Finding all and every continuous function $ mathbb{R}tomathbb{R} $ such that $ f(x+y)=f(x)cdot f(y) $...












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This question already has an answer here:




  • Is there a name for function with the exponential property $f(x+y)=f(x) cdot f(y)$?

    5 answers




How to find all and every continuous function $ f $ from $ mathbb{R} $ in $ mathbb{R} $ that verifies $$ f(x+y)=f(x)cdot f(y)quadforall x,yinmathbb{R} $$



All I could find is $ f(x)=fleft(frac{1}{-x}right) $... but I don't like that answer at all... any hints please??










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marked as duplicate by José Carlos Santos, Ross Millikan, Community Dec 2 '18 at 18:25


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • 3




    $begingroup$
    How about $e^x$?
    $endgroup$
    – lulu
    Dec 2 '18 at 15:41










  • $begingroup$
    Very logical I like it .. but what is the procedure to prove that?
    $endgroup$
    – Abderrazzak
    Dec 2 '18 at 15:45










  • $begingroup$
    @Abderrazzak The exponential $ exp_e:mathbb{R}tomathbb{R}_{>0} $ is usually defined as a monotonic function such that $ exp_e(x_1+x_2)=exp_e(x_1)exp_e(x_2) $ (and such that $ exp_e(1)=e $); at most you should check the (uniqueness and the) existence of such a function.
    $endgroup$
    – marco21
    Dec 2 '18 at 16:26
















0












$begingroup$



This question already has an answer here:




  • Is there a name for function with the exponential property $f(x+y)=f(x) cdot f(y)$?

    5 answers




How to find all and every continuous function $ f $ from $ mathbb{R} $ in $ mathbb{R} $ that verifies $$ f(x+y)=f(x)cdot f(y)quadforall x,yinmathbb{R} $$



All I could find is $ f(x)=fleft(frac{1}{-x}right) $... but I don't like that answer at all... any hints please??










share|cite|improve this question











$endgroup$



marked as duplicate by José Carlos Santos, Ross Millikan, Community Dec 2 '18 at 18:25


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • 3




    $begingroup$
    How about $e^x$?
    $endgroup$
    – lulu
    Dec 2 '18 at 15:41










  • $begingroup$
    Very logical I like it .. but what is the procedure to prove that?
    $endgroup$
    – Abderrazzak
    Dec 2 '18 at 15:45










  • $begingroup$
    @Abderrazzak The exponential $ exp_e:mathbb{R}tomathbb{R}_{>0} $ is usually defined as a monotonic function such that $ exp_e(x_1+x_2)=exp_e(x_1)exp_e(x_2) $ (and such that $ exp_e(1)=e $); at most you should check the (uniqueness and the) existence of such a function.
    $endgroup$
    – marco21
    Dec 2 '18 at 16:26














0












0








0





$begingroup$



This question already has an answer here:




  • Is there a name for function with the exponential property $f(x+y)=f(x) cdot f(y)$?

    5 answers




How to find all and every continuous function $ f $ from $ mathbb{R} $ in $ mathbb{R} $ that verifies $$ f(x+y)=f(x)cdot f(y)quadforall x,yinmathbb{R} $$



All I could find is $ f(x)=fleft(frac{1}{-x}right) $... but I don't like that answer at all... any hints please??










share|cite|improve this question











$endgroup$





This question already has an answer here:




  • Is there a name for function with the exponential property $f(x+y)=f(x) cdot f(y)$?

    5 answers




How to find all and every continuous function $ f $ from $ mathbb{R} $ in $ mathbb{R} $ that verifies $$ f(x+y)=f(x)cdot f(y)quadforall x,yinmathbb{R} $$



All I could find is $ f(x)=fleft(frac{1}{-x}right) $... but I don't like that answer at all... any hints please??





This question already has an answer here:




  • Is there a name for function with the exponential property $f(x+y)=f(x) cdot f(y)$?

    5 answers








analysis functions continuity






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edited Dec 2 '18 at 17:02









marco21

308211




308211










asked Dec 2 '18 at 15:40









AbderrazzakAbderrazzak

12




12




marked as duplicate by José Carlos Santos, Ross Millikan, Community Dec 2 '18 at 18:25


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.









marked as duplicate by José Carlos Santos, Ross Millikan, Community Dec 2 '18 at 18:25


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.










  • 3




    $begingroup$
    How about $e^x$?
    $endgroup$
    – lulu
    Dec 2 '18 at 15:41










  • $begingroup$
    Very logical I like it .. but what is the procedure to prove that?
    $endgroup$
    – Abderrazzak
    Dec 2 '18 at 15:45










  • $begingroup$
    @Abderrazzak The exponential $ exp_e:mathbb{R}tomathbb{R}_{>0} $ is usually defined as a monotonic function such that $ exp_e(x_1+x_2)=exp_e(x_1)exp_e(x_2) $ (and such that $ exp_e(1)=e $); at most you should check the (uniqueness and the) existence of such a function.
    $endgroup$
    – marco21
    Dec 2 '18 at 16:26














  • 3




    $begingroup$
    How about $e^x$?
    $endgroup$
    – lulu
    Dec 2 '18 at 15:41










  • $begingroup$
    Very logical I like it .. but what is the procedure to prove that?
    $endgroup$
    – Abderrazzak
    Dec 2 '18 at 15:45










  • $begingroup$
    @Abderrazzak The exponential $ exp_e:mathbb{R}tomathbb{R}_{>0} $ is usually defined as a monotonic function such that $ exp_e(x_1+x_2)=exp_e(x_1)exp_e(x_2) $ (and such that $ exp_e(1)=e $); at most you should check the (uniqueness and the) existence of such a function.
    $endgroup$
    – marco21
    Dec 2 '18 at 16:26








3




3




$begingroup$
How about $e^x$?
$endgroup$
– lulu
Dec 2 '18 at 15:41




$begingroup$
How about $e^x$?
$endgroup$
– lulu
Dec 2 '18 at 15:41












$begingroup$
Very logical I like it .. but what is the procedure to prove that?
$endgroup$
– Abderrazzak
Dec 2 '18 at 15:45




$begingroup$
Very logical I like it .. but what is the procedure to prove that?
$endgroup$
– Abderrazzak
Dec 2 '18 at 15:45












$begingroup$
@Abderrazzak The exponential $ exp_e:mathbb{R}tomathbb{R}_{>0} $ is usually defined as a monotonic function such that $ exp_e(x_1+x_2)=exp_e(x_1)exp_e(x_2) $ (and such that $ exp_e(1)=e $); at most you should check the (uniqueness and the) existence of such a function.
$endgroup$
– marco21
Dec 2 '18 at 16:26




$begingroup$
@Abderrazzak The exponential $ exp_e:mathbb{R}tomathbb{R}_{>0} $ is usually defined as a monotonic function such that $ exp_e(x_1+x_2)=exp_e(x_1)exp_e(x_2) $ (and such that $ exp_e(1)=e $); at most you should check the (uniqueness and the) existence of such a function.
$endgroup$
– marco21
Dec 2 '18 at 16:26










1 Answer
1






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This is not the complete answer, but it finds all differentiable functions $f:mathbb Rrightarrowmathbb R$ satisfying $f(x+y)=f(x)cdot f(y), forall x,yinmathbb R$.



Since $f$ is differentiable,



$f'(x)=lim_{hto0}frac{f(x+h)-f(x)}h=lim_{hto0}frac{f(x)cdot f(h)-f(x)}h=f(x)cdotlim_{hto0}frac{f(h)-1}h=f(x)cdot f'(0), forall xinmathbb R$



$implies f'(x)=f(x)cdot f'(0)$



Integrate this equation to obtain $f(x)=ke^{f'(0)x}$.



Since $f(x+y)=f(x)cdot f(y), k=k^2implies k=0,1$. Thus, the differentiable solutions of this functional equation are $f(x)=0, e^{kx}; kinmathbb R$.






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  • $begingroup$
    I understand this part ... but is this function unique .. if so how can I prove that ?
    $endgroup$
    – Abderrazzak
    Dec 6 '18 at 6:28


















1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









0












$begingroup$

This is not the complete answer, but it finds all differentiable functions $f:mathbb Rrightarrowmathbb R$ satisfying $f(x+y)=f(x)cdot f(y), forall x,yinmathbb R$.



Since $f$ is differentiable,



$f'(x)=lim_{hto0}frac{f(x+h)-f(x)}h=lim_{hto0}frac{f(x)cdot f(h)-f(x)}h=f(x)cdotlim_{hto0}frac{f(h)-1}h=f(x)cdot f'(0), forall xinmathbb R$



$implies f'(x)=f(x)cdot f'(0)$



Integrate this equation to obtain $f(x)=ke^{f'(0)x}$.



Since $f(x+y)=f(x)cdot f(y), k=k^2implies k=0,1$. Thus, the differentiable solutions of this functional equation are $f(x)=0, e^{kx}; kinmathbb R$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I understand this part ... but is this function unique .. if so how can I prove that ?
    $endgroup$
    – Abderrazzak
    Dec 6 '18 at 6:28
















0












$begingroup$

This is not the complete answer, but it finds all differentiable functions $f:mathbb Rrightarrowmathbb R$ satisfying $f(x+y)=f(x)cdot f(y), forall x,yinmathbb R$.



Since $f$ is differentiable,



$f'(x)=lim_{hto0}frac{f(x+h)-f(x)}h=lim_{hto0}frac{f(x)cdot f(h)-f(x)}h=f(x)cdotlim_{hto0}frac{f(h)-1}h=f(x)cdot f'(0), forall xinmathbb R$



$implies f'(x)=f(x)cdot f'(0)$



Integrate this equation to obtain $f(x)=ke^{f'(0)x}$.



Since $f(x+y)=f(x)cdot f(y), k=k^2implies k=0,1$. Thus, the differentiable solutions of this functional equation are $f(x)=0, e^{kx}; kinmathbb R$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I understand this part ... but is this function unique .. if so how can I prove that ?
    $endgroup$
    – Abderrazzak
    Dec 6 '18 at 6:28














0












0








0





$begingroup$

This is not the complete answer, but it finds all differentiable functions $f:mathbb Rrightarrowmathbb R$ satisfying $f(x+y)=f(x)cdot f(y), forall x,yinmathbb R$.



Since $f$ is differentiable,



$f'(x)=lim_{hto0}frac{f(x+h)-f(x)}h=lim_{hto0}frac{f(x)cdot f(h)-f(x)}h=f(x)cdotlim_{hto0}frac{f(h)-1}h=f(x)cdot f'(0), forall xinmathbb R$



$implies f'(x)=f(x)cdot f'(0)$



Integrate this equation to obtain $f(x)=ke^{f'(0)x}$.



Since $f(x+y)=f(x)cdot f(y), k=k^2implies k=0,1$. Thus, the differentiable solutions of this functional equation are $f(x)=0, e^{kx}; kinmathbb R$.






share|cite|improve this answer









$endgroup$



This is not the complete answer, but it finds all differentiable functions $f:mathbb Rrightarrowmathbb R$ satisfying $f(x+y)=f(x)cdot f(y), forall x,yinmathbb R$.



Since $f$ is differentiable,



$f'(x)=lim_{hto0}frac{f(x+h)-f(x)}h=lim_{hto0}frac{f(x)cdot f(h)-f(x)}h=f(x)cdotlim_{hto0}frac{f(h)-1}h=f(x)cdot f'(0), forall xinmathbb R$



$implies f'(x)=f(x)cdot f'(0)$



Integrate this equation to obtain $f(x)=ke^{f'(0)x}$.



Since $f(x+y)=f(x)cdot f(y), k=k^2implies k=0,1$. Thus, the differentiable solutions of this functional equation are $f(x)=0, e^{kx}; kinmathbb R$.







share|cite|improve this answer












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share|cite|improve this answer










answered Dec 2 '18 at 16:18









Shubham JohriShubham Johri

4,992717




4,992717












  • $begingroup$
    I understand this part ... but is this function unique .. if so how can I prove that ?
    $endgroup$
    – Abderrazzak
    Dec 6 '18 at 6:28


















  • $begingroup$
    I understand this part ... but is this function unique .. if so how can I prove that ?
    $endgroup$
    – Abderrazzak
    Dec 6 '18 at 6:28
















$begingroup$
I understand this part ... but is this function unique .. if so how can I prove that ?
$endgroup$
– Abderrazzak
Dec 6 '18 at 6:28




$begingroup$
I understand this part ... but is this function unique .. if so how can I prove that ?
$endgroup$
– Abderrazzak
Dec 6 '18 at 6:28



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