# How would you explain why "e" is important? (And when it applies?) [duplicate]

Possible Duplicate:
Intuitive Understanding of the constant “e”

Let's say you want to explain this to your teenage son. I understand the technical definition of $e$

$$e=\lim_{n\to\infty}\left( 1 + \frac1n \right)^n$$

But, I don't want to get lost in technical babble. While specific examples are welcome, I also want to understand the big picture. I want to first know the general significance of why and when e is used. Anyone got a link? Is there a typical "pattern" that it models? In what general sorts of situations does "e" arise?

I only know e from the classic continuous compounding example. But, why does it appear in other applications of growth, science, etc? Are all these examples just variations on this same limit that defines e? This increasingly smaller interval when you apply an infinitely smaller percentage growth (1 + 1/n) but infinitely many times (to the power of n)

– user856
Jan 8, 2013 at 0:41
• Try talking for an hour with someone without using any word containing the letter "e." Just about impossible. Jan 8, 2013 at 0:55
• @WillJagy ... One word: Gadsby. Jan 8, 2013 at 1:59
• @AvatarOfChronos, good, good. The subtitle should have been "Fiction book using only abcdfghijklmnopqrstuvwxyz" Jan 8, 2013 at 2:08

There is also a difference between an interest in understanding

• the mathematical constant "$e$", which is an irrational, transcendental number, but real number, just as is $\pi$, but a number, nonetheless, which happens to be the value of your limit, and happens to be the value of the infinite sum $$e = \displaystyle \sum_{n=0}^\infty \frac{1}{n!}$$And more: $$e^x = \sum_{n=0}^\infty{x^n \over n!} = 1 + x + {x^2\over2!}+{x^3\over3!}+{x^4\over4!}+\cdots$$ vs.

• exponential functions (base $e$: (e.g., $f(x) = e^x$, or $e^{i\theta} = \cos \theta + i\sin \theta$), functions to which you seem to refer when you speak of its use in representing exponential growth, and other applications which crop up everywhere, so it seems. Certainly, functions involving $e$ tell us something about $e$, but they tell us so much more than that.

That is, the importance of $e$ itself isn't so much for its significance as a particular real number, but its significance as a base for exponential functions, and in terms of the ways that functions involving $e$ as a base appear in surprising ways, are powerful, and have applications in many domains.

So it's hard to know exactly what you are interested in: the number $e$, or the many functions involving $e$.

• I'm interested in both, I suppose Jan 8, 2013 at 14:14
• @amWhy: That's $e$erily another nice answer! :-) +1 May 8, 2013 at 2:36

Not an easy task (the teenage part).

Any system where the rate of change of a quantity is proportional to the amount of the quantity has solutions involving exponentials. This is true for continuous as well as discrete systems. Chemical reactions, electronic circuit behavior, physical systems, etc, are often well approximated by such systems.

If you differentiate an exponential function, you get itself times a constant: $$\frac{d}{dx} 2^x = \left(\text{constant}\cdot 2^x\right).$$ In other words, the function grows at a rate proportional to its present size.

Only if the base of the exponential function is $e$ is the "constant" equal to $1$, so that you get $$\frac{d}{dx} e^x = 1\cdot e^x.$$ In other words, the function grows at a rate equal to its present size.

It's the same as the reason why radians are used in calculus. You have $$\frac{d}{dx} \sin x = (\text{constant}\cdot\cos x).$$ If you use degrees, the "constant" is $\pi/180$. If you use radians, the "constant" is $1$, but only if you use radians.

There's more to the story than that. For example, how does the Poisson distribution arise as the limit of binomial distributions? But the above should show you in what sense $e$ is "natural", and in what sense radians are "natural".

• Someone edited this to say $180/\pi$ instead of $\pi/180,$ and I changed it back. Did I miss something? $$\frac d {dx} \text{sine of x degrees} = \frac d{dx} \text{sine of } \frac{\pi x}{180} \text{ radians} = \left( \text{cosine of } \frac{\pi x}{180} \text{ radians} \right) \times \frac \pi {180}$$ The constant by which the cosine is multiplied at the end is $\pi/180. \qquad$ May 25, 2017 at 15:42

Considering functions of the form $f(x)=a\,b^x$ with $b>0$, we will always have an initial value of $a$. Only with the base $b=e$ will we also have an initial rate-of-change equal to $a$.

Demonstrate this with pictures: (This image is from lecture notes I am coauthoring.) Even better would be a GeoGebra applet modeled after images like these where $a$ and $b$ are manipulable.

Let $a > 0$ and consider the exponential function $y = a^x$. Now draw the tangent line to the graph at the $y$-axis. If $a < 1$ this line has negative slope; if $a > 1$ the line has positive slope. There is a unique value of $a$ which gives this tangent line slope 1. That value is $a = e$.

You can derive all of the goodies about the exponential function right from this principle.

$e$ appears in a lot of places in calculus, but before calculus, it has little significance.

• Limits appear in pre-calculus. Compound interest is a valid topic for a pre-calculus course. One of the numbers that appears when studying limits and compound interest is $e$.
– robjohn
Jan 8, 2013 at 1:05