Show (F$_{k}$) converges uniformly to some continuous function
Suppose ${0<r<1}$. For each k $in$ $mathbb{N}$, define F$_{k}$ $in$ C$bigl($[-r,r]$bigr)$ by F$_{k}$(x) = $sum_{n=1}^k$ x$^{n}$.
Show (F$_{k}$) converges uniformly to some continuous function f $in$ C$bigl($[-r,r]$bigr)$.
Somehow this has something to do with compact sets and metric spaces, as that was the section this homework was posted in. I'm not sure how they relate, however.
real-analysis convergence metric-spaces compactness riemann-sum
add a comment |
Suppose ${0<r<1}$. For each k $in$ $mathbb{N}$, define F$_{k}$ $in$ C$bigl($[-r,r]$bigr)$ by F$_{k}$(x) = $sum_{n=1}^k$ x$^{n}$.
Show (F$_{k}$) converges uniformly to some continuous function f $in$ C$bigl($[-r,r]$bigr)$.
Somehow this has something to do with compact sets and metric spaces, as that was the section this homework was posted in. I'm not sure how they relate, however.
real-analysis convergence metric-spaces compactness riemann-sum
I suppose you should use the Dini theorem, by first calculating the limit function and verify its continuity, then Dini theorem gets you there. Dini theorem is related to the compactness of domain.
– xbh
Dec 5 '18 at 3:37
@xbh we haven't studied that theorem, so on this homework that wouldn't be the answer my prof is looking for.
– kendal
Dec 5 '18 at 3:39
Then first compute the limit function, then prove the uniform convergence by definition. If that's the case, you might not use the compactness at all.
– xbh
Dec 5 '18 at 3:47
The sup norm of $F_j-F_k$ for $jgeq k$ can be directly bounded above using a geometric series, showing that ${F_k}$ is uniformly Cauchy. Then use the completeness of $mathcal{C}([-r,r])$.
– carmichael561
Dec 5 '18 at 3:48
add a comment |
Suppose ${0<r<1}$. For each k $in$ $mathbb{N}$, define F$_{k}$ $in$ C$bigl($[-r,r]$bigr)$ by F$_{k}$(x) = $sum_{n=1}^k$ x$^{n}$.
Show (F$_{k}$) converges uniformly to some continuous function f $in$ C$bigl($[-r,r]$bigr)$.
Somehow this has something to do with compact sets and metric spaces, as that was the section this homework was posted in. I'm not sure how they relate, however.
real-analysis convergence metric-spaces compactness riemann-sum
Suppose ${0<r<1}$. For each k $in$ $mathbb{N}$, define F$_{k}$ $in$ C$bigl($[-r,r]$bigr)$ by F$_{k}$(x) = $sum_{n=1}^k$ x$^{n}$.
Show (F$_{k}$) converges uniformly to some continuous function f $in$ C$bigl($[-r,r]$bigr)$.
Somehow this has something to do with compact sets and metric spaces, as that was the section this homework was posted in. I'm not sure how they relate, however.
real-analysis convergence metric-spaces compactness riemann-sum
real-analysis convergence metric-spaces compactness riemann-sum
edited Dec 7 '18 at 18:35
Math Avengers
709
709
asked Dec 5 '18 at 3:33
kendalkendal
337
337
I suppose you should use the Dini theorem, by first calculating the limit function and verify its continuity, then Dini theorem gets you there. Dini theorem is related to the compactness of domain.
– xbh
Dec 5 '18 at 3:37
@xbh we haven't studied that theorem, so on this homework that wouldn't be the answer my prof is looking for.
– kendal
Dec 5 '18 at 3:39
Then first compute the limit function, then prove the uniform convergence by definition. If that's the case, you might not use the compactness at all.
– xbh
Dec 5 '18 at 3:47
The sup norm of $F_j-F_k$ for $jgeq k$ can be directly bounded above using a geometric series, showing that ${F_k}$ is uniformly Cauchy. Then use the completeness of $mathcal{C}([-r,r])$.
– carmichael561
Dec 5 '18 at 3:48
add a comment |
I suppose you should use the Dini theorem, by first calculating the limit function and verify its continuity, then Dini theorem gets you there. Dini theorem is related to the compactness of domain.
– xbh
Dec 5 '18 at 3:37
@xbh we haven't studied that theorem, so on this homework that wouldn't be the answer my prof is looking for.
– kendal
Dec 5 '18 at 3:39
Then first compute the limit function, then prove the uniform convergence by definition. If that's the case, you might not use the compactness at all.
– xbh
Dec 5 '18 at 3:47
The sup norm of $F_j-F_k$ for $jgeq k$ can be directly bounded above using a geometric series, showing that ${F_k}$ is uniformly Cauchy. Then use the completeness of $mathcal{C}([-r,r])$.
– carmichael561
Dec 5 '18 at 3:48
I suppose you should use the Dini theorem, by first calculating the limit function and verify its continuity, then Dini theorem gets you there. Dini theorem is related to the compactness of domain.
– xbh
Dec 5 '18 at 3:37
I suppose you should use the Dini theorem, by first calculating the limit function and verify its continuity, then Dini theorem gets you there. Dini theorem is related to the compactness of domain.
– xbh
Dec 5 '18 at 3:37
@xbh we haven't studied that theorem, so on this homework that wouldn't be the answer my prof is looking for.
– kendal
Dec 5 '18 at 3:39
@xbh we haven't studied that theorem, so on this homework that wouldn't be the answer my prof is looking for.
– kendal
Dec 5 '18 at 3:39
Then first compute the limit function, then prove the uniform convergence by definition. If that's the case, you might not use the compactness at all.
– xbh
Dec 5 '18 at 3:47
Then first compute the limit function, then prove the uniform convergence by definition. If that's the case, you might not use the compactness at all.
– xbh
Dec 5 '18 at 3:47
The sup norm of $F_j-F_k$ for $jgeq k$ can be directly bounded above using a geometric series, showing that ${F_k}$ is uniformly Cauchy. Then use the completeness of $mathcal{C}([-r,r])$.
– carmichael561
Dec 5 '18 at 3:48
The sup norm of $F_j-F_k$ for $jgeq k$ can be directly bounded above using a geometric series, showing that ${F_k}$ is uniformly Cauchy. Then use the completeness of $mathcal{C}([-r,r])$.
– carmichael561
Dec 5 '18 at 3:48
add a comment |
1 Answer
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$$F_k(x)=sum_{n=1}^kx^n = frac{x(1-x^k)}{1-x}$$
converges to
$$F(x) = frac{x}{1-x}$$
Then
$$left|F_k(x)-F(x)right| = left|sum_{n=k+1}^infty x^nright| = |x^k F(x) | le r^k F(r)$$
The last value tends to $0$ when $k to infty$ and is independent of $x$. Uniform convergence follows.
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
add a comment |
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$$F_k(x)=sum_{n=1}^kx^n = frac{x(1-x^k)}{1-x}$$
converges to
$$F(x) = frac{x}{1-x}$$
Then
$$left|F_k(x)-F(x)right| = left|sum_{n=k+1}^infty x^nright| = |x^k F(x) | le r^k F(r)$$
The last value tends to $0$ when $k to infty$ and is independent of $x$. Uniform convergence follows.
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
add a comment |
$$F_k(x)=sum_{n=1}^kx^n = frac{x(1-x^k)}{1-x}$$
converges to
$$F(x) = frac{x}{1-x}$$
Then
$$left|F_k(x)-F(x)right| = left|sum_{n=k+1}^infty x^nright| = |x^k F(x) | le r^k F(r)$$
The last value tends to $0$ when $k to infty$ and is independent of $x$. Uniform convergence follows.
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
add a comment |
$$F_k(x)=sum_{n=1}^kx^n = frac{x(1-x^k)}{1-x}$$
converges to
$$F(x) = frac{x}{1-x}$$
Then
$$left|F_k(x)-F(x)right| = left|sum_{n=k+1}^infty x^nright| = |x^k F(x) | le r^k F(r)$$
The last value tends to $0$ when $k to infty$ and is independent of $x$. Uniform convergence follows.
$$F_k(x)=sum_{n=1}^kx^n = frac{x(1-x^k)}{1-x}$$
converges to
$$F(x) = frac{x}{1-x}$$
Then
$$left|F_k(x)-F(x)right| = left|sum_{n=k+1}^infty x^nright| = |x^k F(x) | le r^k F(r)$$
The last value tends to $0$ when $k to infty$ and is independent of $x$. Uniform convergence follows.
answered Dec 5 '18 at 8:54
DamienDamien
52714
52714
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
add a comment |
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
The limit function $F$ is continuous by a widely-used (& not difficult) result that if $(X,d)$ and $(Y,e)$ are metric spaces and a sequence $f_n:Xto Y$ of continuous functions converges uniformly, with respect to the metrics $d,e$, to $f:Xto Y,$ then $f$ is continuous. One method of proof is to show that if $lim_{jto infty}d(x_j,x)=0$ then $lim_{jto infty}e(f(x_j),f(x))=0$.... But in this case we have an explicit formula for $F$, and it is clearly continuous anyway.....+1
– DanielWainfleet
Dec 5 '18 at 10:30
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
@DanielWainfleet Thank you for the information. I am an engineer and basically I am doing maths as an engineer ... My maths lessons are very far
– Damien
Dec 5 '18 at 10:37
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
Another result, called The Dini Theorem (perhaps to distinguish it from some other important results, especially in Measure Theory, by Dini): Let $X$ be a compact Hausdorff space (e.g. a closed bounded subset of $Bbb R$ ) and let $f_n:Xto Bbb R$ be a sequence of continuous functions converging pointwise to a $ continuous$ $f:Xto Bbb R.$ If $forall n in Bbb N;forall xin X; (f_{n+1}(x)leq f_n(x),)$ then $f_n$ converges uniformly to $f$.
– DanielWainfleet
Dec 6 '18 at 0:08
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
@DanielWainfleet Thank you again. In my answer, in particular, I used the fact that F() is bounded, which is related to the conditions of Dini Theorem. But I also used the particular characteristics of the series
– Damien
Dec 6 '18 at 8:43
add a comment |
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I suppose you should use the Dini theorem, by first calculating the limit function and verify its continuity, then Dini theorem gets you there. Dini theorem is related to the compactness of domain.
– xbh
Dec 5 '18 at 3:37
@xbh we haven't studied that theorem, so on this homework that wouldn't be the answer my prof is looking for.
– kendal
Dec 5 '18 at 3:39
Then first compute the limit function, then prove the uniform convergence by definition. If that's the case, you might not use the compactness at all.
– xbh
Dec 5 '18 at 3:47
The sup norm of $F_j-F_k$ for $jgeq k$ can be directly bounded above using a geometric series, showing that ${F_k}$ is uniformly Cauchy. Then use the completeness of $mathcal{C}([-r,r])$.
– carmichael561
Dec 5 '18 at 3:48