# Most beautiful definition?

by tgt
Tags: beautiful, definition
 P: 468 What's the most beautiful definition you've ever seen? For me, it has to be the definition of a free basis in group theory.
 HW Helper P: 3,531 Perpendicular distance from a point to a line in coordinate geometry I was the only one in my class to appreciate the formula though.
 P: 21 Well call me premature if you will, but I reckon it is: e is a number such that: d/dx (e^x) = e^x I mean so much can be drawn from this...
P: 468
Most beautiful definition?

 Quote by prasannaworld Well call me premature if you will, but I reckon it is: e is a number such that: d/dx (e^x) = e^x I mean so much can be drawn from this...
Is that a definition?
P: 21
 Quote by tgt Is that a definition?
YES IT IS!

Give me one "Definition" that boils to this one...

Using this definition one can derive the Maclaurin Series for e...

Using this definition one can use l'Hopital's Rule to derive:
e = lim (1+1/n)^n
x->inf

And by defining ln(x) to be the inverse function of e^x (i.e. Logarithm base e), one can go further and get Integral of ln(x) is 1/x - which some claim to be the first definition...
P: 124
 Quote by prasannaworld Give me one "Definition" that boils to this one...
you could also define e to be e = lim (1+1/n)^n, x->inf and then derive the other results, which I think is a more common definition.

Ontopic: I dont get how a definition can be beautiful? Sure, a proof or a theorem can be elegant, but what is a "beautiful" definition? :o
PF Gold
P: 3,189
 Well call me premature if you will, but I reckon it is: e is a number such that: d/dx (e^x) = e^x I mean so much can be drawn from this...
The number 0 also satisfies the condition above, so e is not unique in that case. (well if you consider $$x\neq 0$$.)
P: 21
 Quote by fluidistic The number 0 also satisfies the condition above, so e is not unique in that case. (well if you consider $$x\neq 0$$.)
True... I still view that as the standard definition. To make it better how about: xER; obviously 0 can no longer work.

Also on topic: I believe a "beautiful" definition in simple refers to one that is simple but a lot can be done with it/derived from it...
 HW Helper P: 1,021 It still doesn't define e^x uniquely, because any c.e^x with c in R is good too. You can define f(x) = e^x as the function satisfying f(x)' = f(x) and f(0) = 1.
HW Helper
P: 3,684
I think the most beautiful definition for me (simple though it is!) would be Gauss' definition of congruence classes mod m.

 Quote by TD It still doesn't define e^x uniquely, because any c.e^x with c in R is good too. You can define f(x) = e^x as the function satisfying f(x)' = f(x) and f(0) = 1.
I think the intent was to define e, not e^x. e is the unique positive solution of a^x = d/dx a^x.
 HW Helper P: 1,021 Oh of course, I misread!
 P: 308 Beautiful is the way that you write down the definition in SYMBOLS and not the definition itself
 PF Gold P: 3,189 $$\vec{L}=\vec{r}\times \vec{p}$$ definition of angular momentum.
P: 468
 Quote by Kurret but what is a "beautiful" definition? :o
Just like a beautiful proof. When it has a lot in it (i.e get something out of it every time you think about it) and gets to your heart.
P: 124
 Quote by tgt Just like a beautiful proof. When it has a lot in it (i.e get something out of it every time you think about it) and gets to your heart.
I think a beautiful proof is totally different. Imo a beautiful proof is one that has some elegant and creative "trick" in it, that usually makes the proof short without a lot of messy computation, and usually makes you think "how did he think of that?"...

Altough, when thinking about it, the construction of the real numbers with dedekind cuts is imo very cool and elegant, so that would maybe qualify as a beautiful definition for me..