Show that $(exp(x)cdot exp(x')=exp(x+x')$ and $ exp(x)>exp(0),x>0)Rightarrow exp$ is strictly...
$begingroup$
$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$
My calculations so far:
Assume
$exists _{x,x'inmathbb{R}}:$
$x<x'$ and $exp(x)>exp(x')$
$x<x'Rightarrow x'-x>0 Rightarrow exp(x-x')>exp(0)$
Can I somehow invoke a contradiction with this?
real-analysis
$endgroup$
add a comment |
$begingroup$
$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$
My calculations so far:
Assume
$exists _{x,x'inmathbb{R}}:$
$x<x'$ and $exp(x)>exp(x')$
$x<x'Rightarrow x'-x>0 Rightarrow exp(x-x')>exp(0)$
Can I somehow invoke a contradiction with this?
real-analysis
$endgroup$
1
$begingroup$
Try setting $x= y-x'$ where $y>x'$.
$endgroup$
– irchans
Jan 12 at 19:52
$begingroup$
I read this first as "Show that $A$, and $(BRightarrow C)$, not as the (presumably intended) "Show that $(A$ and $B)Rightarrow C$."
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
Oh sorry I will eddit the question
$endgroup$
– RM777
Jan 12 at 20:13
add a comment |
$begingroup$
$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$
My calculations so far:
Assume
$exists _{x,x'inmathbb{R}}:$
$x<x'$ and $exp(x)>exp(x')$
$x<x'Rightarrow x'-x>0 Rightarrow exp(x-x')>exp(0)$
Can I somehow invoke a contradiction with this?
real-analysis
$endgroup$
$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$
My calculations so far:
Assume
$exists _{x,x'inmathbb{R}}:$
$x<x'$ and $exp(x)>exp(x')$
$x<x'Rightarrow x'-x>0 Rightarrow exp(x-x')>exp(0)$
Can I somehow invoke a contradiction with this?
real-analysis
real-analysis
edited Jan 12 at 20:15
RM777
asked Jan 12 at 19:41
RM777RM777
38312
38312
1
$begingroup$
Try setting $x= y-x'$ where $y>x'$.
$endgroup$
– irchans
Jan 12 at 19:52
$begingroup$
I read this first as "Show that $A$, and $(BRightarrow C)$, not as the (presumably intended) "Show that $(A$ and $B)Rightarrow C$."
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
Oh sorry I will eddit the question
$endgroup$
– RM777
Jan 12 at 20:13
add a comment |
1
$begingroup$
Try setting $x= y-x'$ where $y>x'$.
$endgroup$
– irchans
Jan 12 at 19:52
$begingroup$
I read this first as "Show that $A$, and $(BRightarrow C)$, not as the (presumably intended) "Show that $(A$ and $B)Rightarrow C$."
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
Oh sorry I will eddit the question
$endgroup$
– RM777
Jan 12 at 20:13
1
1
$begingroup$
Try setting $x= y-x'$ where $y>x'$.
$endgroup$
– irchans
Jan 12 at 19:52
$begingroup$
Try setting $x= y-x'$ where $y>x'$.
$endgroup$
– irchans
Jan 12 at 19:52
$begingroup$
I read this first as "Show that $A$, and $(BRightarrow C)$, not as the (presumably intended) "Show that $(A$ and $B)Rightarrow C$."
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
I read this first as "Show that $A$, and $(BRightarrow C)$, not as the (presumably intended) "Show that $(A$ and $B)Rightarrow C$."
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
Oh sorry I will eddit the question
$endgroup$
– RM777
Jan 12 at 20:13
$begingroup$
Oh sorry I will eddit the question
$endgroup$
– RM777
Jan 12 at 20:13
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Let $y>x leftrightarrow y-x>0$. Then
$exp(y)=exp(y-x+x)=exp(y-x) exp(x)> exp(0) exp(x) =exp(x)$.
In the second and third step you use the two assumptions.
$endgroup$
add a comment |
$begingroup$
First let $x=x'=0$ therefore $$exp(0)cdotexp(0)=exp(0)$$which yields to $$exp(0)=0text{ or }1$$ if $exp(0)=0$ then we have $$exp(x){=exp(x+0)\=exp(0)cdotexp(x)\=0quad,quad forall xin Bbb R}$$which is non-sense. Therefore $exp(0)=1$. Now let $x'>x$. Then there exists $epsilon>0$ for which $x'=x+epsilon$. This leads to$$exp(x'){=exp(x+epsilon)\=exp(x)exp(epsilon)\>exp(x)exp(0)\=exp(x)}$$therefore$$exp(x')>exp(x)$$which completes our proof.
$endgroup$
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
add a comment |
$begingroup$
In general, for any $f(cdot)$ with $f(x+y)=f(x)f(y)$ where $f(y)>f(0)=1$ for every $y>0$, you can immediately prove that $f(cdot)$ is strictly increasing.
For this, I start by proving $f(x)>0$ for every $x$. To see this, use $f(x+y)=f(x)f(y)$, take $y=-x$ for an $x>0$ and get $f(x)f(-x)=f(0)>0$. Since $f(x)>0$, we deduce $f(-x)>0$.
Now, fix an $xinmathbb{R}$, and let $z>x$. Then, $z=x+epsilon$ for an $epsilon>0$. This gives, $f(z)=f(x)f(epsilon)>f(x)$, as $f(x)>0$ and $f(epsilon)>1$. Done.
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Let $y>x leftrightarrow y-x>0$. Then
$exp(y)=exp(y-x+x)=exp(y-x) exp(x)> exp(0) exp(x) =exp(x)$.
In the second and third step you use the two assumptions.
$endgroup$
add a comment |
$begingroup$
Let $y>x leftrightarrow y-x>0$. Then
$exp(y)=exp(y-x+x)=exp(y-x) exp(x)> exp(0) exp(x) =exp(x)$.
In the second and third step you use the two assumptions.
$endgroup$
add a comment |
$begingroup$
Let $y>x leftrightarrow y-x>0$. Then
$exp(y)=exp(y-x+x)=exp(y-x) exp(x)> exp(0) exp(x) =exp(x)$.
In the second and third step you use the two assumptions.
$endgroup$
Let $y>x leftrightarrow y-x>0$. Then
$exp(y)=exp(y-x+x)=exp(y-x) exp(x)> exp(0) exp(x) =exp(x)$.
In the second and third step you use the two assumptions.
answered Jan 12 at 19:59
MaxMax
586
586
add a comment |
add a comment |
$begingroup$
First let $x=x'=0$ therefore $$exp(0)cdotexp(0)=exp(0)$$which yields to $$exp(0)=0text{ or }1$$ if $exp(0)=0$ then we have $$exp(x){=exp(x+0)\=exp(0)cdotexp(x)\=0quad,quad forall xin Bbb R}$$which is non-sense. Therefore $exp(0)=1$. Now let $x'>x$. Then there exists $epsilon>0$ for which $x'=x+epsilon$. This leads to$$exp(x'){=exp(x+epsilon)\=exp(x)exp(epsilon)\>exp(x)exp(0)\=exp(x)}$$therefore$$exp(x')>exp(x)$$which completes our proof.
$endgroup$
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
add a comment |
$begingroup$
First let $x=x'=0$ therefore $$exp(0)cdotexp(0)=exp(0)$$which yields to $$exp(0)=0text{ or }1$$ if $exp(0)=0$ then we have $$exp(x){=exp(x+0)\=exp(0)cdotexp(x)\=0quad,quad forall xin Bbb R}$$which is non-sense. Therefore $exp(0)=1$. Now let $x'>x$. Then there exists $epsilon>0$ for which $x'=x+epsilon$. This leads to$$exp(x'){=exp(x+epsilon)\=exp(x)exp(epsilon)\>exp(x)exp(0)\=exp(x)}$$therefore$$exp(x')>exp(x)$$which completes our proof.
$endgroup$
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
add a comment |
$begingroup$
First let $x=x'=0$ therefore $$exp(0)cdotexp(0)=exp(0)$$which yields to $$exp(0)=0text{ or }1$$ if $exp(0)=0$ then we have $$exp(x){=exp(x+0)\=exp(0)cdotexp(x)\=0quad,quad forall xin Bbb R}$$which is non-sense. Therefore $exp(0)=1$. Now let $x'>x$. Then there exists $epsilon>0$ for which $x'=x+epsilon$. This leads to$$exp(x'){=exp(x+epsilon)\=exp(x)exp(epsilon)\>exp(x)exp(0)\=exp(x)}$$therefore$$exp(x')>exp(x)$$which completes our proof.
$endgroup$
First let $x=x'=0$ therefore $$exp(0)cdotexp(0)=exp(0)$$which yields to $$exp(0)=0text{ or }1$$ if $exp(0)=0$ then we have $$exp(x){=exp(x+0)\=exp(0)cdotexp(x)\=0quad,quad forall xin Bbb R}$$which is non-sense. Therefore $exp(0)=1$. Now let $x'>x$. Then there exists $epsilon>0$ for which $x'=x+epsilon$. This leads to$$exp(x'){=exp(x+epsilon)\=exp(x)exp(epsilon)\>exp(x)exp(0)\=exp(x)}$$therefore$$exp(x')>exp(x)$$which completes our proof.
answered Jan 12 at 19:57
Mostafa AyazMostafa Ayaz
15.8k3939
15.8k3939
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
add a comment |
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
You haven't used the definition of $exp$ at all, so how can you prove anything about it?
$endgroup$
– TonyK
Jan 12 at 20:03
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
It needs not. Any function with such properties is strictly increasing.
$endgroup$
– Mostafa Ayaz
Jan 12 at 20:04
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
@TonyK Everything on the left side of the implication can be taken for granted. We have proved this property with the Cauchy-Product already in class
$endgroup$
– RM777
Jan 12 at 20:11
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
Perhaps I misinterpreted the question $-$ see my comment there.
$endgroup$
– TonyK
Jan 12 at 20:12
add a comment |
$begingroup$
In general, for any $f(cdot)$ with $f(x+y)=f(x)f(y)$ where $f(y)>f(0)=1$ for every $y>0$, you can immediately prove that $f(cdot)$ is strictly increasing.
For this, I start by proving $f(x)>0$ for every $x$. To see this, use $f(x+y)=f(x)f(y)$, take $y=-x$ for an $x>0$ and get $f(x)f(-x)=f(0)>0$. Since $f(x)>0$, we deduce $f(-x)>0$.
Now, fix an $xinmathbb{R}$, and let $z>x$. Then, $z=x+epsilon$ for an $epsilon>0$. This gives, $f(z)=f(x)f(epsilon)>f(x)$, as $f(x)>0$ and $f(epsilon)>1$. Done.
$endgroup$
add a comment |
$begingroup$
In general, for any $f(cdot)$ with $f(x+y)=f(x)f(y)$ where $f(y)>f(0)=1$ for every $y>0$, you can immediately prove that $f(cdot)$ is strictly increasing.
For this, I start by proving $f(x)>0$ for every $x$. To see this, use $f(x+y)=f(x)f(y)$, take $y=-x$ for an $x>0$ and get $f(x)f(-x)=f(0)>0$. Since $f(x)>0$, we deduce $f(-x)>0$.
Now, fix an $xinmathbb{R}$, and let $z>x$. Then, $z=x+epsilon$ for an $epsilon>0$. This gives, $f(z)=f(x)f(epsilon)>f(x)$, as $f(x)>0$ and $f(epsilon)>1$. Done.
$endgroup$
add a comment |
$begingroup$
In general, for any $f(cdot)$ with $f(x+y)=f(x)f(y)$ where $f(y)>f(0)=1$ for every $y>0$, you can immediately prove that $f(cdot)$ is strictly increasing.
For this, I start by proving $f(x)>0$ for every $x$. To see this, use $f(x+y)=f(x)f(y)$, take $y=-x$ for an $x>0$ and get $f(x)f(-x)=f(0)>0$. Since $f(x)>0$, we deduce $f(-x)>0$.
Now, fix an $xinmathbb{R}$, and let $z>x$. Then, $z=x+epsilon$ for an $epsilon>0$. This gives, $f(z)=f(x)f(epsilon)>f(x)$, as $f(x)>0$ and $f(epsilon)>1$. Done.
$endgroup$
In general, for any $f(cdot)$ with $f(x+y)=f(x)f(y)$ where $f(y)>f(0)=1$ for every $y>0$, you can immediately prove that $f(cdot)$ is strictly increasing.
For this, I start by proving $f(x)>0$ for every $x$. To see this, use $f(x+y)=f(x)f(y)$, take $y=-x$ for an $x>0$ and get $f(x)f(-x)=f(0)>0$. Since $f(x)>0$, we deduce $f(-x)>0$.
Now, fix an $xinmathbb{R}$, and let $z>x$. Then, $z=x+epsilon$ for an $epsilon>0$. This gives, $f(z)=f(x)f(epsilon)>f(x)$, as $f(x)>0$ and $f(epsilon)>1$. Done.
answered Jan 12 at 20:22
AaronAaron
1,912415
1,912415
add a comment |
add a comment |
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$begingroup$
Try setting $x= y-x'$ where $y>x'$.
$endgroup$
– irchans
Jan 12 at 19:52
$begingroup$
I read this first as "Show that $A$, and $(BRightarrow C)$, not as the (presumably intended) "Show that $(A$ and $B)Rightarrow C$."
$endgroup$
– TonyK
Jan 12 at 20:12
$begingroup$
Oh sorry I will eddit the question
$endgroup$
– RM777
Jan 12 at 20:13