Does the given statements means the same?
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- 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.
- 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.
real-analysis functions examples-counterexamples monotone-functions
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- 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.
- 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.
real-analysis functions examples-counterexamples monotone-functions
4
What does it mean for a function to be increasing at a point ?
â 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/â¦
â 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.
â M. Winter
Aug 11 at 7:56
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up vote
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favorite
- 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.
- 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.
real-analysis functions examples-counterexamples monotone-functions
- 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.
- 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.
real-analysis functions examples-counterexamples monotone-functions
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 ?
â 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/â¦
â 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.
â M. Winter
Aug 11 at 7:56
add a comment |Â
4
What does it mean for a function to be increasing at a point ?
â 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/â¦
â 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.
â M. Winter
Aug 11 at 7:56
4
4
What does it mean for a function to be increasing at a point ?
â Kolja
Aug 10 at 21:18
What does it mean for a function to be increasing at a point ?
â 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/â¦
â 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/â¦
â 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.
â 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.
â M. Winter
Aug 11 at 7:56
add a comment |Â
2 Answers
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The two statements are not the same. One assumes differentibity and the other does not.
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).
â M. Winter
Aug 11 at 8:27
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up vote
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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
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
The two statements are not the same. One assumes differentibity and the other does not.
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).
â M. Winter
Aug 11 at 8:27
add a comment |Â
up vote
2
down vote
The two statements are not the same. One assumes differentibity and the other does not.
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).
â M. Winter
Aug 11 at 8:27
add a comment |Â
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.
The two statements are not the same. One assumes differentibity and the other does not.
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).
â M. Winter
Aug 11 at 8:27
add a comment |Â
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).
â 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).
â 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).
â M. Winter
Aug 11 at 8:27
add a comment |Â
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.
add a comment |Â
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.
add a comment |Â
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.
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.
edited Aug 10 at 22:13
answered Aug 10 at 21:41
mfl
24.6k12040
24.6k12040
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4
What does it mean for a function to be increasing at a point ?
â 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/â¦
â 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.
â M. Winter
Aug 11 at 7:56