Accumulation points: an example of a sequence that has exactly two accumulation points
The definition in my text defines A.P. as: A point p is an A.P. of a set S if it is the limit of a sequence of points of S-{p}.
I am confused by my classnotes and information about A.P. in my textbook is not enough for me to understand the topic.
real-analysis sequences-and-series
add a comment |
The definition in my text defines A.P. as: A point p is an A.P. of a set S if it is the limit of a sequence of points of S-{p}.
I am confused by my classnotes and information about A.P. in my textbook is not enough for me to understand the topic.
real-analysis sequences-and-series
So: "accumulation point" is for a set not for a sequence ??
– GEdgar
Oct 24 '12 at 21:24
add a comment |
The definition in my text defines A.P. as: A point p is an A.P. of a set S if it is the limit of a sequence of points of S-{p}.
I am confused by my classnotes and information about A.P. in my textbook is not enough for me to understand the topic.
real-analysis sequences-and-series
The definition in my text defines A.P. as: A point p is an A.P. of a set S if it is the limit of a sequence of points of S-{p}.
I am confused by my classnotes and information about A.P. in my textbook is not enough for me to understand the topic.
real-analysis sequences-and-series
real-analysis sequences-and-series
asked Oct 24 '12 at 20:48
Maximiliano
43021225
43021225
So: "accumulation point" is for a set not for a sequence ??
– GEdgar
Oct 24 '12 at 21:24
add a comment |
So: "accumulation point" is for a set not for a sequence ??
– GEdgar
Oct 24 '12 at 21:24
So: "accumulation point" is for a set not for a sequence ??
– GEdgar
Oct 24 '12 at 21:24
So: "accumulation point" is for a set not for a sequence ??
– GEdgar
Oct 24 '12 at 21:24
add a comment |
4 Answers
4
active
oldest
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The sequence $(-1)^nleft(1+frac{1}{n}right)$ has exactly two A.Ps $1$ and $-1$.
More generally if $(a_n)_{nin mathbb{N}}$ is a sequence s.t. $a_n to a neq0 $ and the set ${a_n:n in mathbb{N}}$ is infinite then the sequence $((-1)^na_n)_{nin mathbb{N}}$ has exactly two accumulation points $a$ and $-a$.
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
add a comment |
Exactly two accumulation points can be done using the sequence
$$frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots,$$
and in many other ways. In this case the accumulation points are $0$ and $1$.
Here I intend the set $S$ of the definition to be ${frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots}$. We could work with a somewhat larger set for $S$, by, in addition to the points mentioned, throwing in all the integers, or something else that would not produce additional accumulation points.
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
add a comment |
This seems a little vague.
What is your question? Do you want an example of the sequence or do you want more info. about accumulation points?
Intuitively, accumulations points are the points of the set S which are not isolated.
e.g. 1 is an A.P. of (0,1) but 2 is not an A.P of $(0,1) cup$ {2} (because it's isolated.) To prove this, the set $X = $ ($(0,1) cup$ {2} ) - {2} = $(0,1)$ but 2 is not the limit of a sequence in $(0,1)$ so 2 is not an A.P. of $(0,1) cup$ {2}
from there its pretty easy to construct such a sequence.
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
add a comment |
Point $p$ is accumulation point of a set $S$, if every punctured neighborhood of the point $p$ contains points of $S.$ (In other words, if there exists sequence ${x_n} subset S$ that converges to $p.$)
Set $$S={left lbrace sin{dfrac{1}{n}}, cos{dfrac{1}{n}} right rbrace}_{n in mathbb{N}}=left lbrace sin{1}, cos{1},sin{dfrac{1}{2}}, cos{dfrac{1}{2}}, ldots, sin{dfrac{1}{n}}, cos{dfrac{1}{n}} , ldots right rbrace$$ has exactly two accumulation points.
add a comment |
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4 Answers
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4 Answers
4
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oldest
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The sequence $(-1)^nleft(1+frac{1}{n}right)$ has exactly two A.Ps $1$ and $-1$.
More generally if $(a_n)_{nin mathbb{N}}$ is a sequence s.t. $a_n to a neq0 $ and the set ${a_n:n in mathbb{N}}$ is infinite then the sequence $((-1)^na_n)_{nin mathbb{N}}$ has exactly two accumulation points $a$ and $-a$.
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
add a comment |
The sequence $(-1)^nleft(1+frac{1}{n}right)$ has exactly two A.Ps $1$ and $-1$.
More generally if $(a_n)_{nin mathbb{N}}$ is a sequence s.t. $a_n to a neq0 $ and the set ${a_n:n in mathbb{N}}$ is infinite then the sequence $((-1)^na_n)_{nin mathbb{N}}$ has exactly two accumulation points $a$ and $-a$.
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
add a comment |
The sequence $(-1)^nleft(1+frac{1}{n}right)$ has exactly two A.Ps $1$ and $-1$.
More generally if $(a_n)_{nin mathbb{N}}$ is a sequence s.t. $a_n to a neq0 $ and the set ${a_n:n in mathbb{N}}$ is infinite then the sequence $((-1)^na_n)_{nin mathbb{N}}$ has exactly two accumulation points $a$ and $-a$.
The sequence $(-1)^nleft(1+frac{1}{n}right)$ has exactly two A.Ps $1$ and $-1$.
More generally if $(a_n)_{nin mathbb{N}}$ is a sequence s.t. $a_n to a neq0 $ and the set ${a_n:n in mathbb{N}}$ is infinite then the sequence $((-1)^na_n)_{nin mathbb{N}}$ has exactly two accumulation points $a$ and $-a$.
edited Nov 26 at 0:49
answered Oct 24 '12 at 21:10
P..
13.4k22348
13.4k22348
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
add a comment |
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
how can this be proved?
– Maximiliano
Oct 24 '12 at 21:14
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
Prove that $a_{2n}rightarrow a , a_{2n+1}rightarrow −a ,a_{2n}neq a ,a_{2n+1}neq −a$, for (almost) all $ninmathbb N$. Also show that $a$ and $−a$ are the only numbers that can be the limit points of subsequences of $(a_n)_{ninmathbb N}$.
– P..
Nov 26 at 0:53
add a comment |
Exactly two accumulation points can be done using the sequence
$$frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots,$$
and in many other ways. In this case the accumulation points are $0$ and $1$.
Here I intend the set $S$ of the definition to be ${frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots}$. We could work with a somewhat larger set for $S$, by, in addition to the points mentioned, throwing in all the integers, or something else that would not produce additional accumulation points.
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
add a comment |
Exactly two accumulation points can be done using the sequence
$$frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots,$$
and in many other ways. In this case the accumulation points are $0$ and $1$.
Here I intend the set $S$ of the definition to be ${frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots}$. We could work with a somewhat larger set for $S$, by, in addition to the points mentioned, throwing in all the integers, or something else that would not produce additional accumulation points.
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
add a comment |
Exactly two accumulation points can be done using the sequence
$$frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots,$$
and in many other ways. In this case the accumulation points are $0$ and $1$.
Here I intend the set $S$ of the definition to be ${frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots}$. We could work with a somewhat larger set for $S$, by, in addition to the points mentioned, throwing in all the integers, or something else that would not produce additional accumulation points.
Exactly two accumulation points can be done using the sequence
$$frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots,$$
and in many other ways. In this case the accumulation points are $0$ and $1$.
Here I intend the set $S$ of the definition to be ${frac{1}{3}, frac{2}{3},frac{1}{4},frac{3}{4}, frac{1}{5},frac{4}{5},frac{1}{6}, frac{5}{6},frac{1}{7}, frac{6}{7},dots}$. We could work with a somewhat larger set for $S$, by, in addition to the points mentioned, throwing in all the integers, or something else that would not produce additional accumulation points.
edited Oct 24 '12 at 21:09
answered Oct 24 '12 at 20:54
André Nicolas
451k36421805
451k36421805
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
add a comment |
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
I presume your set is the unit interval?
– Lord_Farin
Oct 24 '12 at 20:57
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
No, my set $S$ is the set ${frac{1}{3},frac{2}{3},frac{1}{4},dots}$.
– André Nicolas
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
Of course; I just wanted to explicate stuff in case OP desired that $p in S$.
– Lord_Farin
Oct 24 '12 at 21:05
add a comment |
This seems a little vague.
What is your question? Do you want an example of the sequence or do you want more info. about accumulation points?
Intuitively, accumulations points are the points of the set S which are not isolated.
e.g. 1 is an A.P. of (0,1) but 2 is not an A.P of $(0,1) cup$ {2} (because it's isolated.) To prove this, the set $X = $ ($(0,1) cup$ {2} ) - {2} = $(0,1)$ but 2 is not the limit of a sequence in $(0,1)$ so 2 is not an A.P. of $(0,1) cup$ {2}
from there its pretty easy to construct such a sequence.
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
add a comment |
This seems a little vague.
What is your question? Do you want an example of the sequence or do you want more info. about accumulation points?
Intuitively, accumulations points are the points of the set S which are not isolated.
e.g. 1 is an A.P. of (0,1) but 2 is not an A.P of $(0,1) cup$ {2} (because it's isolated.) To prove this, the set $X = $ ($(0,1) cup$ {2} ) - {2} = $(0,1)$ but 2 is not the limit of a sequence in $(0,1)$ so 2 is not an A.P. of $(0,1) cup$ {2}
from there its pretty easy to construct such a sequence.
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
add a comment |
This seems a little vague.
What is your question? Do you want an example of the sequence or do you want more info. about accumulation points?
Intuitively, accumulations points are the points of the set S which are not isolated.
e.g. 1 is an A.P. of (0,1) but 2 is not an A.P of $(0,1) cup$ {2} (because it's isolated.) To prove this, the set $X = $ ($(0,1) cup$ {2} ) - {2} = $(0,1)$ but 2 is not the limit of a sequence in $(0,1)$ so 2 is not an A.P. of $(0,1) cup$ {2}
from there its pretty easy to construct such a sequence.
This seems a little vague.
What is your question? Do you want an example of the sequence or do you want more info. about accumulation points?
Intuitively, accumulations points are the points of the set S which are not isolated.
e.g. 1 is an A.P. of (0,1) but 2 is not an A.P of $(0,1) cup$ {2} (because it's isolated.) To prove this, the set $X = $ ($(0,1) cup$ {2} ) - {2} = $(0,1)$ but 2 is not the limit of a sequence in $(0,1)$ so 2 is not an A.P. of $(0,1) cup$ {2}
from there its pretty easy to construct such a sequence.
answered Oct 24 '12 at 20:57
Henrique Tyrrell
784623
784623
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
add a comment |
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
Yes an example of a sequence
– Maximiliano
Oct 24 '12 at 20:58
add a comment |
Point $p$ is accumulation point of a set $S$, if every punctured neighborhood of the point $p$ contains points of $S.$ (In other words, if there exists sequence ${x_n} subset S$ that converges to $p.$)
Set $$S={left lbrace sin{dfrac{1}{n}}, cos{dfrac{1}{n}} right rbrace}_{n in mathbb{N}}=left lbrace sin{1}, cos{1},sin{dfrac{1}{2}}, cos{dfrac{1}{2}}, ldots, sin{dfrac{1}{n}}, cos{dfrac{1}{n}} , ldots right rbrace$$ has exactly two accumulation points.
add a comment |
Point $p$ is accumulation point of a set $S$, if every punctured neighborhood of the point $p$ contains points of $S.$ (In other words, if there exists sequence ${x_n} subset S$ that converges to $p.$)
Set $$S={left lbrace sin{dfrac{1}{n}}, cos{dfrac{1}{n}} right rbrace}_{n in mathbb{N}}=left lbrace sin{1}, cos{1},sin{dfrac{1}{2}}, cos{dfrac{1}{2}}, ldots, sin{dfrac{1}{n}}, cos{dfrac{1}{n}} , ldots right rbrace$$ has exactly two accumulation points.
add a comment |
Point $p$ is accumulation point of a set $S$, if every punctured neighborhood of the point $p$ contains points of $S.$ (In other words, if there exists sequence ${x_n} subset S$ that converges to $p.$)
Set $$S={left lbrace sin{dfrac{1}{n}}, cos{dfrac{1}{n}} right rbrace}_{n in mathbb{N}}=left lbrace sin{1}, cos{1},sin{dfrac{1}{2}}, cos{dfrac{1}{2}}, ldots, sin{dfrac{1}{n}}, cos{dfrac{1}{n}} , ldots right rbrace$$ has exactly two accumulation points.
Point $p$ is accumulation point of a set $S$, if every punctured neighborhood of the point $p$ contains points of $S.$ (In other words, if there exists sequence ${x_n} subset S$ that converges to $p.$)
Set $$S={left lbrace sin{dfrac{1}{n}}, cos{dfrac{1}{n}} right rbrace}_{n in mathbb{N}}=left lbrace sin{1}, cos{1},sin{dfrac{1}{2}}, cos{dfrac{1}{2}}, ldots, sin{dfrac{1}{n}}, cos{dfrac{1}{n}} , ldots right rbrace$$ has exactly two accumulation points.
edited Oct 24 '12 at 21:33
answered Oct 24 '12 at 21:05
M. Strochyk
7,67711119
7,67711119
add a comment |
add a comment |
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So: "accumulation point" is for a set not for a sequence ??
– GEdgar
Oct 24 '12 at 21:24