Limits conceptual proof
Clash Royale CLAN TAG#URR8PPP
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Show that if $a_n geq b_n$ Then $lim_n a_n ge lim_n b_n$ provided the limit exists.
I'm trying to derive a contradiction and it feels obvious but I can't come up with a clean proof?
sequences-and-series limits convergence proof-writing
add a comment |Â
up vote
1
down vote
favorite
Show that if $a_n geq b_n$ Then $lim_n a_n ge lim_n b_n$ provided the limit exists.
I'm trying to derive a contradiction and it feels obvious but I can't come up with a clean proof?
sequences-and-series limits convergence proof-writing
1
Since $a_nge b_n$, then $lim a_n ge lim b_n$ qed.
â Jimmy R.
2 hours ago
1
Try writing the definition of the $lim(a_n)=a$ and then add and subtract $b_n$ and add and subtract $b$ to get $|(a_n-b_n)+(b_n-b)+b-a|<varepsilon$ and then use triangle inequality. I think it might work..
â Fareed AF
2 hours ago
See related answer math.stackexchange.com/a/1270897/72031 (especially the second part of the answer).
â Paramanand Singh
57 mins ago
add a comment |Â
up vote
1
down vote
favorite
up vote
1
down vote
favorite
Show that if $a_n geq b_n$ Then $lim_n a_n ge lim_n b_n$ provided the limit exists.
I'm trying to derive a contradiction and it feels obvious but I can't come up with a clean proof?
sequences-and-series limits convergence proof-writing
Show that if $a_n geq b_n$ Then $lim_n a_n ge lim_n b_n$ provided the limit exists.
I'm trying to derive a contradiction and it feels obvious but I can't come up with a clean proof?
sequences-and-series limits convergence proof-writing
sequences-and-series limits convergence proof-writing
edited 1 hour ago
Parcly Taxel
40.1k137098
40.1k137098
asked 2 hours ago
Dis-integrating
948325
948325
1
Since $a_nge b_n$, then $lim a_n ge lim b_n$ qed.
â Jimmy R.
2 hours ago
1
Try writing the definition of the $lim(a_n)=a$ and then add and subtract $b_n$ and add and subtract $b$ to get $|(a_n-b_n)+(b_n-b)+b-a|<varepsilon$ and then use triangle inequality. I think it might work..
â Fareed AF
2 hours ago
See related answer math.stackexchange.com/a/1270897/72031 (especially the second part of the answer).
â Paramanand Singh
57 mins ago
add a comment |Â
1
Since $a_nge b_n$, then $lim a_n ge lim b_n$ qed.
â Jimmy R.
2 hours ago
1
Try writing the definition of the $lim(a_n)=a$ and then add and subtract $b_n$ and add and subtract $b$ to get $|(a_n-b_n)+(b_n-b)+b-a|<varepsilon$ and then use triangle inequality. I think it might work..
â Fareed AF
2 hours ago
See related answer math.stackexchange.com/a/1270897/72031 (especially the second part of the answer).
â Paramanand Singh
57 mins ago
1
1
Since $a_nge b_n$, then $lim a_n ge lim b_n$ qed.
â Jimmy R.
2 hours ago
Since $a_nge b_n$, then $lim a_n ge lim b_n$ qed.
â Jimmy R.
2 hours ago
1
1
Try writing the definition of the $lim(a_n)=a$ and then add and subtract $b_n$ and add and subtract $b$ to get $|(a_n-b_n)+(b_n-b)+b-a|<varepsilon$ and then use triangle inequality. I think it might work..
â Fareed AF
2 hours ago
Try writing the definition of the $lim(a_n)=a$ and then add and subtract $b_n$ and add and subtract $b$ to get $|(a_n-b_n)+(b_n-b)+b-a|<varepsilon$ and then use triangle inequality. I think it might work..
â Fareed AF
2 hours ago
See related answer math.stackexchange.com/a/1270897/72031 (especially the second part of the answer).
â Paramanand Singh
57 mins ago
See related answer math.stackexchange.com/a/1270897/72031 (especially the second part of the answer).
â Paramanand Singh
57 mins ago
add a comment |Â
3 Answers
3
active
oldest
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up vote
2
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accepted
Take $c_n=a_n-b_n geq 0$ and let $a=lim a_n$ and $b=lim b_n$. Then we can define $lim c_n$ as follows:
$$lim c_n=lim a_n-lim b_n=a-b$$
Claim: $a-b geq 0$
Suppose $a-b <0$, then $varepsilon =-(a-b)>0$. Since $c_n rightarrow a-b$, so $$(a-b)-varepsilon <c_n <(a-b)+varepsilon$$ for all $n geq K$. In paricular, $$c_K<(a-b)+varepsilon=(a-b)+(-(a-b))=0$$ contradiction to the fact $c_n geq 0$. Thus $a-b geq 0$ and so $lim a_n geq lim b_n$
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
add a comment |Â
up vote
2
down vote
Here's a hint:
For convenience, define $a=lim_ntoinfty a_n$ and $b=lim_ntoinfty b_n$. Suppose that $a_ngeq b_n$ but $a<b$. Define $epsilon=fracb-a2$. For large enough $n$, we have that
$$|a_n-a|<epsilon$$
Similarly, for large enough $n$, we have
$$|b_n-b|<epsilon$$
Using these inequalities, we can derive a contradiction. What is $a_n$ smaller than? What is $b_n$ larger than?
New contributor
add a comment |Â
up vote
0
down vote
So looking online, I think that the first problem from
http://www.stat.uchicago.edu/~lekheng/courses/104s09/math104s09-hw4sol.pdf is a pretty good resource.
proof. Let's first define $lim a_n = a$ and $lim b_n = b$. Now for the sake of contradiction, let's suppose that $lim a_n < lim b_n$, or equivalently, $a < b$. We can set $epsilon = fracb-a2>0$.
By the definition of limit, there exists an $N_1$ such that for all $n>N_1$, we have that
$(1)$ $|a_n - a| < epsilon$ $iff$ $a-epsilon < a_n < a + epsilon$
Likewise, there exists an $N_2$ such that for all $n>N_2$, we have that
$(2)$ $|b_n - b| < epsilon$ $iff$ $b-epsilon < b_n < b+ epsilon$.
We can let $N = maxN_1, N_2$. From $(1)$ and $(2)$, we get that for $n> N$
$b_n > b - epsilon = b - fracb-a2 = a + fracb-a2 = a + epsilon > a_n$
which contradicts the hypothesis given. Thus if $a_n ge b_n$, it follows that $lim a_n ge lim b_n$
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
accepted
Take $c_n=a_n-b_n geq 0$ and let $a=lim a_n$ and $b=lim b_n$. Then we can define $lim c_n$ as follows:
$$lim c_n=lim a_n-lim b_n=a-b$$
Claim: $a-b geq 0$
Suppose $a-b <0$, then $varepsilon =-(a-b)>0$. Since $c_n rightarrow a-b$, so $$(a-b)-varepsilon <c_n <(a-b)+varepsilon$$ for all $n geq K$. In paricular, $$c_K<(a-b)+varepsilon=(a-b)+(-(a-b))=0$$ contradiction to the fact $c_n geq 0$. Thus $a-b geq 0$ and so $lim a_n geq lim b_n$
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
add a comment |Â
up vote
2
down vote
accepted
Take $c_n=a_n-b_n geq 0$ and let $a=lim a_n$ and $b=lim b_n$. Then we can define $lim c_n$ as follows:
$$lim c_n=lim a_n-lim b_n=a-b$$
Claim: $a-b geq 0$
Suppose $a-b <0$, then $varepsilon =-(a-b)>0$. Since $c_n rightarrow a-b$, so $$(a-b)-varepsilon <c_n <(a-b)+varepsilon$$ for all $n geq K$. In paricular, $$c_K<(a-b)+varepsilon=(a-b)+(-(a-b))=0$$ contradiction to the fact $c_n geq 0$. Thus $a-b geq 0$ and so $lim a_n geq lim b_n$
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
add a comment |Â
up vote
2
down vote
accepted
up vote
2
down vote
accepted
Take $c_n=a_n-b_n geq 0$ and let $a=lim a_n$ and $b=lim b_n$. Then we can define $lim c_n$ as follows:
$$lim c_n=lim a_n-lim b_n=a-b$$
Claim: $a-b geq 0$
Suppose $a-b <0$, then $varepsilon =-(a-b)>0$. Since $c_n rightarrow a-b$, so $$(a-b)-varepsilon <c_n <(a-b)+varepsilon$$ for all $n geq K$. In paricular, $$c_K<(a-b)+varepsilon=(a-b)+(-(a-b))=0$$ contradiction to the fact $c_n geq 0$. Thus $a-b geq 0$ and so $lim a_n geq lim b_n$
Take $c_n=a_n-b_n geq 0$ and let $a=lim a_n$ and $b=lim b_n$. Then we can define $lim c_n$ as follows:
$$lim c_n=lim a_n-lim b_n=a-b$$
Claim: $a-b geq 0$
Suppose $a-b <0$, then $varepsilon =-(a-b)>0$. Since $c_n rightarrow a-b$, so $$(a-b)-varepsilon <c_n <(a-b)+varepsilon$$ for all $n geq K$. In paricular, $$c_K<(a-b)+varepsilon=(a-b)+(-(a-b))=0$$ contradiction to the fact $c_n geq 0$. Thus $a-b geq 0$ and so $lim a_n geq lim b_n$
edited 20 mins ago
K.M
518312
518312
answered 2 hours ago
Chinnapparaj R
4,338725
4,338725
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
add a comment |Â
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
thanks, but somehow i always manage to complete problems myself just after asking somebody for help
â Dis-integrating
1 hour ago
add a comment |Â
up vote
2
down vote
Here's a hint:
For convenience, define $a=lim_ntoinfty a_n$ and $b=lim_ntoinfty b_n$. Suppose that $a_ngeq b_n$ but $a<b$. Define $epsilon=fracb-a2$. For large enough $n$, we have that
$$|a_n-a|<epsilon$$
Similarly, for large enough $n$, we have
$$|b_n-b|<epsilon$$
Using these inequalities, we can derive a contradiction. What is $a_n$ smaller than? What is $b_n$ larger than?
New contributor
add a comment |Â
up vote
2
down vote
Here's a hint:
For convenience, define $a=lim_ntoinfty a_n$ and $b=lim_ntoinfty b_n$. Suppose that $a_ngeq b_n$ but $a<b$. Define $epsilon=fracb-a2$. For large enough $n$, we have that
$$|a_n-a|<epsilon$$
Similarly, for large enough $n$, we have
$$|b_n-b|<epsilon$$
Using these inequalities, we can derive a contradiction. What is $a_n$ smaller than? What is $b_n$ larger than?
New contributor
add a comment |Â
up vote
2
down vote
up vote
2
down vote
Here's a hint:
For convenience, define $a=lim_ntoinfty a_n$ and $b=lim_ntoinfty b_n$. Suppose that $a_ngeq b_n$ but $a<b$. Define $epsilon=fracb-a2$. For large enough $n$, we have that
$$|a_n-a|<epsilon$$
Similarly, for large enough $n$, we have
$$|b_n-b|<epsilon$$
Using these inequalities, we can derive a contradiction. What is $a_n$ smaller than? What is $b_n$ larger than?
New contributor
Here's a hint:
For convenience, define $a=lim_ntoinfty a_n$ and $b=lim_ntoinfty b_n$. Suppose that $a_ngeq b_n$ but $a<b$. Define $epsilon=fracb-a2$. For large enough $n$, we have that
$$|a_n-a|<epsilon$$
Similarly, for large enough $n$, we have
$$|b_n-b|<epsilon$$
Using these inequalities, we can derive a contradiction. What is $a_n$ smaller than? What is $b_n$ larger than?
New contributor
New contributor
answered 2 hours ago
Josh B.
1864
1864
New contributor
New contributor
add a comment |Â
add a comment |Â
up vote
0
down vote
So looking online, I think that the first problem from
http://www.stat.uchicago.edu/~lekheng/courses/104s09/math104s09-hw4sol.pdf is a pretty good resource.
proof. Let's first define $lim a_n = a$ and $lim b_n = b$. Now for the sake of contradiction, let's suppose that $lim a_n < lim b_n$, or equivalently, $a < b$. We can set $epsilon = fracb-a2>0$.
By the definition of limit, there exists an $N_1$ such that for all $n>N_1$, we have that
$(1)$ $|a_n - a| < epsilon$ $iff$ $a-epsilon < a_n < a + epsilon$
Likewise, there exists an $N_2$ such that for all $n>N_2$, we have that
$(2)$ $|b_n - b| < epsilon$ $iff$ $b-epsilon < b_n < b+ epsilon$.
We can let $N = maxN_1, N_2$. From $(1)$ and $(2)$, we get that for $n> N$
$b_n > b - epsilon = b - fracb-a2 = a + fracb-a2 = a + epsilon > a_n$
which contradicts the hypothesis given. Thus if $a_n ge b_n$, it follows that $lim a_n ge lim b_n$
add a comment |Â
up vote
0
down vote
So looking online, I think that the first problem from
http://www.stat.uchicago.edu/~lekheng/courses/104s09/math104s09-hw4sol.pdf is a pretty good resource.
proof. Let's first define $lim a_n = a$ and $lim b_n = b$. Now for the sake of contradiction, let's suppose that $lim a_n < lim b_n$, or equivalently, $a < b$. We can set $epsilon = fracb-a2>0$.
By the definition of limit, there exists an $N_1$ such that for all $n>N_1$, we have that
$(1)$ $|a_n - a| < epsilon$ $iff$ $a-epsilon < a_n < a + epsilon$
Likewise, there exists an $N_2$ such that for all $n>N_2$, we have that
$(2)$ $|b_n - b| < epsilon$ $iff$ $b-epsilon < b_n < b+ epsilon$.
We can let $N = maxN_1, N_2$. From $(1)$ and $(2)$, we get that for $n> N$
$b_n > b - epsilon = b - fracb-a2 = a + fracb-a2 = a + epsilon > a_n$
which contradicts the hypothesis given. Thus if $a_n ge b_n$, it follows that $lim a_n ge lim b_n$
add a comment |Â
up vote
0
down vote
up vote
0
down vote
So looking online, I think that the first problem from
http://www.stat.uchicago.edu/~lekheng/courses/104s09/math104s09-hw4sol.pdf is a pretty good resource.
proof. Let's first define $lim a_n = a$ and $lim b_n = b$. Now for the sake of contradiction, let's suppose that $lim a_n < lim b_n$, or equivalently, $a < b$. We can set $epsilon = fracb-a2>0$.
By the definition of limit, there exists an $N_1$ such that for all $n>N_1$, we have that
$(1)$ $|a_n - a| < epsilon$ $iff$ $a-epsilon < a_n < a + epsilon$
Likewise, there exists an $N_2$ such that for all $n>N_2$, we have that
$(2)$ $|b_n - b| < epsilon$ $iff$ $b-epsilon < b_n < b+ epsilon$.
We can let $N = maxN_1, N_2$. From $(1)$ and $(2)$, we get that for $n> N$
$b_n > b - epsilon = b - fracb-a2 = a + fracb-a2 = a + epsilon > a_n$
which contradicts the hypothesis given. Thus if $a_n ge b_n$, it follows that $lim a_n ge lim b_n$
So looking online, I think that the first problem from
http://www.stat.uchicago.edu/~lekheng/courses/104s09/math104s09-hw4sol.pdf is a pretty good resource.
proof. Let's first define $lim a_n = a$ and $lim b_n = b$. Now for the sake of contradiction, let's suppose that $lim a_n < lim b_n$, or equivalently, $a < b$. We can set $epsilon = fracb-a2>0$.
By the definition of limit, there exists an $N_1$ such that for all $n>N_1$, we have that
$(1)$ $|a_n - a| < epsilon$ $iff$ $a-epsilon < a_n < a + epsilon$
Likewise, there exists an $N_2$ such that for all $n>N_2$, we have that
$(2)$ $|b_n - b| < epsilon$ $iff$ $b-epsilon < b_n < b+ epsilon$.
We can let $N = maxN_1, N_2$. From $(1)$ and $(2)$, we get that for $n> N$
$b_n > b - epsilon = b - fracb-a2 = a + fracb-a2 = a + epsilon > a_n$
which contradicts the hypothesis given. Thus if $a_n ge b_n$, it follows that $lim a_n ge lim b_n$
answered 41 mins ago
K.M
518312
518312
add a comment |Â
add a comment |Â
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1
Since $a_nge b_n$, then $lim a_n ge lim b_n$ qed.
â Jimmy R.
2 hours ago
1
Try writing the definition of the $lim(a_n)=a$ and then add and subtract $b_n$ and add and subtract $b$ to get $|(a_n-b_n)+(b_n-b)+b-a|<varepsilon$ and then use triangle inequality. I think it might work..
â Fareed AF
2 hours ago
See related answer math.stackexchange.com/a/1270897/72031 (especially the second part of the answer).
â Paramanand Singh
57 mins ago