Limits at infinity for sequences and funtions
First recall the definitions
Definition I: Let $(a_n)$ be a sequence of real or complex numbers. We say that $a_nto infty$ as $nto infty$, if
$$
forall R>0 exists Ninmathbb{N} forall ninmathbb{N}:[(ngeq N)implies (a_n>R) ].
$$
Definition II: Let $f:Dto mathbb{C}$, where $Dsubseteq mathbb{R}$ is not bounded above. We say that $f(x)to infty$ as $xto infty$, if
$$
forall R>0, exists Kinmathbb{R}, forall xin D: [(xgeq K)implies (f(x)>R)].
$$
Question: Let $f:[1,infty)to mathbb{C}$, and let $a_n=f(n)$ for $ngeq 1$. It is clear that if $f(x)toinfty$ as $xtoinfty$, then $a_ntoinfty$ as $ntoinfty$ (i.e. Definition II implies Definition I). Does the converse direction hold in general? If we assume that $a_ntoinfty$ as $ntoinfty$, then there are a lot of "gaps" (i.e. the elements of $[1,infty)setminus mathbb{N}$) that should be checked in Definition II.
real-analysis
add a comment |
First recall the definitions
Definition I: Let $(a_n)$ be a sequence of real or complex numbers. We say that $a_nto infty$ as $nto infty$, if
$$
forall R>0 exists Ninmathbb{N} forall ninmathbb{N}:[(ngeq N)implies (a_n>R) ].
$$
Definition II: Let $f:Dto mathbb{C}$, where $Dsubseteq mathbb{R}$ is not bounded above. We say that $f(x)to infty$ as $xto infty$, if
$$
forall R>0, exists Kinmathbb{R}, forall xin D: [(xgeq K)implies (f(x)>R)].
$$
Question: Let $f:[1,infty)to mathbb{C}$, and let $a_n=f(n)$ for $ngeq 1$. It is clear that if $f(x)toinfty$ as $xtoinfty$, then $a_ntoinfty$ as $ntoinfty$ (i.e. Definition II implies Definition I). Does the converse direction hold in general? If we assume that $a_ntoinfty$ as $ntoinfty$, then there are a lot of "gaps" (i.e. the elements of $[1,infty)setminus mathbb{N}$) that should be checked in Definition II.
real-analysis
Yes, I think you are right. We can easily imagine a function that goes to hell and back between integers and yet always is "nice" on the integers. For example $f(x) = xcos(2pi*x)$ for $a_n= f(n) = n$ it's clear $a_nto infty$ but between integers will go way up and way down and it's clear definition 2 fails. .. Oooh.... also if $b_n = f(n + frac 12)=-n-frac 12to -infty$. We can't conclude both $f(x)to pm infty$. And just to be really annoying $c_n = f(n+frac 14)=(n+frac 14)(2npi + frac pi 2) =0$. So $c_nto 0$. ... we're.... boned if we try to conclude that direction.
– fleablood
Nov 27 '18 at 18:43
add a comment |
First recall the definitions
Definition I: Let $(a_n)$ be a sequence of real or complex numbers. We say that $a_nto infty$ as $nto infty$, if
$$
forall R>0 exists Ninmathbb{N} forall ninmathbb{N}:[(ngeq N)implies (a_n>R) ].
$$
Definition II: Let $f:Dto mathbb{C}$, where $Dsubseteq mathbb{R}$ is not bounded above. We say that $f(x)to infty$ as $xto infty$, if
$$
forall R>0, exists Kinmathbb{R}, forall xin D: [(xgeq K)implies (f(x)>R)].
$$
Question: Let $f:[1,infty)to mathbb{C}$, and let $a_n=f(n)$ for $ngeq 1$. It is clear that if $f(x)toinfty$ as $xtoinfty$, then $a_ntoinfty$ as $ntoinfty$ (i.e. Definition II implies Definition I). Does the converse direction hold in general? If we assume that $a_ntoinfty$ as $ntoinfty$, then there are a lot of "gaps" (i.e. the elements of $[1,infty)setminus mathbb{N}$) that should be checked in Definition II.
real-analysis
First recall the definitions
Definition I: Let $(a_n)$ be a sequence of real or complex numbers. We say that $a_nto infty$ as $nto infty$, if
$$
forall R>0 exists Ninmathbb{N} forall ninmathbb{N}:[(ngeq N)implies (a_n>R) ].
$$
Definition II: Let $f:Dto mathbb{C}$, where $Dsubseteq mathbb{R}$ is not bounded above. We say that $f(x)to infty$ as $xto infty$, if
$$
forall R>0, exists Kinmathbb{R}, forall xin D: [(xgeq K)implies (f(x)>R)].
$$
Question: Let $f:[1,infty)to mathbb{C}$, and let $a_n=f(n)$ for $ngeq 1$. It is clear that if $f(x)toinfty$ as $xtoinfty$, then $a_ntoinfty$ as $ntoinfty$ (i.e. Definition II implies Definition I). Does the converse direction hold in general? If we assume that $a_ntoinfty$ as $ntoinfty$, then there are a lot of "gaps" (i.e. the elements of $[1,infty)setminus mathbb{N}$) that should be checked in Definition II.
real-analysis
real-analysis
asked Nov 27 '18 at 17:54
UnknownW
953822
953822
Yes, I think you are right. We can easily imagine a function that goes to hell and back between integers and yet always is "nice" on the integers. For example $f(x) = xcos(2pi*x)$ for $a_n= f(n) = n$ it's clear $a_nto infty$ but between integers will go way up and way down and it's clear definition 2 fails. .. Oooh.... also if $b_n = f(n + frac 12)=-n-frac 12to -infty$. We can't conclude both $f(x)to pm infty$. And just to be really annoying $c_n = f(n+frac 14)=(n+frac 14)(2npi + frac pi 2) =0$. So $c_nto 0$. ... we're.... boned if we try to conclude that direction.
– fleablood
Nov 27 '18 at 18:43
add a comment |
Yes, I think you are right. We can easily imagine a function that goes to hell and back between integers and yet always is "nice" on the integers. For example $f(x) = xcos(2pi*x)$ for $a_n= f(n) = n$ it's clear $a_nto infty$ but between integers will go way up and way down and it's clear definition 2 fails. .. Oooh.... also if $b_n = f(n + frac 12)=-n-frac 12to -infty$. We can't conclude both $f(x)to pm infty$. And just to be really annoying $c_n = f(n+frac 14)=(n+frac 14)(2npi + frac pi 2) =0$. So $c_nto 0$. ... we're.... boned if we try to conclude that direction.
– fleablood
Nov 27 '18 at 18:43
Yes, I think you are right. We can easily imagine a function that goes to hell and back between integers and yet always is "nice" on the integers. For example $f(x) = xcos(2pi*x)$ for $a_n= f(n) = n$ it's clear $a_nto infty$ but between integers will go way up and way down and it's clear definition 2 fails. .. Oooh.... also if $b_n = f(n + frac 12)=-n-frac 12to -infty$. We can't conclude both $f(x)to pm infty$. And just to be really annoying $c_n = f(n+frac 14)=(n+frac 14)(2npi + frac pi 2) =0$. So $c_nto 0$. ... we're.... boned if we try to conclude that direction.
– fleablood
Nov 27 '18 at 18:43
Yes, I think you are right. We can easily imagine a function that goes to hell and back between integers and yet always is "nice" on the integers. For example $f(x) = xcos(2pi*x)$ for $a_n= f(n) = n$ it's clear $a_nto infty$ but between integers will go way up and way down and it's clear definition 2 fails. .. Oooh.... also if $b_n = f(n + frac 12)=-n-frac 12to -infty$. We can't conclude both $f(x)to pm infty$. And just to be really annoying $c_n = f(n+frac 14)=(n+frac 14)(2npi + frac pi 2) =0$. So $c_nto 0$. ... we're.... boned if we try to conclude that direction.
– fleablood
Nov 27 '18 at 18:43
add a comment |
2 Answers
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You are right that $f(n) to infty$ as $n to infty$ does not imply $f(x) to infty$ as $x to infty$. But I would not say things like "Definition II implies Definition I" because the two definitions intrinsically have nothing to do with each other. You've forced a connection with the definition $a_n = f(n)$, but as you pointed out, the behavior of $f$ at natural numbers $n$ does not govern the behavior of $f$ anywhere else.
Concrete counterexample:
Let $f(x) = begin{cases} x & text{if }x in mathbb{N} \ 0 & text{otherwise}end{cases}$.
If $a_n = f(n)$ then $a_n to infty$ as $n to infty$. But $f(x) notto infty$ as $x to infty$.
add a comment |
You are correct.
A good counter example is $f(x) = xcos(2pi x)$ and
$a_n = f(n); b_n = f(n+frac 12);$ $c_n = f(n + frac 14)$
$a_n = ncos (2pi n) = n to infty$
$b_n = (n+frac 12)cos ((2n + 1)pi)=-n -frac 12 to -infty$
$c_n =(n+frac 1r) cos(2npi + frac pi 2) = (n+frac 14)cdot 0 = 0to 0$.
Clearly we can't conclude anything about the limits of $f(x)$ by the limits of $a_n, b_n$ and $c_n$.
(Have you had the theorem that if a sequence converges then all subsequence converge to the same limit? If so this should be clear. If not this should give you a really good idea we the theorem holds.)
And if we actually try to find the limit if $f(x)$.
For Any $R >0$ and all $K in Bbb R$ we can find $x > max (|K|+1, R); x in mathbb N$ so $f(x+frac 12) = -x-frac 12 < -|K|-1 < K$ so $lim f(x) ne infty$.
And $f(x) = x > |K|+1 > K$ so $lim f(x) ne -infty$
and for $0< epsilon < 1$ $|f(x)-K| = ||K|+1 - K| ge 1 > epsilon$ so $lim f(x) ne K$.
So $f(x)$ does not converge to any value nor does it converge to infinity or negative infinity.
SO... Yes, you are correct.
add a comment |
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2 Answers
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2 Answers
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You are right that $f(n) to infty$ as $n to infty$ does not imply $f(x) to infty$ as $x to infty$. But I would not say things like "Definition II implies Definition I" because the two definitions intrinsically have nothing to do with each other. You've forced a connection with the definition $a_n = f(n)$, but as you pointed out, the behavior of $f$ at natural numbers $n$ does not govern the behavior of $f$ anywhere else.
Concrete counterexample:
Let $f(x) = begin{cases} x & text{if }x in mathbb{N} \ 0 & text{otherwise}end{cases}$.
If $a_n = f(n)$ then $a_n to infty$ as $n to infty$. But $f(x) notto infty$ as $x to infty$.
add a comment |
You are right that $f(n) to infty$ as $n to infty$ does not imply $f(x) to infty$ as $x to infty$. But I would not say things like "Definition II implies Definition I" because the two definitions intrinsically have nothing to do with each other. You've forced a connection with the definition $a_n = f(n)$, but as you pointed out, the behavior of $f$ at natural numbers $n$ does not govern the behavior of $f$ anywhere else.
Concrete counterexample:
Let $f(x) = begin{cases} x & text{if }x in mathbb{N} \ 0 & text{otherwise}end{cases}$.
If $a_n = f(n)$ then $a_n to infty$ as $n to infty$. But $f(x) notto infty$ as $x to infty$.
add a comment |
You are right that $f(n) to infty$ as $n to infty$ does not imply $f(x) to infty$ as $x to infty$. But I would not say things like "Definition II implies Definition I" because the two definitions intrinsically have nothing to do with each other. You've forced a connection with the definition $a_n = f(n)$, but as you pointed out, the behavior of $f$ at natural numbers $n$ does not govern the behavior of $f$ anywhere else.
Concrete counterexample:
Let $f(x) = begin{cases} x & text{if }x in mathbb{N} \ 0 & text{otherwise}end{cases}$.
If $a_n = f(n)$ then $a_n to infty$ as $n to infty$. But $f(x) notto infty$ as $x to infty$.
You are right that $f(n) to infty$ as $n to infty$ does not imply $f(x) to infty$ as $x to infty$. But I would not say things like "Definition II implies Definition I" because the two definitions intrinsically have nothing to do with each other. You've forced a connection with the definition $a_n = f(n)$, but as you pointed out, the behavior of $f$ at natural numbers $n$ does not govern the behavior of $f$ anywhere else.
Concrete counterexample:
Let $f(x) = begin{cases} x & text{if }x in mathbb{N} \ 0 & text{otherwise}end{cases}$.
If $a_n = f(n)$ then $a_n to infty$ as $n to infty$. But $f(x) notto infty$ as $x to infty$.
answered Nov 27 '18 at 18:02
angryavian
39k23180
39k23180
add a comment |
add a comment |
You are correct.
A good counter example is $f(x) = xcos(2pi x)$ and
$a_n = f(n); b_n = f(n+frac 12);$ $c_n = f(n + frac 14)$
$a_n = ncos (2pi n) = n to infty$
$b_n = (n+frac 12)cos ((2n + 1)pi)=-n -frac 12 to -infty$
$c_n =(n+frac 1r) cos(2npi + frac pi 2) = (n+frac 14)cdot 0 = 0to 0$.
Clearly we can't conclude anything about the limits of $f(x)$ by the limits of $a_n, b_n$ and $c_n$.
(Have you had the theorem that if a sequence converges then all subsequence converge to the same limit? If so this should be clear. If not this should give you a really good idea we the theorem holds.)
And if we actually try to find the limit if $f(x)$.
For Any $R >0$ and all $K in Bbb R$ we can find $x > max (|K|+1, R); x in mathbb N$ so $f(x+frac 12) = -x-frac 12 < -|K|-1 < K$ so $lim f(x) ne infty$.
And $f(x) = x > |K|+1 > K$ so $lim f(x) ne -infty$
and for $0< epsilon < 1$ $|f(x)-K| = ||K|+1 - K| ge 1 > epsilon$ so $lim f(x) ne K$.
So $f(x)$ does not converge to any value nor does it converge to infinity or negative infinity.
SO... Yes, you are correct.
add a comment |
You are correct.
A good counter example is $f(x) = xcos(2pi x)$ and
$a_n = f(n); b_n = f(n+frac 12);$ $c_n = f(n + frac 14)$
$a_n = ncos (2pi n) = n to infty$
$b_n = (n+frac 12)cos ((2n + 1)pi)=-n -frac 12 to -infty$
$c_n =(n+frac 1r) cos(2npi + frac pi 2) = (n+frac 14)cdot 0 = 0to 0$.
Clearly we can't conclude anything about the limits of $f(x)$ by the limits of $a_n, b_n$ and $c_n$.
(Have you had the theorem that if a sequence converges then all subsequence converge to the same limit? If so this should be clear. If not this should give you a really good idea we the theorem holds.)
And if we actually try to find the limit if $f(x)$.
For Any $R >0$ and all $K in Bbb R$ we can find $x > max (|K|+1, R); x in mathbb N$ so $f(x+frac 12) = -x-frac 12 < -|K|-1 < K$ so $lim f(x) ne infty$.
And $f(x) = x > |K|+1 > K$ so $lim f(x) ne -infty$
and for $0< epsilon < 1$ $|f(x)-K| = ||K|+1 - K| ge 1 > epsilon$ so $lim f(x) ne K$.
So $f(x)$ does not converge to any value nor does it converge to infinity or negative infinity.
SO... Yes, you are correct.
add a comment |
You are correct.
A good counter example is $f(x) = xcos(2pi x)$ and
$a_n = f(n); b_n = f(n+frac 12);$ $c_n = f(n + frac 14)$
$a_n = ncos (2pi n) = n to infty$
$b_n = (n+frac 12)cos ((2n + 1)pi)=-n -frac 12 to -infty$
$c_n =(n+frac 1r) cos(2npi + frac pi 2) = (n+frac 14)cdot 0 = 0to 0$.
Clearly we can't conclude anything about the limits of $f(x)$ by the limits of $a_n, b_n$ and $c_n$.
(Have you had the theorem that if a sequence converges then all subsequence converge to the same limit? If so this should be clear. If not this should give you a really good idea we the theorem holds.)
And if we actually try to find the limit if $f(x)$.
For Any $R >0$ and all $K in Bbb R$ we can find $x > max (|K|+1, R); x in mathbb N$ so $f(x+frac 12) = -x-frac 12 < -|K|-1 < K$ so $lim f(x) ne infty$.
And $f(x) = x > |K|+1 > K$ so $lim f(x) ne -infty$
and for $0< epsilon < 1$ $|f(x)-K| = ||K|+1 - K| ge 1 > epsilon$ so $lim f(x) ne K$.
So $f(x)$ does not converge to any value nor does it converge to infinity or negative infinity.
SO... Yes, you are correct.
You are correct.
A good counter example is $f(x) = xcos(2pi x)$ and
$a_n = f(n); b_n = f(n+frac 12);$ $c_n = f(n + frac 14)$
$a_n = ncos (2pi n) = n to infty$
$b_n = (n+frac 12)cos ((2n + 1)pi)=-n -frac 12 to -infty$
$c_n =(n+frac 1r) cos(2npi + frac pi 2) = (n+frac 14)cdot 0 = 0to 0$.
Clearly we can't conclude anything about the limits of $f(x)$ by the limits of $a_n, b_n$ and $c_n$.
(Have you had the theorem that if a sequence converges then all subsequence converge to the same limit? If so this should be clear. If not this should give you a really good idea we the theorem holds.)
And if we actually try to find the limit if $f(x)$.
For Any $R >0$ and all $K in Bbb R$ we can find $x > max (|K|+1, R); x in mathbb N$ so $f(x+frac 12) = -x-frac 12 < -|K|-1 < K$ so $lim f(x) ne infty$.
And $f(x) = x > |K|+1 > K$ so $lim f(x) ne -infty$
and for $0< epsilon < 1$ $|f(x)-K| = ||K|+1 - K| ge 1 > epsilon$ so $lim f(x) ne K$.
So $f(x)$ does not converge to any value nor does it converge to infinity or negative infinity.
SO... Yes, you are correct.
edited Nov 27 '18 at 19:05
answered Nov 27 '18 at 18:51
fleablood
68.2k22684
68.2k22684
add a comment |
add a comment |
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Yes, I think you are right. We can easily imagine a function that goes to hell and back between integers and yet always is "nice" on the integers. For example $f(x) = xcos(2pi*x)$ for $a_n= f(n) = n$ it's clear $a_nto infty$ but between integers will go way up and way down and it's clear definition 2 fails. .. Oooh.... also if $b_n = f(n + frac 12)=-n-frac 12to -infty$. We can't conclude both $f(x)to pm infty$. And just to be really annoying $c_n = f(n+frac 14)=(n+frac 14)(2npi + frac pi 2) =0$. So $c_nto 0$. ... we're.... boned if we try to conclude that direction.
– fleablood
Nov 27 '18 at 18:43