Does the given statements means the same?

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  1. If a function $f(x)$ is continuous and increasing at point $x=a,$ then there is a nbhd $(x-delta,x+delta),delta>0$ where the function is also increasing.

  2. if $f' (x_0)$ is positive, then for $x$ nearby but smaller than
    $x_0$ the values $f(x)$ will be less than $f(x_0)$, but for $x$
    nearby but larger than $x_0$, the values of $f(x)$ will be larger
    than $f(x_0)$. This says something like $f$ is an increasing
    function near $x_0$, but not quite.






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  • 4




    What does it mean for a function to be increasing at a point ?
    – Kolja
    Aug 10 at 21:18






  • 2




    @Kolja A function $f$ is increasing at $x$ if $f(t)>f(x)$ for every $t>x$ close enough to $x$ and $f(t)<f(x)$ for every $t<x$ close enough to $x$. See math.stackexchange.com/questions/364576/…
    – mfl
    Aug 10 at 21:31










  • @mfl The question is not whether there is a definition (we know there is one), but which one OP wants to use.
    – M. Winter
    Aug 11 at 7:56















up vote
0
down vote

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  1. If a function $f(x)$ is continuous and increasing at point $x=a,$ then there is a nbhd $(x-delta,x+delta),delta>0$ where the function is also increasing.

  2. if $f' (x_0)$ is positive, then for $x$ nearby but smaller than
    $x_0$ the values $f(x)$ will be less than $f(x_0)$, but for $x$
    nearby but larger than $x_0$, the values of $f(x)$ will be larger
    than $f(x_0)$. This says something like $f$ is an increasing
    function near $x_0$, but not quite.






share|cite|improve this question
















  • 4




    What does it mean for a function to be increasing at a point ?
    – Kolja
    Aug 10 at 21:18






  • 2




    @Kolja A function $f$ is increasing at $x$ if $f(t)>f(x)$ for every $t>x$ close enough to $x$ and $f(t)<f(x)$ for every $t<x$ close enough to $x$. See math.stackexchange.com/questions/364576/…
    – mfl
    Aug 10 at 21:31










  • @mfl The question is not whether there is a definition (we know there is one), but which one OP wants to use.
    – M. Winter
    Aug 11 at 7:56













up vote
0
down vote

favorite









up vote
0
down vote

favorite











  1. If a function $f(x)$ is continuous and increasing at point $x=a,$ then there is a nbhd $(x-delta,x+delta),delta>0$ where the function is also increasing.

  2. if $f' (x_0)$ is positive, then for $x$ nearby but smaller than
    $x_0$ the values $f(x)$ will be less than $f(x_0)$, but for $x$
    nearby but larger than $x_0$, the values of $f(x)$ will be larger
    than $f(x_0)$. This says something like $f$ is an increasing
    function near $x_0$, but not quite.






share|cite|improve this question












  1. If a function $f(x)$ is continuous and increasing at point $x=a,$ then there is a nbhd $(x-delta,x+delta),delta>0$ where the function is also increasing.

  2. if $f' (x_0)$ is positive, then for $x$ nearby but smaller than
    $x_0$ the values $f(x)$ will be less than $f(x_0)$, but for $x$
    nearby but larger than $x_0$, the values of $f(x)$ will be larger
    than $f(x_0)$. This says something like $f$ is an increasing
    function near $x_0$, but not quite.








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asked Aug 10 at 21:10









PK Styles

1,394625




1,394625







  • 4




    What does it mean for a function to be increasing at a point ?
    – Kolja
    Aug 10 at 21:18






  • 2




    @Kolja A function $f$ is increasing at $x$ if $f(t)>f(x)$ for every $t>x$ close enough to $x$ and $f(t)<f(x)$ for every $t<x$ close enough to $x$. See math.stackexchange.com/questions/364576/…
    – mfl
    Aug 10 at 21:31










  • @mfl The question is not whether there is a definition (we know there is one), but which one OP wants to use.
    – M. Winter
    Aug 11 at 7:56













  • 4




    What does it mean for a function to be increasing at a point ?
    – Kolja
    Aug 10 at 21:18






  • 2




    @Kolja A function $f$ is increasing at $x$ if $f(t)>f(x)$ for every $t>x$ close enough to $x$ and $f(t)<f(x)$ for every $t<x$ close enough to $x$. See math.stackexchange.com/questions/364576/…
    – mfl
    Aug 10 at 21:31










  • @mfl The question is not whether there is a definition (we know there is one), but which one OP wants to use.
    – M. Winter
    Aug 11 at 7:56








4




4




What does it mean for a function to be increasing at a point ?
– Kolja
Aug 10 at 21:18




What does it mean for a function to be increasing at a point ?
– Kolja
Aug 10 at 21:18




2




2




@Kolja A function $f$ is increasing at $x$ if $f(t)>f(x)$ for every $t>x$ close enough to $x$ and $f(t)<f(x)$ for every $t<x$ close enough to $x$. See math.stackexchange.com/questions/364576/…
– mfl
Aug 10 at 21:31




@Kolja A function $f$ is increasing at $x$ if $f(t)>f(x)$ for every $t>x$ close enough to $x$ and $f(t)<f(x)$ for every $t<x$ close enough to $x$. See math.stackexchange.com/questions/364576/…
– mfl
Aug 10 at 21:31












@mfl The question is not whether there is a definition (we know there is one), but which one OP wants to use.
– M. Winter
Aug 11 at 7:56





@mfl The question is not whether there is a definition (we know there is one), but which one OP wants to use.
– M. Winter
Aug 11 at 7:56











2 Answers
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The two statements are not the same. One assumes differentibity and the other does not.






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  • The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
    – M. Winter
    Aug 11 at 8:27


















up vote
2
down vote













Part 1.-



Consider $f(x)=xleft(sin frac1x +2right), x>0, f(0)=0.$ It is clear that for $x>0$ it is $f(x)>0.$ But $$f'(x)=sinfrac1x+2-frac1xcosfrac1x.$$ So $f$ is increasing at $0$ but not increasing in $(0,delta).$ (Of course, you can modify the function to work on $(-delta,delta).$




Edit



An example can be found in the nice book (see Problem 1.13)
https://stemtec.aut.ac.nz/__data/assets/pdf_file/0003/57639/Counterexamples-in-Calculus-MAA-e-book.pdf




Part 2.-



If $f'(x_0)=lim_xto x_0dfracf(x)-f(x_0)x-x_0>0$ we have that there exists $delta>0$ such that $xin (x_0-delta,x_0)implies f(x)<f(x_0)$ and $xin (x_0,x_0+delta)implies f(x)>f(x_0).$ Thus $f$ is increasing at $x_0.$



Conclusion



Both statements are not the same. The first one is false while the second one holds.






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    2 Answers
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    2 Answers
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    up vote
    2
    down vote













    The two statements are not the same. One assumes differentibity and the other does not.






    share|cite|improve this answer




















    • The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
      – M. Winter
      Aug 11 at 8:27















    up vote
    2
    down vote













    The two statements are not the same. One assumes differentibity and the other does not.






    share|cite|improve this answer




















    • The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
      – M. Winter
      Aug 11 at 8:27













    up vote
    2
    down vote










    up vote
    2
    down vote









    The two statements are not the same. One assumes differentibity and the other does not.






    share|cite|improve this answer












    The two statements are not the same. One assumes differentibity and the other does not.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Aug 10 at 21:37









    Mohammad Riazi-Kermani

    28.2k41852




    28.2k41852











    • The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
      – M. Winter
      Aug 11 at 8:27

















    • The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
      – M. Winter
      Aug 11 at 8:27
















    The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
    – M. Winter
    Aug 11 at 8:27





    The question is: is this relevant? Are "differentiable functions are continuous" and "strictly increasing functions are injective" equivalent? Both are true, hence one is true whenever the other one is. But only one is about differentiability. Maybe "being the same" means talking about the same aspects rather than being equivalent? It is not clear (from the question).
    – M. Winter
    Aug 11 at 8:27











    up vote
    2
    down vote













    Part 1.-



    Consider $f(x)=xleft(sin frac1x +2right), x>0, f(0)=0.$ It is clear that for $x>0$ it is $f(x)>0.$ But $$f'(x)=sinfrac1x+2-frac1xcosfrac1x.$$ So $f$ is increasing at $0$ but not increasing in $(0,delta).$ (Of course, you can modify the function to work on $(-delta,delta).$




    Edit



    An example can be found in the nice book (see Problem 1.13)
    https://stemtec.aut.ac.nz/__data/assets/pdf_file/0003/57639/Counterexamples-in-Calculus-MAA-e-book.pdf




    Part 2.-



    If $f'(x_0)=lim_xto x_0dfracf(x)-f(x_0)x-x_0>0$ we have that there exists $delta>0$ such that $xin (x_0-delta,x_0)implies f(x)<f(x_0)$ and $xin (x_0,x_0+delta)implies f(x)>f(x_0).$ Thus $f$ is increasing at $x_0.$



    Conclusion



    Both statements are not the same. The first one is false while the second one holds.






    share|cite|improve this answer


























      up vote
      2
      down vote













      Part 1.-



      Consider $f(x)=xleft(sin frac1x +2right), x>0, f(0)=0.$ It is clear that for $x>0$ it is $f(x)>0.$ But $$f'(x)=sinfrac1x+2-frac1xcosfrac1x.$$ So $f$ is increasing at $0$ but not increasing in $(0,delta).$ (Of course, you can modify the function to work on $(-delta,delta).$




      Edit



      An example can be found in the nice book (see Problem 1.13)
      https://stemtec.aut.ac.nz/__data/assets/pdf_file/0003/57639/Counterexamples-in-Calculus-MAA-e-book.pdf




      Part 2.-



      If $f'(x_0)=lim_xto x_0dfracf(x)-f(x_0)x-x_0>0$ we have that there exists $delta>0$ such that $xin (x_0-delta,x_0)implies f(x)<f(x_0)$ and $xin (x_0,x_0+delta)implies f(x)>f(x_0).$ Thus $f$ is increasing at $x_0.$



      Conclusion



      Both statements are not the same. The first one is false while the second one holds.






      share|cite|improve this answer
























        up vote
        2
        down vote










        up vote
        2
        down vote









        Part 1.-



        Consider $f(x)=xleft(sin frac1x +2right), x>0, f(0)=0.$ It is clear that for $x>0$ it is $f(x)>0.$ But $$f'(x)=sinfrac1x+2-frac1xcosfrac1x.$$ So $f$ is increasing at $0$ but not increasing in $(0,delta).$ (Of course, you can modify the function to work on $(-delta,delta).$




        Edit



        An example can be found in the nice book (see Problem 1.13)
        https://stemtec.aut.ac.nz/__data/assets/pdf_file/0003/57639/Counterexamples-in-Calculus-MAA-e-book.pdf




        Part 2.-



        If $f'(x_0)=lim_xto x_0dfracf(x)-f(x_0)x-x_0>0$ we have that there exists $delta>0$ such that $xin (x_0-delta,x_0)implies f(x)<f(x_0)$ and $xin (x_0,x_0+delta)implies f(x)>f(x_0).$ Thus $f$ is increasing at $x_0.$



        Conclusion



        Both statements are not the same. The first one is false while the second one holds.






        share|cite|improve this answer














        Part 1.-



        Consider $f(x)=xleft(sin frac1x +2right), x>0, f(0)=0.$ It is clear that for $x>0$ it is $f(x)>0.$ But $$f'(x)=sinfrac1x+2-frac1xcosfrac1x.$$ So $f$ is increasing at $0$ but not increasing in $(0,delta).$ (Of course, you can modify the function to work on $(-delta,delta).$




        Edit



        An example can be found in the nice book (see Problem 1.13)
        https://stemtec.aut.ac.nz/__data/assets/pdf_file/0003/57639/Counterexamples-in-Calculus-MAA-e-book.pdf




        Part 2.-



        If $f'(x_0)=lim_xto x_0dfracf(x)-f(x_0)x-x_0>0$ we have that there exists $delta>0$ such that $xin (x_0-delta,x_0)implies f(x)<f(x_0)$ and $xin (x_0,x_0+delta)implies f(x)>f(x_0).$ Thus $f$ is increasing at $x_0.$



        Conclusion



        Both statements are not the same. The first one is false while the second one holds.







        share|cite|improve this answer














        share|cite|improve this answer



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        edited Aug 10 at 22:13

























        answered Aug 10 at 21:41









        mfl

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