Proving a sequence is bounded from below
up vote
1
down vote
favorite
Let $$a_1=3quad , quad a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}$$
Show that $a_n$ converges.
I know that I need to prove that $a_n$ is monotonic and bounded.
I've assumed that the limit exists, made the calculation and get that $L=2$, hence claiming two claims:
$a_nge 2$ (I know that 0 is a simpler bound)
$a_n$ is monotonically decreasing.
Now, if I can prove the first claim. then I can write $$a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}=frac{3a_n}{4}+frac{a_n}{a_n^2}le frac{3a_n}{4}+frac{a_n}{4}=a_n$$ which proves the second claim.
My only problem is proving the first one. If it is not the right direction, please hint me.
calculus
|
show 1 more comment
up vote
1
down vote
favorite
Let $$a_1=3quad , quad a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}$$
Show that $a_n$ converges.
I know that I need to prove that $a_n$ is monotonic and bounded.
I've assumed that the limit exists, made the calculation and get that $L=2$, hence claiming two claims:
$a_nge 2$ (I know that 0 is a simpler bound)
$a_n$ is monotonically decreasing.
Now, if I can prove the first claim. then I can write $$a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}=frac{3a_n}{4}+frac{a_n}{a_n^2}le frac{3a_n}{4}+frac{a_n}{4}=a_n$$ which proves the second claim.
My only problem is proving the first one. If it is not the right direction, please hint me.
calculus
I think it oscillates above and below 2, but I might be wrong
– mathworker21
Nov 13 at 8:08
@mathworker21, it does not.
– y12
Nov 13 at 8:09
1. You write "I've assumed that the limit exists, made the calculation and get that L=2". That is wrong. Show us your calculation.
– miracle173
Nov 13 at 9:32
2. Why do you think you have to use monotonicity? There are convergent sequences that are no monotone.
– miracle173
Nov 13 at 9:33
@miracle173, let $displaystyle lim_{ntoinfty}a_n=L$, then applying limit to both sides gives the equation $$L=frac{3L}{4}+frac{1}{L}iff L^2=4iff L=pm 2$$ and clearly $a_n>0$, thus $L=2$. Also, I have not written the question as it given - it asks "show that the sequence is monotonic and bounded, thus converges".
– y12
Nov 13 at 10:25
|
show 1 more comment
up vote
1
down vote
favorite
up vote
1
down vote
favorite
Let $$a_1=3quad , quad a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}$$
Show that $a_n$ converges.
I know that I need to prove that $a_n$ is monotonic and bounded.
I've assumed that the limit exists, made the calculation and get that $L=2$, hence claiming two claims:
$a_nge 2$ (I know that 0 is a simpler bound)
$a_n$ is monotonically decreasing.
Now, if I can prove the first claim. then I can write $$a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}=frac{3a_n}{4}+frac{a_n}{a_n^2}le frac{3a_n}{4}+frac{a_n}{4}=a_n$$ which proves the second claim.
My only problem is proving the first one. If it is not the right direction, please hint me.
calculus
Let $$a_1=3quad , quad a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}$$
Show that $a_n$ converges.
I know that I need to prove that $a_n$ is monotonic and bounded.
I've assumed that the limit exists, made the calculation and get that $L=2$, hence claiming two claims:
$a_nge 2$ (I know that 0 is a simpler bound)
$a_n$ is monotonically decreasing.
Now, if I can prove the first claim. then I can write $$a_{n+1}=frac{3a_n}{4}+frac{1}{a_n}=frac{3a_n}{4}+frac{a_n}{a_n^2}le frac{3a_n}{4}+frac{a_n}{4}=a_n$$ which proves the second claim.
My only problem is proving the first one. If it is not the right direction, please hint me.
calculus
calculus
asked Nov 13 at 8:04
y12
324
324
I think it oscillates above and below 2, but I might be wrong
– mathworker21
Nov 13 at 8:08
@mathworker21, it does not.
– y12
Nov 13 at 8:09
1. You write "I've assumed that the limit exists, made the calculation and get that L=2". That is wrong. Show us your calculation.
– miracle173
Nov 13 at 9:32
2. Why do you think you have to use monotonicity? There are convergent sequences that are no monotone.
– miracle173
Nov 13 at 9:33
@miracle173, let $displaystyle lim_{ntoinfty}a_n=L$, then applying limit to both sides gives the equation $$L=frac{3L}{4}+frac{1}{L}iff L^2=4iff L=pm 2$$ and clearly $a_n>0$, thus $L=2$. Also, I have not written the question as it given - it asks "show that the sequence is monotonic and bounded, thus converges".
– y12
Nov 13 at 10:25
|
show 1 more comment
I think it oscillates above and below 2, but I might be wrong
– mathworker21
Nov 13 at 8:08
@mathworker21, it does not.
– y12
Nov 13 at 8:09
1. You write "I've assumed that the limit exists, made the calculation and get that L=2". That is wrong. Show us your calculation.
– miracle173
Nov 13 at 9:32
2. Why do you think you have to use monotonicity? There are convergent sequences that are no monotone.
– miracle173
Nov 13 at 9:33
@miracle173, let $displaystyle lim_{ntoinfty}a_n=L$, then applying limit to both sides gives the equation $$L=frac{3L}{4}+frac{1}{L}iff L^2=4iff L=pm 2$$ and clearly $a_n>0$, thus $L=2$. Also, I have not written the question as it given - it asks "show that the sequence is monotonic and bounded, thus converges".
– y12
Nov 13 at 10:25
I think it oscillates above and below 2, but I might be wrong
– mathworker21
Nov 13 at 8:08
I think it oscillates above and below 2, but I might be wrong
– mathworker21
Nov 13 at 8:08
@mathworker21, it does not.
– y12
Nov 13 at 8:09
@mathworker21, it does not.
– y12
Nov 13 at 8:09
1. You write "I've assumed that the limit exists, made the calculation and get that L=2". That is wrong. Show us your calculation.
– miracle173
Nov 13 at 9:32
1. You write "I've assumed that the limit exists, made the calculation and get that L=2". That is wrong. Show us your calculation.
– miracle173
Nov 13 at 9:32
2. Why do you think you have to use monotonicity? There are convergent sequences that are no monotone.
– miracle173
Nov 13 at 9:33
2. Why do you think you have to use monotonicity? There are convergent sequences that are no monotone.
– miracle173
Nov 13 at 9:33
@miracle173, let $displaystyle lim_{ntoinfty}a_n=L$, then applying limit to both sides gives the equation $$L=frac{3L}{4}+frac{1}{L}iff L^2=4iff L=pm 2$$ and clearly $a_n>0$, thus $L=2$. Also, I have not written the question as it given - it asks "show that the sequence is monotonic and bounded, thus converges".
– y12
Nov 13 at 10:25
@miracle173, let $displaystyle lim_{ntoinfty}a_n=L$, then applying limit to both sides gives the equation $$L=frac{3L}{4}+frac{1}{L}iff L^2=4iff L=pm 2$$ and clearly $a_n>0$, thus $L=2$. Also, I have not written the question as it given - it asks "show that the sequence is monotonic and bounded, thus converges".
– y12
Nov 13 at 10:25
|
show 1 more comment
3 Answers
3
active
oldest
votes
up vote
3
down vote
accepted
To show boundedness from below, you may use AM-GM:
$$frac{3a_n}{4}+frac{1}{a_n}geq 2 sqrt{frac{3a_n}{4}cdot frac{1}{a_n}}= sqrt{3}$$
For convergence you may proceed as follows:
$f(x) = frac{3x}{4} + frac{1}{x}$ has a fixpoint for $x^{star} = 2$
- $f'(x) = frac{3}{4}-frac{1}{x^2}$
- A quick calculation shows that $|f'(x)| < 1$ for $x > frac{2}{sqrt{7}} Rightarrow |f'(x)| leq color{blue}{q} < 1$ for $x geq sqrt{3}$
So, for any starting value $a_0 > 0$ you get
$$|2-a_{n+1}| = |f'(xi_n)||2-a_n| < color{blue}{q} |2-a_n| < color{blue}{q^n}|2-a_1|stackrel{nto infty}{longrightarrow}0 $$
Edit after comment:
Specifically for your question concerning $a_n geq 2$:
$$ color{blue}{a_{n+1}-2} = frac{3a_n}{4}+frac{1}{a_n} - left(frac{3cdot 2}{4}+frac{1}{2} right)= frac{3}{4}left( a_n - 2 right) - frac{a_n - 2}{2a_n}$$
$$ = left(frac{3}{4} - frac{1}{2a_n} right)(a_n -2) stackrel{color{blue}{a_n geq 2}}{geq} frac{1}{2}(a_n -2) color{blue}{geq 0}$$
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
add a comment |
up vote
3
down vote
The inequality $ x geq 2$ implies $frac {3x} 4+frac 1 x geq 2$ because$frac {3x} 4+frac 1 x$ is increasing on $x >frac 2 {sqrt3}$ (and $frac {3times 2} 4+frac 1 2 =2$). This gives (by induction) $a_n geq 2$ for all $n$.
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
add a comment |
up vote
0
down vote
Let $a_n=2+d_n .$ Then $$d_{n+1}=3(2+d_n)/4+ 1/(2+d_n)-2.$$ Use some elementary algebra on this to show that $d_n>0implies d_n>d_{n+1}>0.$
So by induction on $n,$ if $0<a_1-2=d_1$ then $a_n$ decreases strictly to a limit $Lgeq 2.$
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
add a comment |
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
down vote
accepted
To show boundedness from below, you may use AM-GM:
$$frac{3a_n}{4}+frac{1}{a_n}geq 2 sqrt{frac{3a_n}{4}cdot frac{1}{a_n}}= sqrt{3}$$
For convergence you may proceed as follows:
$f(x) = frac{3x}{4} + frac{1}{x}$ has a fixpoint for $x^{star} = 2$
- $f'(x) = frac{3}{4}-frac{1}{x^2}$
- A quick calculation shows that $|f'(x)| < 1$ for $x > frac{2}{sqrt{7}} Rightarrow |f'(x)| leq color{blue}{q} < 1$ for $x geq sqrt{3}$
So, for any starting value $a_0 > 0$ you get
$$|2-a_{n+1}| = |f'(xi_n)||2-a_n| < color{blue}{q} |2-a_n| < color{blue}{q^n}|2-a_1|stackrel{nto infty}{longrightarrow}0 $$
Edit after comment:
Specifically for your question concerning $a_n geq 2$:
$$ color{blue}{a_{n+1}-2} = frac{3a_n}{4}+frac{1}{a_n} - left(frac{3cdot 2}{4}+frac{1}{2} right)= frac{3}{4}left( a_n - 2 right) - frac{a_n - 2}{2a_n}$$
$$ = left(frac{3}{4} - frac{1}{2a_n} right)(a_n -2) stackrel{color{blue}{a_n geq 2}}{geq} frac{1}{2}(a_n -2) color{blue}{geq 0}$$
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
add a comment |
up vote
3
down vote
accepted
To show boundedness from below, you may use AM-GM:
$$frac{3a_n}{4}+frac{1}{a_n}geq 2 sqrt{frac{3a_n}{4}cdot frac{1}{a_n}}= sqrt{3}$$
For convergence you may proceed as follows:
$f(x) = frac{3x}{4} + frac{1}{x}$ has a fixpoint for $x^{star} = 2$
- $f'(x) = frac{3}{4}-frac{1}{x^2}$
- A quick calculation shows that $|f'(x)| < 1$ for $x > frac{2}{sqrt{7}} Rightarrow |f'(x)| leq color{blue}{q} < 1$ for $x geq sqrt{3}$
So, for any starting value $a_0 > 0$ you get
$$|2-a_{n+1}| = |f'(xi_n)||2-a_n| < color{blue}{q} |2-a_n| < color{blue}{q^n}|2-a_1|stackrel{nto infty}{longrightarrow}0 $$
Edit after comment:
Specifically for your question concerning $a_n geq 2$:
$$ color{blue}{a_{n+1}-2} = frac{3a_n}{4}+frac{1}{a_n} - left(frac{3cdot 2}{4}+frac{1}{2} right)= frac{3}{4}left( a_n - 2 right) - frac{a_n - 2}{2a_n}$$
$$ = left(frac{3}{4} - frac{1}{2a_n} right)(a_n -2) stackrel{color{blue}{a_n geq 2}}{geq} frac{1}{2}(a_n -2) color{blue}{geq 0}$$
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
add a comment |
up vote
3
down vote
accepted
up vote
3
down vote
accepted
To show boundedness from below, you may use AM-GM:
$$frac{3a_n}{4}+frac{1}{a_n}geq 2 sqrt{frac{3a_n}{4}cdot frac{1}{a_n}}= sqrt{3}$$
For convergence you may proceed as follows:
$f(x) = frac{3x}{4} + frac{1}{x}$ has a fixpoint for $x^{star} = 2$
- $f'(x) = frac{3}{4}-frac{1}{x^2}$
- A quick calculation shows that $|f'(x)| < 1$ for $x > frac{2}{sqrt{7}} Rightarrow |f'(x)| leq color{blue}{q} < 1$ for $x geq sqrt{3}$
So, for any starting value $a_0 > 0$ you get
$$|2-a_{n+1}| = |f'(xi_n)||2-a_n| < color{blue}{q} |2-a_n| < color{blue}{q^n}|2-a_1|stackrel{nto infty}{longrightarrow}0 $$
Edit after comment:
Specifically for your question concerning $a_n geq 2$:
$$ color{blue}{a_{n+1}-2} = frac{3a_n}{4}+frac{1}{a_n} - left(frac{3cdot 2}{4}+frac{1}{2} right)= frac{3}{4}left( a_n - 2 right) - frac{a_n - 2}{2a_n}$$
$$ = left(frac{3}{4} - frac{1}{2a_n} right)(a_n -2) stackrel{color{blue}{a_n geq 2}}{geq} frac{1}{2}(a_n -2) color{blue}{geq 0}$$
To show boundedness from below, you may use AM-GM:
$$frac{3a_n}{4}+frac{1}{a_n}geq 2 sqrt{frac{3a_n}{4}cdot frac{1}{a_n}}= sqrt{3}$$
For convergence you may proceed as follows:
$f(x) = frac{3x}{4} + frac{1}{x}$ has a fixpoint for $x^{star} = 2$
- $f'(x) = frac{3}{4}-frac{1}{x^2}$
- A quick calculation shows that $|f'(x)| < 1$ for $x > frac{2}{sqrt{7}} Rightarrow |f'(x)| leq color{blue}{q} < 1$ for $x geq sqrt{3}$
So, for any starting value $a_0 > 0$ you get
$$|2-a_{n+1}| = |f'(xi_n)||2-a_n| < color{blue}{q} |2-a_n| < color{blue}{q^n}|2-a_1|stackrel{nto infty}{longrightarrow}0 $$
Edit after comment:
Specifically for your question concerning $a_n geq 2$:
$$ color{blue}{a_{n+1}-2} = frac{3a_n}{4}+frac{1}{a_n} - left(frac{3cdot 2}{4}+frac{1}{2} right)= frac{3}{4}left( a_n - 2 right) - frac{a_n - 2}{2a_n}$$
$$ = left(frac{3}{4} - frac{1}{2a_n} right)(a_n -2) stackrel{color{blue}{a_n geq 2}}{geq} frac{1}{2}(a_n -2) color{blue}{geq 0}$$
edited Nov 13 at 10:50
answered Nov 13 at 8:07
trancelocation
7,7811519
7,7811519
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
add a comment |
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
"show that $a_n$ converges"
– mathworker21
Nov 13 at 8:08
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
A simpler lower bound for $a_n$ is $0$, but from that I cannot show that $a_n$ decreases...
– y12
Nov 13 at 8:10
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
@y12: Yes. But the lower bound $sqrt{3}$ is very useful for showing that the iteration converges for any starting value $a_0 > 0$ to the positive fixpoint of $f$, because $a_1 geq sqrt{3} > frac{2}{sqrt{7}}$.
– trancelocation
Nov 13 at 8:31
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
I'm not allowed to use functions...
– y12
Nov 13 at 8:44
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
@y12: I added something concerning your specific question.
– trancelocation
Nov 13 at 10:51
add a comment |
up vote
3
down vote
The inequality $ x geq 2$ implies $frac {3x} 4+frac 1 x geq 2$ because$frac {3x} 4+frac 1 x$ is increasing on $x >frac 2 {sqrt3}$ (and $frac {3times 2} 4+frac 1 2 =2$). This gives (by induction) $a_n geq 2$ for all $n$.
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
add a comment |
up vote
3
down vote
The inequality $ x geq 2$ implies $frac {3x} 4+frac 1 x geq 2$ because$frac {3x} 4+frac 1 x$ is increasing on $x >frac 2 {sqrt3}$ (and $frac {3times 2} 4+frac 1 2 =2$). This gives (by induction) $a_n geq 2$ for all $n$.
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
add a comment |
up vote
3
down vote
up vote
3
down vote
The inequality $ x geq 2$ implies $frac {3x} 4+frac 1 x geq 2$ because$frac {3x} 4+frac 1 x$ is increasing on $x >frac 2 {sqrt3}$ (and $frac {3times 2} 4+frac 1 2 =2$). This gives (by induction) $a_n geq 2$ for all $n$.
The inequality $ x geq 2$ implies $frac {3x} 4+frac 1 x geq 2$ because$frac {3x} 4+frac 1 x$ is increasing on $x >frac 2 {sqrt3}$ (and $frac {3times 2} 4+frac 1 2 =2$). This gives (by induction) $a_n geq 2$ for all $n$.
edited 2 days ago
miracle173
7,26822247
7,26822247
answered Nov 13 at 8:20
Kavi Rama Murthy
39.3k31748
39.3k31748
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
add a comment |
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
Kavi Rama.You show (nicely) that for $xge 2$ your function is increasing.But $a_n$ is decreasing . Could go below $2$? By AM-GM $ge √3$.Your thoughts , thanks, Peter m.wolframalpha.com/input/…
– Peter Szilas
Nov 13 at 11:19
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
No, it cannot. $a_n geq 2$ implies $frac {3a_n} 4+frac 1 {a_n} geq frac {(3)(2)} 4+frac 1 2 =2$. Use induction to conclude that every $a_n geq 2$.
– Kavi Rama Murthy
Nov 13 at 11:52
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
Kavi Rama.Thanks.
– Peter Szilas
Nov 13 at 12:32
add a comment |
up vote
0
down vote
Let $a_n=2+d_n .$ Then $$d_{n+1}=3(2+d_n)/4+ 1/(2+d_n)-2.$$ Use some elementary algebra on this to show that $d_n>0implies d_n>d_{n+1}>0.$
So by induction on $n,$ if $0<a_1-2=d_1$ then $a_n$ decreases strictly to a limit $Lgeq 2.$
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
add a comment |
up vote
0
down vote
Let $a_n=2+d_n .$ Then $$d_{n+1}=3(2+d_n)/4+ 1/(2+d_n)-2.$$ Use some elementary algebra on this to show that $d_n>0implies d_n>d_{n+1}>0.$
So by induction on $n,$ if $0<a_1-2=d_1$ then $a_n$ decreases strictly to a limit $Lgeq 2.$
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
add a comment |
up vote
0
down vote
up vote
0
down vote
Let $a_n=2+d_n .$ Then $$d_{n+1}=3(2+d_n)/4+ 1/(2+d_n)-2.$$ Use some elementary algebra on this to show that $d_n>0implies d_n>d_{n+1}>0.$
So by induction on $n,$ if $0<a_1-2=d_1$ then $a_n$ decreases strictly to a limit $Lgeq 2.$
Let $a_n=2+d_n .$ Then $$d_{n+1}=3(2+d_n)/4+ 1/(2+d_n)-2.$$ Use some elementary algebra on this to show that $d_n>0implies d_n>d_{n+1}>0.$
So by induction on $n,$ if $0<a_1-2=d_1$ then $a_n$ decreases strictly to a limit $Lgeq 2.$
edited Nov 13 at 8:55
answered Nov 13 at 8:41
DanielWainfleet
33.3k31646
33.3k31646
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
add a comment |
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
You should get (assuming $d_nne -2$) that $d_{n+1}=d_n(frac {3}{4}-frac {1}{4+2d_n}).$ So if $d_n>0$ then $frac {3}{4}d_n>d_{n+1}>0.$
– DanielWainfleet
Nov 13 at 8:52
add a comment |
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f2996466%2fproving-a-sequence-is-bounded-from-below%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
I think it oscillates above and below 2, but I might be wrong
– mathworker21
Nov 13 at 8:08
@mathworker21, it does not.
– y12
Nov 13 at 8:09
1. You write "I've assumed that the limit exists, made the calculation and get that L=2". That is wrong. Show us your calculation.
– miracle173
Nov 13 at 9:32
2. Why do you think you have to use monotonicity? There are convergent sequences that are no monotone.
– miracle173
Nov 13 at 9:33
@miracle173, let $displaystyle lim_{ntoinfty}a_n=L$, then applying limit to both sides gives the equation $$L=frac{3L}{4}+frac{1}{L}iff L^2=4iff L=pm 2$$ and clearly $a_n>0$, thus $L=2$. Also, I have not written the question as it given - it asks "show that the sequence is monotonic and bounded, thus converges".
– y12
Nov 13 at 10:25