How the supremum of a function and its derivative in the unit interval are related in normed spaces
$begingroup$
Let $E={ f: ,fin C^1([0,1]), f(0)=0}$ and the norm defined on $E$ be $$ left|fright|_E = sup_{xin [0,1]} ,|f'(x)|,,, fin E, $$ and $F=C,([0,1])$ with the norm $$ left|fright|_F = sup_{xin [0,1]} ,|f(x)|,,, fin F.$$
I need to prove that $forall fin E, $ $$ left|fright|_F leq left|fright|_E.$$
Since the interval $[0,1]$ is compact, $fin E, f'in F$ are continuous, the maxima of these functions are attained in the unit intarval. Thus, $exists x_1, x_2in [0,1]$ such that we can write: $$ left|fright|_F= max_{xin [0,1]}|f(x)|=|f(x_1)| ,,text{and},,left|fright|_E= max_{xin [0,1]}|f'(x)|=|f'(x_2)|.$$ Also, $f(x+h)=f(x)+f'(x)h +o(h)$. After taking the norm and using the triangle inequality I can not reach the point which would suggest me the solution.
Can you provide a solution proposal or give me a hint. Thanks.
calculus analysis normed-spaces
$endgroup$
add a comment |
$begingroup$
Let $E={ f: ,fin C^1([0,1]), f(0)=0}$ and the norm defined on $E$ be $$ left|fright|_E = sup_{xin [0,1]} ,|f'(x)|,,, fin E, $$ and $F=C,([0,1])$ with the norm $$ left|fright|_F = sup_{xin [0,1]} ,|f(x)|,,, fin F.$$
I need to prove that $forall fin E, $ $$ left|fright|_F leq left|fright|_E.$$
Since the interval $[0,1]$ is compact, $fin E, f'in F$ are continuous, the maxima of these functions are attained in the unit intarval. Thus, $exists x_1, x_2in [0,1]$ such that we can write: $$ left|fright|_F= max_{xin [0,1]}|f(x)|=|f(x_1)| ,,text{and},,left|fright|_E= max_{xin [0,1]}|f'(x)|=|f'(x_2)|.$$ Also, $f(x+h)=f(x)+f'(x)h +o(h)$. After taking the norm and using the triangle inequality I can not reach the point which would suggest me the solution.
Can you provide a solution proposal or give me a hint. Thanks.
calculus analysis normed-spaces
$endgroup$
add a comment |
$begingroup$
Let $E={ f: ,fin C^1([0,1]), f(0)=0}$ and the norm defined on $E$ be $$ left|fright|_E = sup_{xin [0,1]} ,|f'(x)|,,, fin E, $$ and $F=C,([0,1])$ with the norm $$ left|fright|_F = sup_{xin [0,1]} ,|f(x)|,,, fin F.$$
I need to prove that $forall fin E, $ $$ left|fright|_F leq left|fright|_E.$$
Since the interval $[0,1]$ is compact, $fin E, f'in F$ are continuous, the maxima of these functions are attained in the unit intarval. Thus, $exists x_1, x_2in [0,1]$ such that we can write: $$ left|fright|_F= max_{xin [0,1]}|f(x)|=|f(x_1)| ,,text{and},,left|fright|_E= max_{xin [0,1]}|f'(x)|=|f'(x_2)|.$$ Also, $f(x+h)=f(x)+f'(x)h +o(h)$. After taking the norm and using the triangle inequality I can not reach the point which would suggest me the solution.
Can you provide a solution proposal or give me a hint. Thanks.
calculus analysis normed-spaces
$endgroup$
Let $E={ f: ,fin C^1([0,1]), f(0)=0}$ and the norm defined on $E$ be $$ left|fright|_E = sup_{xin [0,1]} ,|f'(x)|,,, fin E, $$ and $F=C,([0,1])$ with the norm $$ left|fright|_F = sup_{xin [0,1]} ,|f(x)|,,, fin F.$$
I need to prove that $forall fin E, $ $$ left|fright|_F leq left|fright|_E.$$
Since the interval $[0,1]$ is compact, $fin E, f'in F$ are continuous, the maxima of these functions are attained in the unit intarval. Thus, $exists x_1, x_2in [0,1]$ such that we can write: $$ left|fright|_F= max_{xin [0,1]}|f(x)|=|f(x_1)| ,,text{and},,left|fright|_E= max_{xin [0,1]}|f'(x)|=|f'(x_2)|.$$ Also, $f(x+h)=f(x)+f'(x)h +o(h)$. After taking the norm and using the triangle inequality I can not reach the point which would suggest me the solution.
Can you provide a solution proposal or give me a hint. Thanks.
calculus analysis normed-spaces
calculus analysis normed-spaces
edited Dec 19 '18 at 20:09
user249018
asked Dec 19 '18 at 19:46
user249018user249018
403137
403137
add a comment |
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1 Answer
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$begingroup$
You were on the right track. Let $x_1 in [0,1]$ such that $|f(x_1)| = |f|_F$.
If $x_1 = 0$ then $f equiv 0$ so the statement holds.
If $x_1 > 0$, the mean value theorem implies that $exists c in langle 0, x_1rangle$ such that
$$f(x_1) - f(0) = f'(c)(x_1 - 0)$$
so
$$|f|_F = |f(x_1)| = |f'(c)||x_1| le |f'(c)| le |f|_E$$
$endgroup$
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
1
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
add a comment |
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$begingroup$
You were on the right track. Let $x_1 in [0,1]$ such that $|f(x_1)| = |f|_F$.
If $x_1 = 0$ then $f equiv 0$ so the statement holds.
If $x_1 > 0$, the mean value theorem implies that $exists c in langle 0, x_1rangle$ such that
$$f(x_1) - f(0) = f'(c)(x_1 - 0)$$
so
$$|f|_F = |f(x_1)| = |f'(c)||x_1| le |f'(c)| le |f|_E$$
$endgroup$
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
1
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
add a comment |
$begingroup$
You were on the right track. Let $x_1 in [0,1]$ such that $|f(x_1)| = |f|_F$.
If $x_1 = 0$ then $f equiv 0$ so the statement holds.
If $x_1 > 0$, the mean value theorem implies that $exists c in langle 0, x_1rangle$ such that
$$f(x_1) - f(0) = f'(c)(x_1 - 0)$$
so
$$|f|_F = |f(x_1)| = |f'(c)||x_1| le |f'(c)| le |f|_E$$
$endgroup$
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
1
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
add a comment |
$begingroup$
You were on the right track. Let $x_1 in [0,1]$ such that $|f(x_1)| = |f|_F$.
If $x_1 = 0$ then $f equiv 0$ so the statement holds.
If $x_1 > 0$, the mean value theorem implies that $exists c in langle 0, x_1rangle$ such that
$$f(x_1) - f(0) = f'(c)(x_1 - 0)$$
so
$$|f|_F = |f(x_1)| = |f'(c)||x_1| le |f'(c)| le |f|_E$$
$endgroup$
You were on the right track. Let $x_1 in [0,1]$ such that $|f(x_1)| = |f|_F$.
If $x_1 = 0$ then $f equiv 0$ so the statement holds.
If $x_1 > 0$, the mean value theorem implies that $exists c in langle 0, x_1rangle$ such that
$$f(x_1) - f(0) = f'(c)(x_1 - 0)$$
so
$$|f|_F = |f(x_1)| = |f'(c)||x_1| le |f'(c)| le |f|_E$$
answered Dec 19 '18 at 20:00
mechanodroidmechanodroid
27.9k62447
27.9k62447
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
1
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
add a comment |
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
1
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Many thanks. It looks so easy and yet I could not make it.
$endgroup$
– user249018
Dec 19 '18 at 20:19
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
$begingroup$
Under the same conditions can you give me a hint to prove that the function $s(f)=f'+f²$ is differentiable.
$endgroup$
– user249018
Dec 19 '18 at 20:45
1
1
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
@user249018 The differential of $s : E to F$ at point $f in E$ should be given by $Ds(f)h := h' + 2fh$. Indeed, it is bounded with $|Ds(f)| le 1+2|f|_E$ and begin{align} lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}&= lim_{hto 0}frac{|s(f+h) - s(f) - Ds(f)h|_F}{|h|_E}\ &= lim_{hto 0}frac{|f'+h' +(f+h)^2- f'-f^2 - h'-2fh|_F}{|h|_E}\ &= lim_{hto 0}frac{|h^2|_F}{|h|_E}\ &= lim_{hto 0}frac{|h|_F^2}{|h|_E}\ &le lim_{hto 0}frac{|h|_E^2}{|h|_E}\ &= lim_{hto 0} |h|_E\ &= 0 end{align}
$endgroup$
– mechanodroid
Dec 19 '18 at 21:07
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
Thanks. How you come to define the differential in such a way ? I tried it with $Ds(f):=2f$ but did not work, thats why I needed some support. Is it very obvious or one should try different options such that the rest of the first order Taylor development become negligeable in respect to the function $h$ ? Anyway thank you for the very clear answer.
$endgroup$
– user249018
Dec 19 '18 at 22:23
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
$begingroup$
@user249018 I tried to define it such that $s(f+h) - s(f) - Ds(f)h = o(h^2)$, it was mostly trial and error.
$endgroup$
– mechanodroid
Dec 20 '18 at 7:25
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