Composition of two uniformly convergent sequences of functions is uniformly convergent?
I am trying to prove or provide a counter-example for the following:
Let $f_k$ and $g_k$ be sequences of continuous functions on $[0,1]to[0,1]$ converging uniformly to $f:[0,1]to mathbb{R}$ and $g:[0,1]to mathbb{R}$ respectively. Does $f_k circ g_k$ coverge uniformly to $fcirc g$?
What I've done so far:
I know I need to prove that $forall epsilon>0, exists Nin mathbb{N}$ such that $||f_k(g_k(x)) - f(g(x))|| < epsilon$ for all $x in [0,1]$ and $kgeq N$.
At first, I thought I can prove this easily since it follows trivially from the definition of $f_k$ uniformly converging to $f$. However, I noticed that is only true for all $x in [0,1]$ and $g$ maps onto all of $mathbb{R}$, not just $[0,1]$. So does that mean it's not necessarily true? Can anyone provide a counter-example?
real-analysis sequences-and-series uniform-convergence
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I am trying to prove or provide a counter-example for the following:
Let $f_k$ and $g_k$ be sequences of continuous functions on $[0,1]to[0,1]$ converging uniformly to $f:[0,1]to mathbb{R}$ and $g:[0,1]to mathbb{R}$ respectively. Does $f_k circ g_k$ coverge uniformly to $fcirc g$?
What I've done so far:
I know I need to prove that $forall epsilon>0, exists Nin mathbb{N}$ such that $||f_k(g_k(x)) - f(g(x))|| < epsilon$ for all $x in [0,1]$ and $kgeq N$.
At first, I thought I can prove this easily since it follows trivially from the definition of $f_k$ uniformly converging to $f$. However, I noticed that is only true for all $x in [0,1]$ and $g$ maps onto all of $mathbb{R}$, not just $[0,1]$. So does that mean it's not necessarily true? Can anyone provide a counter-example?
real-analysis sequences-and-series uniform-convergence
Without loss of generality, you may assume that the domain and range of both $f$ and $g$ are $[0,1]$. (Uniform limit of sequence of continuous functions with compact support, so you may rescale them without changing much of the problem.)
– GNUSupporter 8964民主女神 地下教會
Dec 2 '18 at 5:52
add a comment |
I am trying to prove or provide a counter-example for the following:
Let $f_k$ and $g_k$ be sequences of continuous functions on $[0,1]to[0,1]$ converging uniformly to $f:[0,1]to mathbb{R}$ and $g:[0,1]to mathbb{R}$ respectively. Does $f_k circ g_k$ coverge uniformly to $fcirc g$?
What I've done so far:
I know I need to prove that $forall epsilon>0, exists Nin mathbb{N}$ such that $||f_k(g_k(x)) - f(g(x))|| < epsilon$ for all $x in [0,1]$ and $kgeq N$.
At first, I thought I can prove this easily since it follows trivially from the definition of $f_k$ uniformly converging to $f$. However, I noticed that is only true for all $x in [0,1]$ and $g$ maps onto all of $mathbb{R}$, not just $[0,1]$. So does that mean it's not necessarily true? Can anyone provide a counter-example?
real-analysis sequences-and-series uniform-convergence
I am trying to prove or provide a counter-example for the following:
Let $f_k$ and $g_k$ be sequences of continuous functions on $[0,1]to[0,1]$ converging uniformly to $f:[0,1]to mathbb{R}$ and $g:[0,1]to mathbb{R}$ respectively. Does $f_k circ g_k$ coverge uniformly to $fcirc g$?
What I've done so far:
I know I need to prove that $forall epsilon>0, exists Nin mathbb{N}$ such that $||f_k(g_k(x)) - f(g(x))|| < epsilon$ for all $x in [0,1]$ and $kgeq N$.
At first, I thought I can prove this easily since it follows trivially from the definition of $f_k$ uniformly converging to $f$. However, I noticed that is only true for all $x in [0,1]$ and $g$ maps onto all of $mathbb{R}$, not just $[0,1]$. So does that mean it's not necessarily true? Can anyone provide a counter-example?
real-analysis sequences-and-series uniform-convergence
real-analysis sequences-and-series uniform-convergence
asked Dec 2 '18 at 5:41
darcy
311
311
Without loss of generality, you may assume that the domain and range of both $f$ and $g$ are $[0,1]$. (Uniform limit of sequence of continuous functions with compact support, so you may rescale them without changing much of the problem.)
– GNUSupporter 8964民主女神 地下教會
Dec 2 '18 at 5:52
add a comment |
Without loss of generality, you may assume that the domain and range of both $f$ and $g$ are $[0,1]$. (Uniform limit of sequence of continuous functions with compact support, so you may rescale them without changing much of the problem.)
– GNUSupporter 8964民主女神 地下教會
Dec 2 '18 at 5:52
Without loss of generality, you may assume that the domain and range of both $f$ and $g$ are $[0,1]$. (Uniform limit of sequence of continuous functions with compact support, so you may rescale them without changing much of the problem.)
– GNUSupporter 8964民主女神 地下教會
Dec 2 '18 at 5:52
Without loss of generality, you may assume that the domain and range of both $f$ and $g$ are $[0,1]$. (Uniform limit of sequence of continuous functions with compact support, so you may rescale them without changing much of the problem.)
– GNUSupporter 8964民主女神 地下教會
Dec 2 '18 at 5:52
add a comment |
2 Answers
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Let $|h|_infty = sup_{x in [0,1]} |h(x)|$.
Note that $f$ is uniformly continuous since $[0,1]$ is compact. Hence since $|g-g_k|_infty to 0$, we see that $|f circ g-f circ g_k|_infty to 0$.
Then
begin{eqnarray}
|f circ g(x)-f_k circ g_k (x)| &le& |f circ g(x)-f circ g_k (x)| + |f circ g_k(x)-f_k circ g_k (x)| \
&le & |f circ g-f circ g_k|_infty + |f-f_k|_infty
end{eqnarray}
Hence $|f circ g-f_k circ g_k|_infty to 0$.
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For each $x in [0,1]$, $f_n(x) to f(x)$. Since $f_n(x) in [0,1]$, $f(x) in [0,1]$. Idem for $g$.
Let $epsilon > 0$.
begin{align}
& exists N_1 in Bbb{N}: forall n ge N_1, forall y in [0,1], |f_n(y)-f(y)| < epsilon tag1 label1 \
& exists delta > 0 : forall |y - y'| le delta, |f(y) - f(y')| < epsilon tag2 label2 \
& exists N_2 in Bbb{N}: forall k ge N_2, forall x in [0,1], |g_k(x)-g(x)| < delta tag3 label3
end{align}
eqref{1} and eqref{3} are the definition of uniform continuity; eqref{2} is due to the facts that the uniform limit $f$ of a sequence of continuous functions $(f_n)_n$ is continuous, and that $f$ is uniformly continuous on closed and bounded interval $[0,1]$.
Put eqref{1}-eqref{3} together. Take $N = max{N_1,N_2}$. For all $n ge N$,
begin{align}
& quad |f_n(g_n(x)) - f(g(x))| \
&le |f_n(g_n(x)) - f(g_n(x))| + |f(g_n(x)) - f(g(x))| \
&le epsilon + epsilon = 2epsilon.
end{align}
In the last inequality, we applied eqref{1} with $y = g_n(x)$ in the first term, and eqref{3} with $k = n$ composed with eqref{2} with $y = g_n(x)$ and $y' = g(x)$ in the second term.
Hence $f_n circ g_n$ converges uniformly to $f circ g$.
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Let $|h|_infty = sup_{x in [0,1]} |h(x)|$.
Note that $f$ is uniformly continuous since $[0,1]$ is compact. Hence since $|g-g_k|_infty to 0$, we see that $|f circ g-f circ g_k|_infty to 0$.
Then
begin{eqnarray}
|f circ g(x)-f_k circ g_k (x)| &le& |f circ g(x)-f circ g_k (x)| + |f circ g_k(x)-f_k circ g_k (x)| \
&le & |f circ g-f circ g_k|_infty + |f-f_k|_infty
end{eqnarray}
Hence $|f circ g-f_k circ g_k|_infty to 0$.
add a comment |
Let $|h|_infty = sup_{x in [0,1]} |h(x)|$.
Note that $f$ is uniformly continuous since $[0,1]$ is compact. Hence since $|g-g_k|_infty to 0$, we see that $|f circ g-f circ g_k|_infty to 0$.
Then
begin{eqnarray}
|f circ g(x)-f_k circ g_k (x)| &le& |f circ g(x)-f circ g_k (x)| + |f circ g_k(x)-f_k circ g_k (x)| \
&le & |f circ g-f circ g_k|_infty + |f-f_k|_infty
end{eqnarray}
Hence $|f circ g-f_k circ g_k|_infty to 0$.
add a comment |
Let $|h|_infty = sup_{x in [0,1]} |h(x)|$.
Note that $f$ is uniformly continuous since $[0,1]$ is compact. Hence since $|g-g_k|_infty to 0$, we see that $|f circ g-f circ g_k|_infty to 0$.
Then
begin{eqnarray}
|f circ g(x)-f_k circ g_k (x)| &le& |f circ g(x)-f circ g_k (x)| + |f circ g_k(x)-f_k circ g_k (x)| \
&le & |f circ g-f circ g_k|_infty + |f-f_k|_infty
end{eqnarray}
Hence $|f circ g-f_k circ g_k|_infty to 0$.
Let $|h|_infty = sup_{x in [0,1]} |h(x)|$.
Note that $f$ is uniformly continuous since $[0,1]$ is compact. Hence since $|g-g_k|_infty to 0$, we see that $|f circ g-f circ g_k|_infty to 0$.
Then
begin{eqnarray}
|f circ g(x)-f_k circ g_k (x)| &le& |f circ g(x)-f circ g_k (x)| + |f circ g_k(x)-f_k circ g_k (x)| \
&le & |f circ g-f circ g_k|_infty + |f-f_k|_infty
end{eqnarray}
Hence $|f circ g-f_k circ g_k|_infty to 0$.
answered Dec 2 '18 at 7:14
copper.hat
126k559159
126k559159
add a comment |
add a comment |
For each $x in [0,1]$, $f_n(x) to f(x)$. Since $f_n(x) in [0,1]$, $f(x) in [0,1]$. Idem for $g$.
Let $epsilon > 0$.
begin{align}
& exists N_1 in Bbb{N}: forall n ge N_1, forall y in [0,1], |f_n(y)-f(y)| < epsilon tag1 label1 \
& exists delta > 0 : forall |y - y'| le delta, |f(y) - f(y')| < epsilon tag2 label2 \
& exists N_2 in Bbb{N}: forall k ge N_2, forall x in [0,1], |g_k(x)-g(x)| < delta tag3 label3
end{align}
eqref{1} and eqref{3} are the definition of uniform continuity; eqref{2} is due to the facts that the uniform limit $f$ of a sequence of continuous functions $(f_n)_n$ is continuous, and that $f$ is uniformly continuous on closed and bounded interval $[0,1]$.
Put eqref{1}-eqref{3} together. Take $N = max{N_1,N_2}$. For all $n ge N$,
begin{align}
& quad |f_n(g_n(x)) - f(g(x))| \
&le |f_n(g_n(x)) - f(g_n(x))| + |f(g_n(x)) - f(g(x))| \
&le epsilon + epsilon = 2epsilon.
end{align}
In the last inequality, we applied eqref{1} with $y = g_n(x)$ in the first term, and eqref{3} with $k = n$ composed with eqref{2} with $y = g_n(x)$ and $y' = g(x)$ in the second term.
Hence $f_n circ g_n$ converges uniformly to $f circ g$.
add a comment |
For each $x in [0,1]$, $f_n(x) to f(x)$. Since $f_n(x) in [0,1]$, $f(x) in [0,1]$. Idem for $g$.
Let $epsilon > 0$.
begin{align}
& exists N_1 in Bbb{N}: forall n ge N_1, forall y in [0,1], |f_n(y)-f(y)| < epsilon tag1 label1 \
& exists delta > 0 : forall |y - y'| le delta, |f(y) - f(y')| < epsilon tag2 label2 \
& exists N_2 in Bbb{N}: forall k ge N_2, forall x in [0,1], |g_k(x)-g(x)| < delta tag3 label3
end{align}
eqref{1} and eqref{3} are the definition of uniform continuity; eqref{2} is due to the facts that the uniform limit $f$ of a sequence of continuous functions $(f_n)_n$ is continuous, and that $f$ is uniformly continuous on closed and bounded interval $[0,1]$.
Put eqref{1}-eqref{3} together. Take $N = max{N_1,N_2}$. For all $n ge N$,
begin{align}
& quad |f_n(g_n(x)) - f(g(x))| \
&le |f_n(g_n(x)) - f(g_n(x))| + |f(g_n(x)) - f(g(x))| \
&le epsilon + epsilon = 2epsilon.
end{align}
In the last inequality, we applied eqref{1} with $y = g_n(x)$ in the first term, and eqref{3} with $k = n$ composed with eqref{2} with $y = g_n(x)$ and $y' = g(x)$ in the second term.
Hence $f_n circ g_n$ converges uniformly to $f circ g$.
add a comment |
For each $x in [0,1]$, $f_n(x) to f(x)$. Since $f_n(x) in [0,1]$, $f(x) in [0,1]$. Idem for $g$.
Let $epsilon > 0$.
begin{align}
& exists N_1 in Bbb{N}: forall n ge N_1, forall y in [0,1], |f_n(y)-f(y)| < epsilon tag1 label1 \
& exists delta > 0 : forall |y - y'| le delta, |f(y) - f(y')| < epsilon tag2 label2 \
& exists N_2 in Bbb{N}: forall k ge N_2, forall x in [0,1], |g_k(x)-g(x)| < delta tag3 label3
end{align}
eqref{1} and eqref{3} are the definition of uniform continuity; eqref{2} is due to the facts that the uniform limit $f$ of a sequence of continuous functions $(f_n)_n$ is continuous, and that $f$ is uniformly continuous on closed and bounded interval $[0,1]$.
Put eqref{1}-eqref{3} together. Take $N = max{N_1,N_2}$. For all $n ge N$,
begin{align}
& quad |f_n(g_n(x)) - f(g(x))| \
&le |f_n(g_n(x)) - f(g_n(x))| + |f(g_n(x)) - f(g(x))| \
&le epsilon + epsilon = 2epsilon.
end{align}
In the last inequality, we applied eqref{1} with $y = g_n(x)$ in the first term, and eqref{3} with $k = n$ composed with eqref{2} with $y = g_n(x)$ and $y' = g(x)$ in the second term.
Hence $f_n circ g_n$ converges uniformly to $f circ g$.
For each $x in [0,1]$, $f_n(x) to f(x)$. Since $f_n(x) in [0,1]$, $f(x) in [0,1]$. Idem for $g$.
Let $epsilon > 0$.
begin{align}
& exists N_1 in Bbb{N}: forall n ge N_1, forall y in [0,1], |f_n(y)-f(y)| < epsilon tag1 label1 \
& exists delta > 0 : forall |y - y'| le delta, |f(y) - f(y')| < epsilon tag2 label2 \
& exists N_2 in Bbb{N}: forall k ge N_2, forall x in [0,1], |g_k(x)-g(x)| < delta tag3 label3
end{align}
eqref{1} and eqref{3} are the definition of uniform continuity; eqref{2} is due to the facts that the uniform limit $f$ of a sequence of continuous functions $(f_n)_n$ is continuous, and that $f$ is uniformly continuous on closed and bounded interval $[0,1]$.
Put eqref{1}-eqref{3} together. Take $N = max{N_1,N_2}$. For all $n ge N$,
begin{align}
& quad |f_n(g_n(x)) - f(g(x))| \
&le |f_n(g_n(x)) - f(g_n(x))| + |f(g_n(x)) - f(g(x))| \
&le epsilon + epsilon = 2epsilon.
end{align}
In the last inequality, we applied eqref{1} with $y = g_n(x)$ in the first term, and eqref{3} with $k = n$ composed with eqref{2} with $y = g_n(x)$ and $y' = g(x)$ in the second term.
Hence $f_n circ g_n$ converges uniformly to $f circ g$.
answered Dec 2 '18 at 6:48
GNUSupporter 8964民主女神 地下教會
12.8k72445
12.8k72445
add a comment |
add a comment |
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Without loss of generality, you may assume that the domain and range of both $f$ and $g$ are $[0,1]$. (Uniform limit of sequence of continuous functions with compact support, so you may rescale them without changing much of the problem.)
– GNUSupporter 8964民主女神 地下教會
Dec 2 '18 at 5:52