Showing that e^a * e^b = e^(a+b)

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Homework Help Overview

The discussion revolves around proving the relationship \( e^a \cdot e^b = e^{a+b} \) using the definition of the exponential function as a power series. Participants are exploring the manipulation of infinite series to establish this equality.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to manipulate the series definitions of \( e^{z_1} \) and \( e^{z_2} \) to show their product equals the series for \( e^{(z_1 + z_2)} \). Some participants question the validity of multiplying infinite series directly and provide examples to illustrate potential pitfalls. Others express uncertainty about the completeness of their approach and whether expanding the series sufficiently demonstrates the equality.

Discussion Status

The discussion is active, with participants providing feedback on the original poster's attempts. There is recognition of the challenges in manipulating series, and some guidance is offered regarding the need for careful handling of infinite series. Multiple interpretations of how to approach the problem are being explored.

Contextual Notes

Participants note a lack of pure mathematical background, which may affect their confidence in determining what constitutes a complete proof. There is also mention of potential grading criteria related to the expansion of series terms.

dacruick
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Homework Statement


By definition, e^{z} = \sum\frac{1}{n!}z^{n}

Use this to show the relationship in the question title


The Attempt at a Solution


Well, what I've tried to do is as follows:
e^{z_{1}}e^{z_{2}} = \sum(z_{1}z_{2})^{n}/{n!^{2}}

And set that equal to
e^{(z_{1}+z_{2})} = \sum\frac{1}{n!}(z_{1}+z_{2})^{n}

What I'm left with is this expression that
(z_{1}+z_{2})^{n} = (z_{1}z_{2})^{n}/n!

those are all sums from n=0 to n=∞ still btw. Sorry for my LameTex inexperience.

My question is, is there any way that I can show that those two are equal? Or have I gone about this all wrong. Thanks!

dacruick
 
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You can't multiply infinite series like you've done. In fact you can't even multiply finite polynomials like that. For example consider the multiplication of the following quadratics:

(x^2 + x + 1)(y^2 + y + 1) = (xy)^2 + x^2y + xy^2 + x^2 + y^2 + x + y + 1 \neq (xy)^2 + xy + 1
 
Poopsilon said:
You can't multiply infinite series like you've done. In fact you can't even multiply finite polynomials like that. For example consider the multiplication of the following quadratics:

(x^2 + x + 1)(y^2 + y + 1) = (xy)^2 + x^2y + xy^2 + x^2 + y^2 + x + y + 1 \neq (xy)^2 + xy + 1

I'm so stupid...Haha, not just because of the foolishness you stated but because as soon as you said that I'm pretty sure I figured out how to do it. Maybe it had something to do with that cup of coffee I just made. You'll have to tussle with Starbucks for partial credit of my pending success.
 
Starbucks is far too successful already, I want all of it.
 
Haha get your lawyer ready then :P.

So I have the answer, I've expanded both of the series for n = 0, 1, 2, and 3 and I can sufficiently see that the terms will cancel out on both sides.

However, I question the completeness of my answer. I assume that the only way to get marks for this badboy is to expand both of the series' in terms of 'n'. I don't readily see how to do this. Would it work if I just multiplied each of the first 4 values of 'n' by the last 2 terms of each series, and then see if those cancelled?

I have next to no pure mathematical background, and I'm unclear as to what constitutes "showing" that these two series are equal.
 

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