $f$ is continuous on each of the intervals $I_k$ for $k=0,ldots,n$.
Use the fact that $f[F]=F$ to show that $f$ must map each of the intervals $I_k$ to one of the intervals $(0,a_1)$, $(a_k,a_{k+1})$ for $k=1,ldots,n-1$, or $(a_n,1)$.
Get a contradiction by showing that $f[I_0]$ is a half-open interval, not an open interval.
$endgroup$
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
add a comment |
$begingroup$
Suppose that $f$ is discontinuous only at a finite set $A$, and let $B = A cup {0}$. Set $|B| = n$.
$[0,1)setminus B$ is a disjoint union of $n$ open intervals ${I_k}$ on which $f$ is continuous, so they are mapped by $f$ to a disjoint union of open intervals ${f(I_k)}$ that must line up end-to-end (as their complement is finite).
But lining up $n$ open intervals end-to-end in $(0,1)$ gives us $n-1$ endpoints, which cannot be in bijection with $B$.
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add a comment |
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2 Answers
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2 Answers
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$begingroup$
HINT: Suppose that $f:[0,1)to(0,1)$ is a bijection with only finitely many discontinuities. Let $F$ be the set of points at which $f$ is not continuous. Then $[0,1)setminus F$ is a union of intervals, and $f$ is continuous on each of these intervals. Moreover, all of these intervals are open except possibly one of the form $[0,a)$, in the case in which $f$ is continuous at $0$. Let $mathscr{I}$ be the set of these intervals.
Use the continuity of $fupharpoonright I$ and the fact that $f$ is a bijection to show that $f[I]$ is an interval for each $Iinmathscr{I}$. Moreover, $f[I]$ is an open interval if $I$ is.
Show that if $0in F$, so that each $Iinmathscr{I}$ is an open interval, then on the one hand $|f[F]|=|mathscr{I}|-1$, since $(0,1)setminus f[F]$ is the union of $|mathscr{I}$ open intervals, but on the other hand $|F|=|mathscr{I}|$. Conclude that $0notin F$, so $f$ is continuous at $0$.
Let $F={a_1,ldots,a_n}$, where $0<a_1<ldots<a_n<1$. Let $I_0=[0,a_1)$, for $k=1,ldots,n-1$ let $I_k=(a_k,a_{k+1})$, and let $I_n=(a_n,1)$. Then $f$ is continuous on each of the intervals $I_k$ for $k=0,ldots,n$.
Use the fact that $f[F]=F$ to show that $f$ must map each of the intervals $I_k$ to one of the intervals $(0,a_1)$, $(a_k,a_{k+1})$ for $k=1,ldots,n-1$, or $(a_n,1)$.
Get a contradiction by showing that $f[I_0]$ is a half-open interval, not an open interval.
$endgroup$
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
add a comment |
$begingroup$
HINT: Suppose that $f:[0,1)to(0,1)$ is a bijection with only finitely many discontinuities. Let $F$ be the set of points at which $f$ is not continuous. Then $[0,1)setminus F$ is a union of intervals, and $f$ is continuous on each of these intervals. Moreover, all of these intervals are open except possibly one of the form $[0,a)$, in the case in which $f$ is continuous at $0$. Let $mathscr{I}$ be the set of these intervals.
Use the continuity of $fupharpoonright I$ and the fact that $f$ is a bijection to show that $f[I]$ is an interval for each $Iinmathscr{I}$. Moreover, $f[I]$ is an open interval if $I$ is.
Show that if $0in F$, so that each $Iinmathscr{I}$ is an open interval, then on the one hand $|f[F]|=|mathscr{I}|-1$, since $(0,1)setminus f[F]$ is the union of $|mathscr{I}$ open intervals, but on the other hand $|F|=|mathscr{I}|$. Conclude that $0notin F$, so $f$ is continuous at $0$.
Let $F={a_1,ldots,a_n}$, where $0<a_1<ldots<a_n<1$. Let $I_0=[0,a_1)$, for $k=1,ldots,n-1$ let $I_k=(a_k,a_{k+1})$, and let $I_n=(a_n,1)$. Then $f$ is continuous on each of the intervals $I_k$ for $k=0,ldots,n$.
Use the fact that $f[F]=F$ to show that $f$ must map each of the intervals $I_k$ to one of the intervals $(0,a_1)$, $(a_k,a_{k+1})$ for $k=1,ldots,n-1$, or $(a_n,1)$.
Get a contradiction by showing that $f[I_0]$ is a half-open interval, not an open interval.
$endgroup$
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
add a comment |
$begingroup$
HINT: Suppose that $f:[0,1)to(0,1)$ is a bijection with only finitely many discontinuities. Let $F$ be the set of points at which $f$ is not continuous. Then $[0,1)setminus F$ is a union of intervals, and $f$ is continuous on each of these intervals. Moreover, all of these intervals are open except possibly one of the form $[0,a)$, in the case in which $f$ is continuous at $0$. Let $mathscr{I}$ be the set of these intervals.
Use the continuity of $fupharpoonright I$ and the fact that $f$ is a bijection to show that $f[I]$ is an interval for each $Iinmathscr{I}$. Moreover, $f[I]$ is an open interval if $I$ is.
Show that if $0in F$, so that each $Iinmathscr{I}$ is an open interval, then on the one hand $|f[F]|=|mathscr{I}|-1$, since $(0,1)setminus f[F]$ is the union of $|mathscr{I}$ open intervals, but on the other hand $|F|=|mathscr{I}|$. Conclude that $0notin F$, so $f$ is continuous at $0$.
Let $F={a_1,ldots,a_n}$, where $0<a_1<ldots<a_n<1$. Let $I_0=[0,a_1)$, for $k=1,ldots,n-1$ let $I_k=(a_k,a_{k+1})$, and let $I_n=(a_n,1)$. Then $f$ is continuous on each of the intervals $I_k$ for $k=0,ldots,n$.
Use the fact that $f[F]=F$ to show that $f$ must map each of the intervals $I_k$ to one of the intervals $(0,a_1)$, $(a_k,a_{k+1})$ for $k=1,ldots,n-1$, or $(a_n,1)$.
Get a contradiction by showing that $f[I_0]$ is a half-open interval, not an open interval.
$endgroup$
HINT: Suppose that $f:[0,1)to(0,1)$ is a bijection with only finitely many discontinuities. Let $F$ be the set of points at which $f$ is not continuous. Then $[0,1)setminus F$ is a union of intervals, and $f$ is continuous on each of these intervals. Moreover, all of these intervals are open except possibly one of the form $[0,a)$, in the case in which $f$ is continuous at $0$. Let $mathscr{I}$ be the set of these intervals.
Use the continuity of $fupharpoonright I$ and the fact that $f$ is a bijection to show that $f[I]$ is an interval for each $Iinmathscr{I}$. Moreover, $f[I]$ is an open interval if $I$ is.
Show that if $0in F$, so that each $Iinmathscr{I}$ is an open interval, then on the one hand $|f[F]|=|mathscr{I}|-1$, since $(0,1)setminus f[F]$ is the union of $|mathscr{I}$ open intervals, but on the other hand $|F|=|mathscr{I}|$. Conclude that $0notin F$, so $f$ is continuous at $0$.
Let $F={a_1,ldots,a_n}$, where $0<a_1<ldots<a_n<1$. Let $I_0=[0,a_1)$, for $k=1,ldots,n-1$ let $I_k=(a_k,a_{k+1})$, and let $I_n=(a_n,1)$. Then $f$ is continuous on each of the intervals $I_k$ for $k=0,ldots,n$.
Use the fact that $f[F]=F$ to show that $f$ must map each of the intervals $I_k$ to one of the intervals $(0,a_1)$, $(a_k,a_{k+1})$ for $k=1,ldots,n-1$, or $(a_n,1)$.
Get a contradiction by showing that $f[I_0]$ is a half-open interval, not an open interval.
edited Sep 7 '16 at 19:08
answered Sep 7 '16 at 17:22
Brian M. ScottBrian M. Scott
461k40518920
461k40518920
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
add a comment |
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
Do you actually get $f[F]=[F]$? I can get $|f[F-{0}]|=|F|$ which also shows that $0notin F$, but I don't know how you get the equality of the two sets $f[F]$ and $[F]$. Could you please explain to me?
$endgroup$
– Darío G
Sep 7 '16 at 18:31
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
$begingroup$
@Wore: You’re absolutely right; I don’t know what (or if!) I was thinking. I’ve corrected that bit now; thanks!
$endgroup$
– Brian M. Scott
Sep 7 '16 at 19:09
add a comment |
$begingroup$
Suppose that $f$ is discontinuous only at a finite set $A$, and let $B = A cup {0}$. Set $|B| = n$.
$[0,1)setminus B$ is a disjoint union of $n$ open intervals ${I_k}$ on which $f$ is continuous, so they are mapped by $f$ to a disjoint union of open intervals ${f(I_k)}$ that must line up end-to-end (as their complement is finite).
But lining up $n$ open intervals end-to-end in $(0,1)$ gives us $n-1$ endpoints, which cannot be in bijection with $B$.
$endgroup$
add a comment |
$begingroup$
Suppose that $f$ is discontinuous only at a finite set $A$, and let $B = A cup {0}$. Set $|B| = n$.
$[0,1)setminus B$ is a disjoint union of $n$ open intervals ${I_k}$ on which $f$ is continuous, so they are mapped by $f$ to a disjoint union of open intervals ${f(I_k)}$ that must line up end-to-end (as their complement is finite).
But lining up $n$ open intervals end-to-end in $(0,1)$ gives us $n-1$ endpoints, which cannot be in bijection with $B$.
$endgroup$
add a comment |
$begingroup$
Suppose that $f$ is discontinuous only at a finite set $A$, and let $B = A cup {0}$. Set $|B| = n$.
$[0,1)setminus B$ is a disjoint union of $n$ open intervals ${I_k}$ on which $f$ is continuous, so they are mapped by $f$ to a disjoint union of open intervals ${f(I_k)}$ that must line up end-to-end (as their complement is finite).
But lining up $n$ open intervals end-to-end in $(0,1)$ gives us $n-1$ endpoints, which cannot be in bijection with $B$.
$endgroup$
Suppose that $f$ is discontinuous only at a finite set $A$, and let $B = A cup {0}$. Set $|B| = n$.
$[0,1)setminus B$ is a disjoint union of $n$ open intervals ${I_k}$ on which $f$ is continuous, so they are mapped by $f$ to a disjoint union of open intervals ${f(I_k)}$ that must line up end-to-end (as their complement is finite).
But lining up $n$ open intervals end-to-end in $(0,1)$ gives us $n-1$ endpoints, which cannot be in bijection with $B$.
answered Dec 23 '18 at 20:35
SladeSlade
25.4k12665
25.4k12665
add a comment |
add a comment |
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$begingroup$
@Ian such a bijection from $[0,a)$ to $(b,c)$ can be discontinuous at $0$, right?
$endgroup$
– Darío G
Sep 7 '16 at 17:08
$begingroup$
See my answer here (after the "edit"): math.stackexchange.com/questions/966299/…
$endgroup$
– Moishe Kohan
Sep 10 '16 at 3:46