Show that $(exp(x)cdot exp(x')=exp(x+x')$ and $ exp(x)>exp(0),x>0)Rightarrow exp$ is strictly...












1












$begingroup$


$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$



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?










share|cite|improve this question











$endgroup$








  • 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
















1












$begingroup$


$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$



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?










share|cite|improve this question











$endgroup$








  • 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














1












1








1





$begingroup$


$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$



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?










share|cite|improve this question











$endgroup$




$exp(x)=sum_{n=0}^{infty}frac{1}{n!}x^n$



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






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share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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














  • 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










3 Answers
3






active

oldest

votes


















2












$begingroup$

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.






share|cite|improve this answer









$endgroup$





















    4












    $begingroup$

    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.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      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



















    0












    $begingroup$

    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.






    share|cite|improve this answer









    $endgroup$













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      3 Answers
      3






      active

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      3 Answers
      3






      active

      oldest

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      active

      oldest

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      active

      oldest

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      2












      $begingroup$

      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.






      share|cite|improve this answer









      $endgroup$


















        2












        $begingroup$

        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.






        share|cite|improve this answer









        $endgroup$
















          2












          2








          2





          $begingroup$

          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.






          share|cite|improve this answer









          $endgroup$



          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.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 12 at 19:59









          MaxMax

          586




          586























              4












              $begingroup$

              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.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                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
















              4












              $begingroup$

              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.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                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














              4












              4








              4





              $begingroup$

              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.






              share|cite|improve this answer









              $endgroup$



              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.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              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


















              • $begingroup$
                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











              0












              $begingroup$

              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.






              share|cite|improve this answer









              $endgroup$


















                0












                $begingroup$

                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.






                share|cite|improve this answer









                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  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.






                  share|cite|improve this answer









                  $endgroup$



                  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.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Jan 12 at 20:22









                  AaronAaron

                  1,912415




                  1,912415






























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