Is the set ${|f(0)|: int_{0}^{1}|f(t)|dtle1}$ bounded?












4














Let $x_0 in [0,1]$ and define $T:C[0,1] rightarrow mathbb{R}$ by $T_{x_0}(f)=f(x_0)$. Let $||cdot||_1$ be a norm on $C[0,1]$. Is $T_0$ bounded or not? That is, is the set
$$
left{|T_{0}(f)|:||f||_1 leq 1right}={|f(0)|:||f||_1 leq 1,f in C[0,1]}
$$
bounded? Since $||f||_1:=int_{0}^{1}|f(t)|dt$, the question may be equivalent to the following:



Let $f:[0,1] rightarrow mathbb{R}$ be continuous. Is the set $$left{|f(0)|: int_{0}^{1}|f(t)|dt leq 1right}$$ bounded?



I guess the answer is no. Because, for example, we can have a function whose graph is a narrow spike at the origin but with infinite height. The area enclosed by the graph may be 1 but the value at the origin $f(0)$ which is its height is infinite.



But how can I prove this formally?










share|cite|improve this question
























  • As written, this makes little sense. What is the precise definition of the set? There's no condition placed on anything. Is it a set of functions?
    – Alex R.
    Nov 29 at 21:32












  • This is not a set... you need to assume something about $int_0^1 | f(t)|dt$. I mean it should be a true/false statement...
    – Yanko
    Nov 29 at 21:32










  • Is the RHS trying to say that $f$ is absolutely integrable on $[0,1]$? If so, you have the right idea, consider the function $1/sqrt{x}$ and its integral on $[0,1]$
    – rubikscube09
    Nov 29 at 21:35












  • Sorry for the confusion. Just edited the description to make it clearer.
    – bbw
    Nov 29 at 21:37






  • 1




    Is it really a problem? What about a constant coefficient?
    – Jean-Claude Arbaut
    Nov 29 at 21:51
















4














Let $x_0 in [0,1]$ and define $T:C[0,1] rightarrow mathbb{R}$ by $T_{x_0}(f)=f(x_0)$. Let $||cdot||_1$ be a norm on $C[0,1]$. Is $T_0$ bounded or not? That is, is the set
$$
left{|T_{0}(f)|:||f||_1 leq 1right}={|f(0)|:||f||_1 leq 1,f in C[0,1]}
$$
bounded? Since $||f||_1:=int_{0}^{1}|f(t)|dt$, the question may be equivalent to the following:



Let $f:[0,1] rightarrow mathbb{R}$ be continuous. Is the set $$left{|f(0)|: int_{0}^{1}|f(t)|dt leq 1right}$$ bounded?



I guess the answer is no. Because, for example, we can have a function whose graph is a narrow spike at the origin but with infinite height. The area enclosed by the graph may be 1 but the value at the origin $f(0)$ which is its height is infinite.



But how can I prove this formally?










share|cite|improve this question
























  • As written, this makes little sense. What is the precise definition of the set? There's no condition placed on anything. Is it a set of functions?
    – Alex R.
    Nov 29 at 21:32












  • This is not a set... you need to assume something about $int_0^1 | f(t)|dt$. I mean it should be a true/false statement...
    – Yanko
    Nov 29 at 21:32










  • Is the RHS trying to say that $f$ is absolutely integrable on $[0,1]$? If so, you have the right idea, consider the function $1/sqrt{x}$ and its integral on $[0,1]$
    – rubikscube09
    Nov 29 at 21:35












  • Sorry for the confusion. Just edited the description to make it clearer.
    – bbw
    Nov 29 at 21:37






  • 1




    Is it really a problem? What about a constant coefficient?
    – Jean-Claude Arbaut
    Nov 29 at 21:51














4












4








4







Let $x_0 in [0,1]$ and define $T:C[0,1] rightarrow mathbb{R}$ by $T_{x_0}(f)=f(x_0)$. Let $||cdot||_1$ be a norm on $C[0,1]$. Is $T_0$ bounded or not? That is, is the set
$$
left{|T_{0}(f)|:||f||_1 leq 1right}={|f(0)|:||f||_1 leq 1,f in C[0,1]}
$$
bounded? Since $||f||_1:=int_{0}^{1}|f(t)|dt$, the question may be equivalent to the following:



Let $f:[0,1] rightarrow mathbb{R}$ be continuous. Is the set $$left{|f(0)|: int_{0}^{1}|f(t)|dt leq 1right}$$ bounded?



I guess the answer is no. Because, for example, we can have a function whose graph is a narrow spike at the origin but with infinite height. The area enclosed by the graph may be 1 but the value at the origin $f(0)$ which is its height is infinite.



But how can I prove this formally?










share|cite|improve this question















Let $x_0 in [0,1]$ and define $T:C[0,1] rightarrow mathbb{R}$ by $T_{x_0}(f)=f(x_0)$. Let $||cdot||_1$ be a norm on $C[0,1]$. Is $T_0$ bounded or not? That is, is the set
$$
left{|T_{0}(f)|:||f||_1 leq 1right}={|f(0)|:||f||_1 leq 1,f in C[0,1]}
$$
bounded? Since $||f||_1:=int_{0}^{1}|f(t)|dt$, the question may be equivalent to the following:



Let $f:[0,1] rightarrow mathbb{R}$ be continuous. Is the set $$left{|f(0)|: int_{0}^{1}|f(t)|dt leq 1right}$$ bounded?



I guess the answer is no. Because, for example, we can have a function whose graph is a narrow spike at the origin but with infinite height. The area enclosed by the graph may be 1 but the value at the origin $f(0)$ which is its height is infinite.



But how can I prove this formally?







calculus real-analysis functional-analysis analysis






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Nov 29 at 21:45









Jean-Claude Arbaut

14.7k63464




14.7k63464










asked Nov 29 at 21:30









bbw

47038




47038












  • As written, this makes little sense. What is the precise definition of the set? There's no condition placed on anything. Is it a set of functions?
    – Alex R.
    Nov 29 at 21:32












  • This is not a set... you need to assume something about $int_0^1 | f(t)|dt$. I mean it should be a true/false statement...
    – Yanko
    Nov 29 at 21:32










  • Is the RHS trying to say that $f$ is absolutely integrable on $[0,1]$? If so, you have the right idea, consider the function $1/sqrt{x}$ and its integral on $[0,1]$
    – rubikscube09
    Nov 29 at 21:35












  • Sorry for the confusion. Just edited the description to make it clearer.
    – bbw
    Nov 29 at 21:37






  • 1




    Is it really a problem? What about a constant coefficient?
    – Jean-Claude Arbaut
    Nov 29 at 21:51


















  • As written, this makes little sense. What is the precise definition of the set? There's no condition placed on anything. Is it a set of functions?
    – Alex R.
    Nov 29 at 21:32












  • This is not a set... you need to assume something about $int_0^1 | f(t)|dt$. I mean it should be a true/false statement...
    – Yanko
    Nov 29 at 21:32










  • Is the RHS trying to say that $f$ is absolutely integrable on $[0,1]$? If so, you have the right idea, consider the function $1/sqrt{x}$ and its integral on $[0,1]$
    – rubikscube09
    Nov 29 at 21:35












  • Sorry for the confusion. Just edited the description to make it clearer.
    – bbw
    Nov 29 at 21:37






  • 1




    Is it really a problem? What about a constant coefficient?
    – Jean-Claude Arbaut
    Nov 29 at 21:51
















As written, this makes little sense. What is the precise definition of the set? There's no condition placed on anything. Is it a set of functions?
– Alex R.
Nov 29 at 21:32






As written, this makes little sense. What is the precise definition of the set? There's no condition placed on anything. Is it a set of functions?
– Alex R.
Nov 29 at 21:32














This is not a set... you need to assume something about $int_0^1 | f(t)|dt$. I mean it should be a true/false statement...
– Yanko
Nov 29 at 21:32




This is not a set... you need to assume something about $int_0^1 | f(t)|dt$. I mean it should be a true/false statement...
– Yanko
Nov 29 at 21:32












Is the RHS trying to say that $f$ is absolutely integrable on $[0,1]$? If so, you have the right idea, consider the function $1/sqrt{x}$ and its integral on $[0,1]$
– rubikscube09
Nov 29 at 21:35






Is the RHS trying to say that $f$ is absolutely integrable on $[0,1]$? If so, you have the right idea, consider the function $1/sqrt{x}$ and its integral on $[0,1]$
– rubikscube09
Nov 29 at 21:35














Sorry for the confusion. Just edited the description to make it clearer.
– bbw
Nov 29 at 21:37




Sorry for the confusion. Just edited the description to make it clearer.
– bbw
Nov 29 at 21:37




1




1




Is it really a problem? What about a constant coefficient?
– Jean-Claude Arbaut
Nov 29 at 21:51




Is it really a problem? What about a constant coefficient?
– Jean-Claude Arbaut
Nov 29 at 21:51










4 Answers
4






active

oldest

votes


















2














No, it is not bounded. Define, for each $ninmathbb N$,$$begin{array}{rccc}f_ncolon&[0,1]&longrightarrow&mathbb R\&t&mapsto&begin{cases}n-n^2t&text{ if }tleqslantfrac1n\0&text{ otherwise.}end{cases}end{array}$$Then$$int_0^1bigllvert f_n(t)bigrrvert,mathrm dt=frac12,$$but $f_n(0)=n$.






share|cite|improve this answer





























    1














    We can take a function $f_n$ such that $f_n$ is affine on $(0,1/n)$, $f_n(0)=2n$ and $f_n(1/n)=0$ and $0$ for the other values of the interval. Then $leftlVert f_nrightrVert_1=1$.



    For a formal example: define for each positive integer $n$ the function $f_n$ in the following way: $f_n(t)=-2n^2t+2n$ for $0leqslant tleqslant 1/n$ and $f_n(t)=0$ for $1/nlt tleqslant 1$.






    share|cite|improve this answer





























      0














      Consider the function $f_n(x)=2n(1/n-x)1_{[0,1/n]}(x).$ Then $int_0^1 |f_n(x)|dx=1$ but $f_n(0)=2n.$






      share|cite|improve this answer





























        0














        For every $a>0$, the function
        $$f_a(x)=frac{2 a e^{-a^2 x^2}}{sqrt{pi} textrm{erf}(a)}$$ with the error function $textrm{erf}(a)=frac{2}{pi}int_0^a e^{-t^2}dt$ is in the set ${|f(0)|: int_{0}^{1}|f(t)|dt=1}$ and evaluates to $f_a(0)=frac{2 a }{sqrt{pi} textrm{erf}(a)}$. Because $textrm{erf}(a) rightarrow 1$ as $arightarrow infty$, we obtain $f_a(0)$ arbitrarily large as we increase $a$. As a consequence, $f_a(0)$ and thus your set are unbounded.



        For every $a>0$, the function $f_a(x)$ is in $C[0,1]$ and even infinitely differentiable on $[0,1]$.






        share|cite|improve this answer





















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          4 Answers
          4






          active

          oldest

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          4 Answers
          4






          active

          oldest

          votes









          active

          oldest

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          active

          oldest

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          2














          No, it is not bounded. Define, for each $ninmathbb N$,$$begin{array}{rccc}f_ncolon&[0,1]&longrightarrow&mathbb R\&t&mapsto&begin{cases}n-n^2t&text{ if }tleqslantfrac1n\0&text{ otherwise.}end{cases}end{array}$$Then$$int_0^1bigllvert f_n(t)bigrrvert,mathrm dt=frac12,$$but $f_n(0)=n$.






          share|cite|improve this answer


























            2














            No, it is not bounded. Define, for each $ninmathbb N$,$$begin{array}{rccc}f_ncolon&[0,1]&longrightarrow&mathbb R\&t&mapsto&begin{cases}n-n^2t&text{ if }tleqslantfrac1n\0&text{ otherwise.}end{cases}end{array}$$Then$$int_0^1bigllvert f_n(t)bigrrvert,mathrm dt=frac12,$$but $f_n(0)=n$.






            share|cite|improve this answer
























              2












              2








              2






              No, it is not bounded. Define, for each $ninmathbb N$,$$begin{array}{rccc}f_ncolon&[0,1]&longrightarrow&mathbb R\&t&mapsto&begin{cases}n-n^2t&text{ if }tleqslantfrac1n\0&text{ otherwise.}end{cases}end{array}$$Then$$int_0^1bigllvert f_n(t)bigrrvert,mathrm dt=frac12,$$but $f_n(0)=n$.






              share|cite|improve this answer












              No, it is not bounded. Define, for each $ninmathbb N$,$$begin{array}{rccc}f_ncolon&[0,1]&longrightarrow&mathbb R\&t&mapsto&begin{cases}n-n^2t&text{ if }tleqslantfrac1n\0&text{ otherwise.}end{cases}end{array}$$Then$$int_0^1bigllvert f_n(t)bigrrvert,mathrm dt=frac12,$$but $f_n(0)=n$.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Nov 29 at 21:50









              José Carlos Santos

              149k22117219




              149k22117219























                  1














                  We can take a function $f_n$ such that $f_n$ is affine on $(0,1/n)$, $f_n(0)=2n$ and $f_n(1/n)=0$ and $0$ for the other values of the interval. Then $leftlVert f_nrightrVert_1=1$.



                  For a formal example: define for each positive integer $n$ the function $f_n$ in the following way: $f_n(t)=-2n^2t+2n$ for $0leqslant tleqslant 1/n$ and $f_n(t)=0$ for $1/nlt tleqslant 1$.






                  share|cite|improve this answer


























                    1














                    We can take a function $f_n$ such that $f_n$ is affine on $(0,1/n)$, $f_n(0)=2n$ and $f_n(1/n)=0$ and $0$ for the other values of the interval. Then $leftlVert f_nrightrVert_1=1$.



                    For a formal example: define for each positive integer $n$ the function $f_n$ in the following way: $f_n(t)=-2n^2t+2n$ for $0leqslant tleqslant 1/n$ and $f_n(t)=0$ for $1/nlt tleqslant 1$.






                    share|cite|improve this answer
























                      1












                      1








                      1






                      We can take a function $f_n$ such that $f_n$ is affine on $(0,1/n)$, $f_n(0)=2n$ and $f_n(1/n)=0$ and $0$ for the other values of the interval. Then $leftlVert f_nrightrVert_1=1$.



                      For a formal example: define for each positive integer $n$ the function $f_n$ in the following way: $f_n(t)=-2n^2t+2n$ for $0leqslant tleqslant 1/n$ and $f_n(t)=0$ for $1/nlt tleqslant 1$.






                      share|cite|improve this answer












                      We can take a function $f_n$ such that $f_n$ is affine on $(0,1/n)$, $f_n(0)=2n$ and $f_n(1/n)=0$ and $0$ for the other values of the interval. Then $leftlVert f_nrightrVert_1=1$.



                      For a formal example: define for each positive integer $n$ the function $f_n$ in the following way: $f_n(t)=-2n^2t+2n$ for $0leqslant tleqslant 1/n$ and $f_n(t)=0$ for $1/nlt tleqslant 1$.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Nov 29 at 21:51









                      Davide Giraudo

                      125k16150259




                      125k16150259























                          0














                          Consider the function $f_n(x)=2n(1/n-x)1_{[0,1/n]}(x).$ Then $int_0^1 |f_n(x)|dx=1$ but $f_n(0)=2n.$






                          share|cite|improve this answer


























                            0














                            Consider the function $f_n(x)=2n(1/n-x)1_{[0,1/n]}(x).$ Then $int_0^1 |f_n(x)|dx=1$ but $f_n(0)=2n.$






                            share|cite|improve this answer
























                              0












                              0








                              0






                              Consider the function $f_n(x)=2n(1/n-x)1_{[0,1/n]}(x).$ Then $int_0^1 |f_n(x)|dx=1$ but $f_n(0)=2n.$






                              share|cite|improve this answer












                              Consider the function $f_n(x)=2n(1/n-x)1_{[0,1/n]}(x).$ Then $int_0^1 |f_n(x)|dx=1$ but $f_n(0)=2n.$







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Nov 29 at 21:52









                              John_Wick

                              1,356111




                              1,356111























                                  0














                                  For every $a>0$, the function
                                  $$f_a(x)=frac{2 a e^{-a^2 x^2}}{sqrt{pi} textrm{erf}(a)}$$ with the error function $textrm{erf}(a)=frac{2}{pi}int_0^a e^{-t^2}dt$ is in the set ${|f(0)|: int_{0}^{1}|f(t)|dt=1}$ and evaluates to $f_a(0)=frac{2 a }{sqrt{pi} textrm{erf}(a)}$. Because $textrm{erf}(a) rightarrow 1$ as $arightarrow infty$, we obtain $f_a(0)$ arbitrarily large as we increase $a$. As a consequence, $f_a(0)$ and thus your set are unbounded.



                                  For every $a>0$, the function $f_a(x)$ is in $C[0,1]$ and even infinitely differentiable on $[0,1]$.






                                  share|cite|improve this answer


























                                    0














                                    For every $a>0$, the function
                                    $$f_a(x)=frac{2 a e^{-a^2 x^2}}{sqrt{pi} textrm{erf}(a)}$$ with the error function $textrm{erf}(a)=frac{2}{pi}int_0^a e^{-t^2}dt$ is in the set ${|f(0)|: int_{0}^{1}|f(t)|dt=1}$ and evaluates to $f_a(0)=frac{2 a }{sqrt{pi} textrm{erf}(a)}$. Because $textrm{erf}(a) rightarrow 1$ as $arightarrow infty$, we obtain $f_a(0)$ arbitrarily large as we increase $a$. As a consequence, $f_a(0)$ and thus your set are unbounded.



                                    For every $a>0$, the function $f_a(x)$ is in $C[0,1]$ and even infinitely differentiable on $[0,1]$.






                                    share|cite|improve this answer
























                                      0












                                      0








                                      0






                                      For every $a>0$, the function
                                      $$f_a(x)=frac{2 a e^{-a^2 x^2}}{sqrt{pi} textrm{erf}(a)}$$ with the error function $textrm{erf}(a)=frac{2}{pi}int_0^a e^{-t^2}dt$ is in the set ${|f(0)|: int_{0}^{1}|f(t)|dt=1}$ and evaluates to $f_a(0)=frac{2 a }{sqrt{pi} textrm{erf}(a)}$. Because $textrm{erf}(a) rightarrow 1$ as $arightarrow infty$, we obtain $f_a(0)$ arbitrarily large as we increase $a$. As a consequence, $f_a(0)$ and thus your set are unbounded.



                                      For every $a>0$, the function $f_a(x)$ is in $C[0,1]$ and even infinitely differentiable on $[0,1]$.






                                      share|cite|improve this answer












                                      For every $a>0$, the function
                                      $$f_a(x)=frac{2 a e^{-a^2 x^2}}{sqrt{pi} textrm{erf}(a)}$$ with the error function $textrm{erf}(a)=frac{2}{pi}int_0^a e^{-t^2}dt$ is in the set ${|f(0)|: int_{0}^{1}|f(t)|dt=1}$ and evaluates to $f_a(0)=frac{2 a }{sqrt{pi} textrm{erf}(a)}$. Because $textrm{erf}(a) rightarrow 1$ as $arightarrow infty$, we obtain $f_a(0)$ arbitrarily large as we increase $a$. As a consequence, $f_a(0)$ and thus your set are unbounded.



                                      For every $a>0$, the function $f_a(x)$ is in $C[0,1]$ and even infinitely differentiable on $[0,1]$.







                                      share|cite|improve this answer












                                      share|cite|improve this answer



                                      share|cite|improve this answer










                                      answered Nov 29 at 21:53









                                      M1183

                                      943




                                      943






























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