Prove that $sup S=sup R,$ if $SsubseteqBbb{R}$ is bounded from above and $Rsubseteq S$.
$begingroup$
Please, is this correct?
Let $S$ be a subset of the real numbers bounded from above and let $Rsubseteq S$ satisfy the following condition: $forall;xin S,;exists; rin R;text{such that};xleq r$.
I want to prove that begin{align} sup S=sup R.end{align}
PROOF
Let $xin S$ be arbitrary, then $exists; rin R;text{such that};xleq r.$ The number, $r$, is an upper bound for the set $S.$ So, $sup Sleq r,;text{for some} ; rin R.$ By definition of $sup,;;yleqsup R, forall; yin R.$ In particular, $y=r,;rleqsup R.$ Thus,
begin{align}tag{1} sup Sleq rleqsup R.end{align}
Since $Rsubseteq S$, we have
begin{align}tag{2} sup Rleq sup S.end{align}
Thus, begin{align} sup S=sup R.end{align}
real-analysis analysis
$endgroup$
add a comment |
$begingroup$
Please, is this correct?
Let $S$ be a subset of the real numbers bounded from above and let $Rsubseteq S$ satisfy the following condition: $forall;xin S,;exists; rin R;text{such that};xleq r$.
I want to prove that begin{align} sup S=sup R.end{align}
PROOF
Let $xin S$ be arbitrary, then $exists; rin R;text{such that};xleq r.$ The number, $r$, is an upper bound for the set $S.$ So, $sup Sleq r,;text{for some} ; rin R.$ By definition of $sup,;;yleqsup R, forall; yin R.$ In particular, $y=r,;rleqsup R.$ Thus,
begin{align}tag{1} sup Sleq rleqsup R.end{align}
Since $Rsubseteq S$, we have
begin{align}tag{2} sup Rleq sup S.end{align}
Thus, begin{align} sup S=sup R.end{align}
real-analysis analysis
$endgroup$
1
$begingroup$
Your $r$ is not necessarily an upper bound for $S$. $r$ depends on $x$. So, how do you conclude that $r$ is an upper bound for $S$? Choose, for example, $S = (0,1)$ and $R= S cap mathbb{Q}$.
$endgroup$
– trancelocation
Jan 10 at 4:22
$begingroup$
@trancelocation: You are indeed, right!
$endgroup$
– Omojola Micheal
Jan 10 at 4:24
add a comment |
$begingroup$
Please, is this correct?
Let $S$ be a subset of the real numbers bounded from above and let $Rsubseteq S$ satisfy the following condition: $forall;xin S,;exists; rin R;text{such that};xleq r$.
I want to prove that begin{align} sup S=sup R.end{align}
PROOF
Let $xin S$ be arbitrary, then $exists; rin R;text{such that};xleq r.$ The number, $r$, is an upper bound for the set $S.$ So, $sup Sleq r,;text{for some} ; rin R.$ By definition of $sup,;;yleqsup R, forall; yin R.$ In particular, $y=r,;rleqsup R.$ Thus,
begin{align}tag{1} sup Sleq rleqsup R.end{align}
Since $Rsubseteq S$, we have
begin{align}tag{2} sup Rleq sup S.end{align}
Thus, begin{align} sup S=sup R.end{align}
real-analysis analysis
$endgroup$
Please, is this correct?
Let $S$ be a subset of the real numbers bounded from above and let $Rsubseteq S$ satisfy the following condition: $forall;xin S,;exists; rin R;text{such that};xleq r$.
I want to prove that begin{align} sup S=sup R.end{align}
PROOF
Let $xin S$ be arbitrary, then $exists; rin R;text{such that};xleq r.$ The number, $r$, is an upper bound for the set $S.$ So, $sup Sleq r,;text{for some} ; rin R.$ By definition of $sup,;;yleqsup R, forall; yin R.$ In particular, $y=r,;rleqsup R.$ Thus,
begin{align}tag{1} sup Sleq rleqsup R.end{align}
Since $Rsubseteq S$, we have
begin{align}tag{2} sup Rleq sup S.end{align}
Thus, begin{align} sup S=sup R.end{align}
real-analysis analysis
real-analysis analysis
edited Jan 10 at 4:38
Omojola Micheal
asked Jan 10 at 4:15
Omojola MichealOmojola Micheal
2,049424
2,049424
1
$begingroup$
Your $r$ is not necessarily an upper bound for $S$. $r$ depends on $x$. So, how do you conclude that $r$ is an upper bound for $S$? Choose, for example, $S = (0,1)$ and $R= S cap mathbb{Q}$.
$endgroup$
– trancelocation
Jan 10 at 4:22
$begingroup$
@trancelocation: You are indeed, right!
$endgroup$
– Omojola Micheal
Jan 10 at 4:24
add a comment |
1
$begingroup$
Your $r$ is not necessarily an upper bound for $S$. $r$ depends on $x$. So, how do you conclude that $r$ is an upper bound for $S$? Choose, for example, $S = (0,1)$ and $R= S cap mathbb{Q}$.
$endgroup$
– trancelocation
Jan 10 at 4:22
$begingroup$
@trancelocation: You are indeed, right!
$endgroup$
– Omojola Micheal
Jan 10 at 4:24
1
1
$begingroup$
Your $r$ is not necessarily an upper bound for $S$. $r$ depends on $x$. So, how do you conclude that $r$ is an upper bound for $S$? Choose, for example, $S = (0,1)$ and $R= S cap mathbb{Q}$.
$endgroup$
– trancelocation
Jan 10 at 4:22
$begingroup$
Your $r$ is not necessarily an upper bound for $S$. $r$ depends on $x$. So, how do you conclude that $r$ is an upper bound for $S$? Choose, for example, $S = (0,1)$ and $R= S cap mathbb{Q}$.
$endgroup$
– trancelocation
Jan 10 at 4:22
$begingroup$
@trancelocation: You are indeed, right!
$endgroup$
– Omojola Micheal
Jan 10 at 4:24
$begingroup$
@trancelocation: You are indeed, right!
$endgroup$
– Omojola Micheal
Jan 10 at 4:24
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
It seems you confuse "$color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$" with "$color{blue}{exists}; rin R; color{blue}{forall};xin S,;text{such that};xleq r$".
The sets $S = (0,1)$ and $R= S cap mathbb{Q}$ satisfy the condition: $color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$.
But your conclusion $sup S leq r$ is not true for any $r in R$.
To show that $sup R = sup S$ you may proceed as follows:
- Show $forall epsilon > 0 exists x_{epsilon}in R: sup S - epsilon < x_{epsilon}$
$$Rightarrow forall epsilon >0: sup S - epsilon < sup R Rightarrow sup S leq sup R$$
That's exactly what was to be shown.
$endgroup$
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
add a comment |
$begingroup$
Suppose $sup R<sup S$. Then $sup R$ is not an upper bound of $S$, thus $exists xin S|sup R<xlesup S$. But this is a contradiction, since $forall rin R, rlesup R<x$, and it implies there does not exist any $rge x$. This means $sup R=sup S$.
$endgroup$
add a comment |
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2 Answers
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2 Answers
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$begingroup$
It seems you confuse "$color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$" with "$color{blue}{exists}; rin R; color{blue}{forall};xin S,;text{such that};xleq r$".
The sets $S = (0,1)$ and $R= S cap mathbb{Q}$ satisfy the condition: $color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$.
But your conclusion $sup S leq r$ is not true for any $r in R$.
To show that $sup R = sup S$ you may proceed as follows:
- Show $forall epsilon > 0 exists x_{epsilon}in R: sup S - epsilon < x_{epsilon}$
$$Rightarrow forall epsilon >0: sup S - epsilon < sup R Rightarrow sup S leq sup R$$
That's exactly what was to be shown.
$endgroup$
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
add a comment |
$begingroup$
It seems you confuse "$color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$" with "$color{blue}{exists}; rin R; color{blue}{forall};xin S,;text{such that};xleq r$".
The sets $S = (0,1)$ and $R= S cap mathbb{Q}$ satisfy the condition: $color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$.
But your conclusion $sup S leq r$ is not true for any $r in R$.
To show that $sup R = sup S$ you may proceed as follows:
- Show $forall epsilon > 0 exists x_{epsilon}in R: sup S - epsilon < x_{epsilon}$
$$Rightarrow forall epsilon >0: sup S - epsilon < sup R Rightarrow sup S leq sup R$$
That's exactly what was to be shown.
$endgroup$
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
add a comment |
$begingroup$
It seems you confuse "$color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$" with "$color{blue}{exists}; rin R; color{blue}{forall};xin S,;text{such that};xleq r$".
The sets $S = (0,1)$ and $R= S cap mathbb{Q}$ satisfy the condition: $color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$.
But your conclusion $sup S leq r$ is not true for any $r in R$.
To show that $sup R = sup S$ you may proceed as follows:
- Show $forall epsilon > 0 exists x_{epsilon}in R: sup S - epsilon < x_{epsilon}$
$$Rightarrow forall epsilon >0: sup S - epsilon < sup R Rightarrow sup S leq sup R$$
That's exactly what was to be shown.
$endgroup$
It seems you confuse "$color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$" with "$color{blue}{exists}; rin R; color{blue}{forall};xin S,;text{such that};xleq r$".
The sets $S = (0,1)$ and $R= S cap mathbb{Q}$ satisfy the condition: $color{blue}{forall};xin S,;color{blue}{exists}; rin R;text{such that};xleq r$.
But your conclusion $sup S leq r$ is not true for any $r in R$.
To show that $sup R = sup S$ you may proceed as follows:
- Show $forall epsilon > 0 exists x_{epsilon}in R: sup S - epsilon < x_{epsilon}$
$$Rightarrow forall epsilon >0: sup S - epsilon < sup R Rightarrow sup S leq sup R$$
That's exactly what was to be shown.
edited Jan 10 at 4:43
answered Jan 10 at 4:36
trancelocationtrancelocation
13.6k1829
13.6k1829
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
add a comment |
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
(+1) You claimed that "But your conclusion $sup Sleq r$ is not true for any $rin R$"but I claimed $sup Sleq r$ for some $rin R$" instead.
$endgroup$
– Omojola Micheal
Jan 10 at 4:42
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
@OmojolaMicheal: If it is not true for any $r$ in the given case, then there is also no specific $r$, for which it is suddenly true.
$endgroup$
– trancelocation
Jan 10 at 4:45
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
I get you now. Thanks!
$endgroup$
– Omojola Micheal
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
$begingroup$
You are welcome. :-)
$endgroup$
– trancelocation
Jan 10 at 4:46
add a comment |
$begingroup$
Suppose $sup R<sup S$. Then $sup R$ is not an upper bound of $S$, thus $exists xin S|sup R<xlesup S$. But this is a contradiction, since $forall rin R, rlesup R<x$, and it implies there does not exist any $rge x$. This means $sup R=sup S$.
$endgroup$
add a comment |
$begingroup$
Suppose $sup R<sup S$. Then $sup R$ is not an upper bound of $S$, thus $exists xin S|sup R<xlesup S$. But this is a contradiction, since $forall rin R, rlesup R<x$, and it implies there does not exist any $rge x$. This means $sup R=sup S$.
$endgroup$
add a comment |
$begingroup$
Suppose $sup R<sup S$. Then $sup R$ is not an upper bound of $S$, thus $exists xin S|sup R<xlesup S$. But this is a contradiction, since $forall rin R, rlesup R<x$, and it implies there does not exist any $rge x$. This means $sup R=sup S$.
$endgroup$
Suppose $sup R<sup S$. Then $sup R$ is not an upper bound of $S$, thus $exists xin S|sup R<xlesup S$. But this is a contradiction, since $forall rin R, rlesup R<x$, and it implies there does not exist any $rge x$. This means $sup R=sup S$.
edited Jan 10 at 4:48
answered Jan 10 at 4:43
Shubham JohriShubham Johri
5,515818
5,515818
add a comment |
add a comment |
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$begingroup$
Your $r$ is not necessarily an upper bound for $S$. $r$ depends on $x$. So, how do you conclude that $r$ is an upper bound for $S$? Choose, for example, $S = (0,1)$ and $R= S cap mathbb{Q}$.
$endgroup$
– trancelocation
Jan 10 at 4:22
$begingroup$
@trancelocation: You are indeed, right!
$endgroup$
– Omojola Micheal
Jan 10 at 4:24