Finding the infinitesimal order of a function as $n to infty$
$begingroup$
I have to find the infinitesimal order of a function $f(n)$ as $n to infty$,
this is what I did:
$$
begin{split}
f(n) &= sqrt{n+1} - sqrt{n} + frac{1}n \
&= frac{nsqrt{n+1} - nsqrt{n} + 1}{n} \
&= frac{sqrt{n^3(1+frac{1}{n^2})}-sqrt{n^3}+1}{n} \
&= frac{sqrt{n^3}-sqrt{n^3}+1}{n} \
&= frac{1}{n}
end{split}
$$
So the infinitesimal order is: 1
Is it correct?
Or should I have used some notable limit, taylor or asymptomatic approximation.
calculus functions infinitesimals
$endgroup$
add a comment |
$begingroup$
I have to find the infinitesimal order of a function $f(n)$ as $n to infty$,
this is what I did:
$$
begin{split}
f(n) &= sqrt{n+1} - sqrt{n} + frac{1}n \
&= frac{nsqrt{n+1} - nsqrt{n} + 1}{n} \
&= frac{sqrt{n^3(1+frac{1}{n^2})}-sqrt{n^3}+1}{n} \
&= frac{sqrt{n^3}-sqrt{n^3}+1}{n} \
&= frac{1}{n}
end{split}
$$
So the infinitesimal order is: 1
Is it correct?
Or should I have used some notable limit, taylor or asymptomatic approximation.
calculus functions infinitesimals
$endgroup$
$begingroup$
Do you have a typo or two? Or are you using "$=$" to mean something other than "equals"?
$endgroup$
– DanielWainfleet
Jan 10 at 15:58
$begingroup$
I had one, I already correct it, it was 1/n instead of 1/2, the = means "equals"
$endgroup$
– El Bryan
Jan 10 at 15:59
2
$begingroup$
@ElBryan : $left( 1 + frac{1}{n^2} right) neq 1$, so you are not using "$=$" to mean just "equals".
$endgroup$
– Eric Towers
Jan 10 at 16:04
$begingroup$
I see, then I'm wrong, I think I should have used "~"
$endgroup$
– El Bryan
Jan 10 at 16:06
$begingroup$
Is the infinitesimal order = 1?
$endgroup$
– El Bryan
Jan 10 at 16:09
add a comment |
$begingroup$
I have to find the infinitesimal order of a function $f(n)$ as $n to infty$,
this is what I did:
$$
begin{split}
f(n) &= sqrt{n+1} - sqrt{n} + frac{1}n \
&= frac{nsqrt{n+1} - nsqrt{n} + 1}{n} \
&= frac{sqrt{n^3(1+frac{1}{n^2})}-sqrt{n^3}+1}{n} \
&= frac{sqrt{n^3}-sqrt{n^3}+1}{n} \
&= frac{1}{n}
end{split}
$$
So the infinitesimal order is: 1
Is it correct?
Or should I have used some notable limit, taylor or asymptomatic approximation.
calculus functions infinitesimals
$endgroup$
I have to find the infinitesimal order of a function $f(n)$ as $n to infty$,
this is what I did:
$$
begin{split}
f(n) &= sqrt{n+1} - sqrt{n} + frac{1}n \
&= frac{nsqrt{n+1} - nsqrt{n} + 1}{n} \
&= frac{sqrt{n^3(1+frac{1}{n^2})}-sqrt{n^3}+1}{n} \
&= frac{sqrt{n^3}-sqrt{n^3}+1}{n} \
&= frac{1}{n}
end{split}
$$
So the infinitesimal order is: 1
Is it correct?
Or should I have used some notable limit, taylor or asymptomatic approximation.
calculus functions infinitesimals
calculus functions infinitesimals
edited Jan 10 at 16:06
gt6989b
35.6k22557
35.6k22557
asked Jan 10 at 15:46
El BryanEl Bryan
417
417
$begingroup$
Do you have a typo or two? Or are you using "$=$" to mean something other than "equals"?
$endgroup$
– DanielWainfleet
Jan 10 at 15:58
$begingroup$
I had one, I already correct it, it was 1/n instead of 1/2, the = means "equals"
$endgroup$
– El Bryan
Jan 10 at 15:59
2
$begingroup$
@ElBryan : $left( 1 + frac{1}{n^2} right) neq 1$, so you are not using "$=$" to mean just "equals".
$endgroup$
– Eric Towers
Jan 10 at 16:04
$begingroup$
I see, then I'm wrong, I think I should have used "~"
$endgroup$
– El Bryan
Jan 10 at 16:06
$begingroup$
Is the infinitesimal order = 1?
$endgroup$
– El Bryan
Jan 10 at 16:09
add a comment |
$begingroup$
Do you have a typo or two? Or are you using "$=$" to mean something other than "equals"?
$endgroup$
– DanielWainfleet
Jan 10 at 15:58
$begingroup$
I had one, I already correct it, it was 1/n instead of 1/2, the = means "equals"
$endgroup$
– El Bryan
Jan 10 at 15:59
2
$begingroup$
@ElBryan : $left( 1 + frac{1}{n^2} right) neq 1$, so you are not using "$=$" to mean just "equals".
$endgroup$
– Eric Towers
Jan 10 at 16:04
$begingroup$
I see, then I'm wrong, I think I should have used "~"
$endgroup$
– El Bryan
Jan 10 at 16:06
$begingroup$
Is the infinitesimal order = 1?
$endgroup$
– El Bryan
Jan 10 at 16:09
$begingroup$
Do you have a typo or two? Or are you using "$=$" to mean something other than "equals"?
$endgroup$
– DanielWainfleet
Jan 10 at 15:58
$begingroup$
Do you have a typo or two? Or are you using "$=$" to mean something other than "equals"?
$endgroup$
– DanielWainfleet
Jan 10 at 15:58
$begingroup$
I had one, I already correct it, it was 1/n instead of 1/2, the = means "equals"
$endgroup$
– El Bryan
Jan 10 at 15:59
$begingroup$
I had one, I already correct it, it was 1/n instead of 1/2, the = means "equals"
$endgroup$
– El Bryan
Jan 10 at 15:59
2
2
$begingroup$
@ElBryan : $left( 1 + frac{1}{n^2} right) neq 1$, so you are not using "$=$" to mean just "equals".
$endgroup$
– Eric Towers
Jan 10 at 16:04
$begingroup$
@ElBryan : $left( 1 + frac{1}{n^2} right) neq 1$, so you are not using "$=$" to mean just "equals".
$endgroup$
– Eric Towers
Jan 10 at 16:04
$begingroup$
I see, then I'm wrong, I think I should have used "~"
$endgroup$
– El Bryan
Jan 10 at 16:06
$begingroup$
I see, then I'm wrong, I think I should have used "~"
$endgroup$
– El Bryan
Jan 10 at 16:06
$begingroup$
Is the infinitesimal order = 1?
$endgroup$
– El Bryan
Jan 10 at 16:09
$begingroup$
Is the infinitesimal order = 1?
$endgroup$
– El Bryan
Jan 10 at 16:09
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Generally it's easier to note that
$$
sqrt{n+1}=sqrt{n}sqrt{1+1/n}sim sqrt{n}left(1+frac{1}{2n}right)=sqrt{n}+frac{1}{2sqrt{n}},
$$
using the Taylor series for $sqrt{1+x}$. Then
$$
sqrt{n+1}-sqrt{n}+frac{1}{n}simfrac{1}{2sqrt{n}}+frac{1}{n},
$$
with omitted terms $O(n^{-3/2})$. The error in your original calculation is that $nsqrt{n+1}=sqrt{n^3+n^2}=sqrt{n^3(1+1/n)}$, not $sqrt{n^3(1+1/n^2)}$.
$endgroup$
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
add a comment |
$begingroup$
I would consider the following instead:
$$
begin{split}
sqrt{n+1} - sqrt{n}
&= left(sqrt{n+1} - sqrt{n}right) times
frac{sqrt{n+1} + sqrt{n}}{sqrt{n+1} + sqrt{n}}\
&= frac{n+1-n}{sqrt{n+1} + sqrt{n}} \
&= frac{1}{sqrt{n+1} + sqrt{n}} \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
therefore,
$$
begin{split}
f(n)
&= sqrt{n+1} - sqrt{n} + frac1n \
&= frac{1}{sqrt{n+1} + sqrt{n}} + frac1n \
&= Thetaleft(n^{-1/2} + n^{-1}right) \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
$endgroup$
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Generally it's easier to note that
$$
sqrt{n+1}=sqrt{n}sqrt{1+1/n}sim sqrt{n}left(1+frac{1}{2n}right)=sqrt{n}+frac{1}{2sqrt{n}},
$$
using the Taylor series for $sqrt{1+x}$. Then
$$
sqrt{n+1}-sqrt{n}+frac{1}{n}simfrac{1}{2sqrt{n}}+frac{1}{n},
$$
with omitted terms $O(n^{-3/2})$. The error in your original calculation is that $nsqrt{n+1}=sqrt{n^3+n^2}=sqrt{n^3(1+1/n)}$, not $sqrt{n^3(1+1/n^2)}$.
$endgroup$
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
add a comment |
$begingroup$
Generally it's easier to note that
$$
sqrt{n+1}=sqrt{n}sqrt{1+1/n}sim sqrt{n}left(1+frac{1}{2n}right)=sqrt{n}+frac{1}{2sqrt{n}},
$$
using the Taylor series for $sqrt{1+x}$. Then
$$
sqrt{n+1}-sqrt{n}+frac{1}{n}simfrac{1}{2sqrt{n}}+frac{1}{n},
$$
with omitted terms $O(n^{-3/2})$. The error in your original calculation is that $nsqrt{n+1}=sqrt{n^3+n^2}=sqrt{n^3(1+1/n)}$, not $sqrt{n^3(1+1/n^2)}$.
$endgroup$
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
add a comment |
$begingroup$
Generally it's easier to note that
$$
sqrt{n+1}=sqrt{n}sqrt{1+1/n}sim sqrt{n}left(1+frac{1}{2n}right)=sqrt{n}+frac{1}{2sqrt{n}},
$$
using the Taylor series for $sqrt{1+x}$. Then
$$
sqrt{n+1}-sqrt{n}+frac{1}{n}simfrac{1}{2sqrt{n}}+frac{1}{n},
$$
with omitted terms $O(n^{-3/2})$. The error in your original calculation is that $nsqrt{n+1}=sqrt{n^3+n^2}=sqrt{n^3(1+1/n)}$, not $sqrt{n^3(1+1/n^2)}$.
$endgroup$
Generally it's easier to note that
$$
sqrt{n+1}=sqrt{n}sqrt{1+1/n}sim sqrt{n}left(1+frac{1}{2n}right)=sqrt{n}+frac{1}{2sqrt{n}},
$$
using the Taylor series for $sqrt{1+x}$. Then
$$
sqrt{n+1}-sqrt{n}+frac{1}{n}simfrac{1}{2sqrt{n}}+frac{1}{n},
$$
with omitted terms $O(n^{-3/2})$. The error in your original calculation is that $nsqrt{n+1}=sqrt{n^3+n^2}=sqrt{n^3(1+1/n)}$, not $sqrt{n^3(1+1/n^2)}$.
answered Jan 10 at 16:12
mjqxxxxmjqxxxx
31.6k24186
31.6k24186
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
add a comment |
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
Your answer is -3/2, the other user answer is -1/2, which one is the correct?
$endgroup$
– El Bryan
Jan 10 at 16:21
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
$begingroup$
No, both answers agree: $f(n)$ is asymptotic to a term of order $n^{-1/2}$ plus additional smaller terms.
$endgroup$
– mjqxxxx
Jan 10 at 20:15
add a comment |
$begingroup$
I would consider the following instead:
$$
begin{split}
sqrt{n+1} - sqrt{n}
&= left(sqrt{n+1} - sqrt{n}right) times
frac{sqrt{n+1} + sqrt{n}}{sqrt{n+1} + sqrt{n}}\
&= frac{n+1-n}{sqrt{n+1} + sqrt{n}} \
&= frac{1}{sqrt{n+1} + sqrt{n}} \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
therefore,
$$
begin{split}
f(n)
&= sqrt{n+1} - sqrt{n} + frac1n \
&= frac{1}{sqrt{n+1} + sqrt{n}} + frac1n \
&= Thetaleft(n^{-1/2} + n^{-1}right) \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
$endgroup$
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
add a comment |
$begingroup$
I would consider the following instead:
$$
begin{split}
sqrt{n+1} - sqrt{n}
&= left(sqrt{n+1} - sqrt{n}right) times
frac{sqrt{n+1} + sqrt{n}}{sqrt{n+1} + sqrt{n}}\
&= frac{n+1-n}{sqrt{n+1} + sqrt{n}} \
&= frac{1}{sqrt{n+1} + sqrt{n}} \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
therefore,
$$
begin{split}
f(n)
&= sqrt{n+1} - sqrt{n} + frac1n \
&= frac{1}{sqrt{n+1} + sqrt{n}} + frac1n \
&= Thetaleft(n^{-1/2} + n^{-1}right) \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
$endgroup$
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
add a comment |
$begingroup$
I would consider the following instead:
$$
begin{split}
sqrt{n+1} - sqrt{n}
&= left(sqrt{n+1} - sqrt{n}right) times
frac{sqrt{n+1} + sqrt{n}}{sqrt{n+1} + sqrt{n}}\
&= frac{n+1-n}{sqrt{n+1} + sqrt{n}} \
&= frac{1}{sqrt{n+1} + sqrt{n}} \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
therefore,
$$
begin{split}
f(n)
&= sqrt{n+1} - sqrt{n} + frac1n \
&= frac{1}{sqrt{n+1} + sqrt{n}} + frac1n \
&= Thetaleft(n^{-1/2} + n^{-1}right) \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
$endgroup$
I would consider the following instead:
$$
begin{split}
sqrt{n+1} - sqrt{n}
&= left(sqrt{n+1} - sqrt{n}right) times
frac{sqrt{n+1} + sqrt{n}}{sqrt{n+1} + sqrt{n}}\
&= frac{n+1-n}{sqrt{n+1} + sqrt{n}} \
&= frac{1}{sqrt{n+1} + sqrt{n}} \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
therefore,
$$
begin{split}
f(n)
&= sqrt{n+1} - sqrt{n} + frac1n \
&= frac{1}{sqrt{n+1} + sqrt{n}} + frac1n \
&= Thetaleft(n^{-1/2} + n^{-1}right) \
&= Thetaleft(n^{-1/2}right)
end{split}
$$
edited Jan 10 at 16:12
answered Jan 10 at 16:10
gt6989bgt6989b
35.6k22557
35.6k22557
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
add a comment |
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
So my answer is wrong?, it was that easy....
$endgroup$
– El Bryan
Jan 10 at 16:12
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
$begingroup$
@ElBryan your approach was incorrect, the error you ignored in your approximation was too big
$endgroup$
– gt6989b
Jan 10 at 16:13
add a comment |
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$begingroup$
Do you have a typo or two? Or are you using "$=$" to mean something other than "equals"?
$endgroup$
– DanielWainfleet
Jan 10 at 15:58
$begingroup$
I had one, I already correct it, it was 1/n instead of 1/2, the = means "equals"
$endgroup$
– El Bryan
Jan 10 at 15:59
2
$begingroup$
@ElBryan : $left( 1 + frac{1}{n^2} right) neq 1$, so you are not using "$=$" to mean just "equals".
$endgroup$
– Eric Towers
Jan 10 at 16:04
$begingroup$
I see, then I'm wrong, I think I should have used "~"
$endgroup$
– El Bryan
Jan 10 at 16:06
$begingroup$
Is the infinitesimal order = 1?
$endgroup$
– El Bryan
Jan 10 at 16:09