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I am trying to prove $\exp(x+y) = \exp(x) \exp(y)$.

I may use that $$\exp(x) = \sum_{n=0}^\infty \frac {x^n}{n!}$$ I further know how to multiply two power series in one point, i.e. if $f(x) = \sum_{n=0}^\infty c_n(x-a)^n$ and $g(x) = \sum_{k=0}^\infty d_n(x-a)^n$ then $$ f(x)g(x) = \sum_{n=0}^\infty e_n(x-a)^n $$ with $$ e_n = \sum_{m=0}^n c_md_{n-m} $$

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What's the question? –  nbubis Jan 23 '13 at 18:54
    
Presumably you tried to multiply the power series for $\exp(x)$ and $\exp(y)$. What did you get stuck on? –  rschwieb Jan 23 '13 at 18:54
    
The title is the question @nbubis. This question is from Tao Analysis II and he gives the hint with the multiplication of power series. I dont see how to apply that hint. –  André Jan 23 '13 at 18:57
    
Ok I got it :) Can I delete this question ? –  André Jan 23 '13 at 19:00
    
This must be a duplicate... –  Fabian Jan 23 '13 at 19:02

4 Answers 4

up vote 10 down vote accepted

\begin{align} \exp(x+y)&=\sum_n\frac{(x+y)^n}{n!} \\\\ &=\sum_{n}\frac{1}{n!}\sum_{a+b=n} {n \choose a} x^ay^b \\\\ &= \sum_{n}\frac{n!}{n!}\sum_{a+b=n}\frac{x^a}{a!}\frac{y^b}{b!} \\\\ &= \sum_{a,b} \frac{x^a}{a!}\frac{y^b}{b!} \\\\ &= \exp(x)\cdot\exp(y) \end{align}

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You can write faster than me :D –  André Jan 23 '13 at 19:16
    
You overcomplicated things in your solution. There is no need to work with a generating function as you have done. Also, you left out some steps in your first equality when going from a product of two generating functions into a single generating function. –  pre-kidney Jan 23 '13 at 19:17
    
I gave the legitimation for that in my question. –  André Jan 23 '13 at 19:19
    
Oh, I see now. Thanks for pointing that out :) –  pre-kidney Jan 23 '13 at 19:20
    
you have used this equality: $$\bigsqcup _n\{(a,b) \mid a+b=n\}=\{(a,b)\mid \}$$ and absolute convergence. –  user59671 May 9 '13 at 11:05

My solution

Let $x,y \in \mathbb R$ and

$f(z) := \sum_{n=0}^\infty \left(\frac {x^n}{n!} \right )z^n$ and $g(z) := \sum_{n=0}^\infty \left(\frac {y^n}{n!} \right )z^n$. Then $\exp(x) \exp(y) = f(1)g(1)$. That is $$ f(z)g(z) = \sum_{n=0}^\infty \left( \sum_{k=0}^m \frac {x^m y^{n-m}}{m! (n-m)!} \right)z^n $$ $$ = \sum_{n=0}^\infty \frac 1 {n!} (x+y)^n z^n $$ thus $f(1)g(1) = \exp(x+y)$.

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$A(t)=\exp(x) = \sum_{n=0}^\infty \frac {x^n}{n!}t^n$
$B(t)=\exp(y) = \sum_{n=0}^\infty \frac {y^n}{n!}t^n$
$C(t) = A(t)*B(t)=\sum_{n=0}^\infty (\sum_{k+z=n}^\ \frac {x^k}{k!}*\frac {y^z}{z!})t^n=\sum_{n=0}^\infty \frac {(x+y)^n}{n!}t^n=exp(x+y)$

and use $t=1$

sry i was too late^^

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Duplicate of the two answers ^^ –  André Jan 23 '13 at 19:23
    
Actually, both you and @nordmann use an extraneous generating function [in the variables $t$,$z$ respectively]. The reason why I say "extraneous" is because if you look at my solution above, no new variable need be introduced. –  pre-kidney Jan 23 '13 at 19:43
    
sry im new to this latex stuff^^ –  nordmann Jan 23 '13 at 23:16

This can actually be done without writing a single sum. Consider the function $$ f(x, y) = \frac{e^x e^y}{e^{x+y}}. $$ Observe that $$ \frac{\partial f}{\partial x} = \frac{e^x e^y e^{x+y} - e^x e^y e^{x+y}}{(e^{x+y})^2} = 0. $$ Similarly, $$ \frac{\partial f}{\partial y} = \frac{e^x e^y e^{x+y} - e^x e^y e^{x+y}}{(e^{x+y})^2} = 0. $$ This shows that $f$ is a constant function. Now, we need only to use the series definition to show $f(0, 0) = 1$. Then, by rearrangement, we have the desired result: $$ e^{x+y} = e^x e^y. $$

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