Real Analysis Inequality Proof Involving Reals and Rationals $0 < |r - q| < varepsilon$
$begingroup$
I'm having difficulties making progress in proving:
$$forall varepsilon > 0, exists q in Q text{ where } 0 < |r - q| < varepsilon $$
To clarify, $r$ is a real number and $q$ is a rational number.
Is there some theorem I should be using? This exercise is presented in the same section/chapter as the Completeness Axiom (each nonempty set has a least upper bound or supremum), the Archimedean Property of Real Numbers ($ exists n in Z^{+}$ such that $na>b $ for positive real numbers $a$ and $b$), and a theorem stating there is a rational and irrational number between any two distinct real numbers.
I'm just not seeing the connection (if any at all). Any help in the right direction would be much appreciated. Thank you!
real-analysis inequality proof-writing real-numbers rational-numbers
$endgroup$
add a comment |
$begingroup$
I'm having difficulties making progress in proving:
$$forall varepsilon > 0, exists q in Q text{ where } 0 < |r - q| < varepsilon $$
To clarify, $r$ is a real number and $q$ is a rational number.
Is there some theorem I should be using? This exercise is presented in the same section/chapter as the Completeness Axiom (each nonempty set has a least upper bound or supremum), the Archimedean Property of Real Numbers ($ exists n in Z^{+}$ such that $na>b $ for positive real numbers $a$ and $b$), and a theorem stating there is a rational and irrational number between any two distinct real numbers.
I'm just not seeing the connection (if any at all). Any help in the right direction would be much appreciated. Thank you!
real-analysis inequality proof-writing real-numbers rational-numbers
$endgroup$
$begingroup$
Dirichlet's approximation theorem should be helpful.
$endgroup$
– rtybase
Jan 12 at 10:40
1
$begingroup$
You know that there is a rational between any two distinct reals. Consider $r$ and $r+epsilon$.
$endgroup$
– Wojowu
Jan 12 at 10:42
add a comment |
$begingroup$
I'm having difficulties making progress in proving:
$$forall varepsilon > 0, exists q in Q text{ where } 0 < |r - q| < varepsilon $$
To clarify, $r$ is a real number and $q$ is a rational number.
Is there some theorem I should be using? This exercise is presented in the same section/chapter as the Completeness Axiom (each nonempty set has a least upper bound or supremum), the Archimedean Property of Real Numbers ($ exists n in Z^{+}$ such that $na>b $ for positive real numbers $a$ and $b$), and a theorem stating there is a rational and irrational number between any two distinct real numbers.
I'm just not seeing the connection (if any at all). Any help in the right direction would be much appreciated. Thank you!
real-analysis inequality proof-writing real-numbers rational-numbers
$endgroup$
I'm having difficulties making progress in proving:
$$forall varepsilon > 0, exists q in Q text{ where } 0 < |r - q| < varepsilon $$
To clarify, $r$ is a real number and $q$ is a rational number.
Is there some theorem I should be using? This exercise is presented in the same section/chapter as the Completeness Axiom (each nonempty set has a least upper bound or supremum), the Archimedean Property of Real Numbers ($ exists n in Z^{+}$ such that $na>b $ for positive real numbers $a$ and $b$), and a theorem stating there is a rational and irrational number between any two distinct real numbers.
I'm just not seeing the connection (if any at all). Any help in the right direction would be much appreciated. Thank you!
real-analysis inequality proof-writing real-numbers rational-numbers
real-analysis inequality proof-writing real-numbers rational-numbers
edited Jan 12 at 10:45
rtybase
10.7k21533
10.7k21533
asked Jan 12 at 10:33
Zen'zZen'z
243
243
$begingroup$
Dirichlet's approximation theorem should be helpful.
$endgroup$
– rtybase
Jan 12 at 10:40
1
$begingroup$
You know that there is a rational between any two distinct reals. Consider $r$ and $r+epsilon$.
$endgroup$
– Wojowu
Jan 12 at 10:42
add a comment |
$begingroup$
Dirichlet's approximation theorem should be helpful.
$endgroup$
– rtybase
Jan 12 at 10:40
1
$begingroup$
You know that there is a rational between any two distinct reals. Consider $r$ and $r+epsilon$.
$endgroup$
– Wojowu
Jan 12 at 10:42
$begingroup$
Dirichlet's approximation theorem should be helpful.
$endgroup$
– rtybase
Jan 12 at 10:40
$begingroup$
Dirichlet's approximation theorem should be helpful.
$endgroup$
– rtybase
Jan 12 at 10:40
1
1
$begingroup$
You know that there is a rational between any two distinct reals. Consider $r$ and $r+epsilon$.
$endgroup$
– Wojowu
Jan 12 at 10:42
$begingroup$
You know that there is a rational between any two distinct reals. Consider $r$ and $r+epsilon$.
$endgroup$
– Wojowu
Jan 12 at 10:42
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Choose some positive integer $n$ such that $n>frac{1}{varepsilon}$. Define $q$ as $frac{[nr]+1}{n}$. Then, $|r-q|=frac{1-{nr}}{n}in (0;varepsilon)$.
$endgroup$
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
add a comment |
$begingroup$
No need to any theorem. Let $rnotinBbb Q$ and $$x=10^kcdot r$$for some value of $kin Bbb R$ then define $$ntriangleqBiglfloor10^kcdot rBigrfloor$$which leads to $$nle 10^kcdot r<n+1$$or $${nover 10^k}-{1over 10^k}<r<{n+1over 10^k}={nover 10^k}+{1over 10^k}$$finally $$0<|r-{nover 10^k}|<{1over 10^k}<epsilon$$ and $q={nover 10^k}$ when $k>-log epsilon$
For the case $rin Bbb Q$, define $q=r+{1over 10^k}$ for large enough $kin Bbb N$.
$endgroup$
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Choose some positive integer $n$ such that $n>frac{1}{varepsilon}$. Define $q$ as $frac{[nr]+1}{n}$. Then, $|r-q|=frac{1-{nr}}{n}in (0;varepsilon)$.
$endgroup$
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
add a comment |
$begingroup$
Choose some positive integer $n$ such that $n>frac{1}{varepsilon}$. Define $q$ as $frac{[nr]+1}{n}$. Then, $|r-q|=frac{1-{nr}}{n}in (0;varepsilon)$.
$endgroup$
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
add a comment |
$begingroup$
Choose some positive integer $n$ such that $n>frac{1}{varepsilon}$. Define $q$ as $frac{[nr]+1}{n}$. Then, $|r-q|=frac{1-{nr}}{n}in (0;varepsilon)$.
$endgroup$
Choose some positive integer $n$ such that $n>frac{1}{varepsilon}$. Define $q$ as $frac{[nr]+1}{n}$. Then, $|r-q|=frac{1-{nr}}{n}in (0;varepsilon)$.
answered Jan 12 at 11:01
richrowrichrow
1615
1615
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
add a comment |
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
Could you clarify the notation of [nr] in defining q as $frac{[nr]+1}{n} $ and the notation of {nr} in defining $|r - q| = frac{1-{nr}}{n} $? Thanks!
$endgroup$
– Zen'z
Jan 13 at 18:27
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For real $x$ we define the floor function of $x$ - $[x]$ as integer $n$ such that $nleq x<n+1$ (it's easy to see that such integer exists and unique).
$endgroup$
– richrow
Jan 13 at 20:05
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
For more information about floor and ceiling functions see here en.m.wikipedia.org/wiki/Floor_and_ceiling_functions or here mathworld.wolfram.com/FloorFunction.html.
$endgroup$
– richrow
Jan 13 at 20:07
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
So the [nr] defined in q and the {nr} in |r - q| both evaluates to integers by definition of the floor function? Also, I don't get how you got to |r - q| = $frac{1 - [nr]}{n}$. Could you help me understand the steps involved? Thanks again!
$endgroup$
– Zen'z
Jan 13 at 20:10
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
$begingroup$
No, ${x}$ is known as fractional part of $x$ and it's defined as ${x}:=x-[x]$. Hence, from definition we always have $0leq {x}<1$.
$endgroup$
– richrow
Jan 13 at 21:40
add a comment |
$begingroup$
No need to any theorem. Let $rnotinBbb Q$ and $$x=10^kcdot r$$for some value of $kin Bbb R$ then define $$ntriangleqBiglfloor10^kcdot rBigrfloor$$which leads to $$nle 10^kcdot r<n+1$$or $${nover 10^k}-{1over 10^k}<r<{n+1over 10^k}={nover 10^k}+{1over 10^k}$$finally $$0<|r-{nover 10^k}|<{1over 10^k}<epsilon$$ and $q={nover 10^k}$ when $k>-log epsilon$
For the case $rin Bbb Q$, define $q=r+{1over 10^k}$ for large enough $kin Bbb N$.
$endgroup$
add a comment |
$begingroup$
No need to any theorem. Let $rnotinBbb Q$ and $$x=10^kcdot r$$for some value of $kin Bbb R$ then define $$ntriangleqBiglfloor10^kcdot rBigrfloor$$which leads to $$nle 10^kcdot r<n+1$$or $${nover 10^k}-{1over 10^k}<r<{n+1over 10^k}={nover 10^k}+{1over 10^k}$$finally $$0<|r-{nover 10^k}|<{1over 10^k}<epsilon$$ and $q={nover 10^k}$ when $k>-log epsilon$
For the case $rin Bbb Q$, define $q=r+{1over 10^k}$ for large enough $kin Bbb N$.
$endgroup$
add a comment |
$begingroup$
No need to any theorem. Let $rnotinBbb Q$ and $$x=10^kcdot r$$for some value of $kin Bbb R$ then define $$ntriangleqBiglfloor10^kcdot rBigrfloor$$which leads to $$nle 10^kcdot r<n+1$$or $${nover 10^k}-{1over 10^k}<r<{n+1over 10^k}={nover 10^k}+{1over 10^k}$$finally $$0<|r-{nover 10^k}|<{1over 10^k}<epsilon$$ and $q={nover 10^k}$ when $k>-log epsilon$
For the case $rin Bbb Q$, define $q=r+{1over 10^k}$ for large enough $kin Bbb N$.
$endgroup$
No need to any theorem. Let $rnotinBbb Q$ and $$x=10^kcdot r$$for some value of $kin Bbb R$ then define $$ntriangleqBiglfloor10^kcdot rBigrfloor$$which leads to $$nle 10^kcdot r<n+1$$or $${nover 10^k}-{1over 10^k}<r<{n+1over 10^k}={nover 10^k}+{1over 10^k}$$finally $$0<|r-{nover 10^k}|<{1over 10^k}<epsilon$$ and $q={nover 10^k}$ when $k>-log epsilon$
For the case $rin Bbb Q$, define $q=r+{1over 10^k}$ for large enough $kin Bbb N$.
answered Jan 12 at 14:16
Mostafa AyazMostafa Ayaz
15.3k3939
15.3k3939
add a comment |
add a comment |
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$begingroup$
Dirichlet's approximation theorem should be helpful.
$endgroup$
– rtybase
Jan 12 at 10:40
1
$begingroup$
You know that there is a rational between any two distinct reals. Consider $r$ and $r+epsilon$.
$endgroup$
– Wojowu
Jan 12 at 10:42