# Irrational questions!...

• Jul 28th 2005, 12:40 AM
Rebesques
Irrational questions!...
We know that $\displaystyle e^x$ is continuous, and so must take rational values. For which $\displaystyle x$, is $\displaystyle e^x$ rational/irrational?? :confused: :o :eek:

(-> Thank/Blame MathGuru, for triggering this question. :cool: )
• Jul 28th 2005, 05:39 PM
WangTaoSG
x=ln(y)
x=ln(y), where y is rational, exp(x) is rational; where y is irrational, exp(x) is irrational
• Jul 28th 2005, 06:59 PM
hpe
Quote:

Originally Posted by Rebesques
We know that $\displaystyle e^x$ is continuous, and so must take rational values. For which $\displaystyle x$, is $\displaystyle e^x$ rational/irrational??

If $\displaystyle x$ is rational, then $\displaystyle e^x$ is not rational, except of course $\displaystyle x = 0$. A proof can be found on PlanetMath.
• Jul 29th 2005, 02:12 AM
Rebesques
Τhanks for the answers :)

I 'll now collect distinct proofs about the irrationality of $\displaystyle e^x$, when $\displaystyle x$ is rational. Here 's a second one, based on the transcedental nature of e, tell me please if it is right:
Let $\displaystyle x=\frac{p}{q}, \ p,q$ a natural and an integer respectively. If $\displaystyle e^{\frac{p}{q}}$ were rational, so would be $\displaystyle (e^{\frac{p}{q}})^q=e^p.$ This implies that the polynomial $\displaystyle z^p-e^p$, has a zero at z=e. I have only a basic knowledge of Galois Theory, but recognize the contradiction here :cool:

Now, what about irrational $\displaystyle x$ and rational $\displaystyle e^x$ ?? :confused:
• Jul 29th 2005, 02:55 AM
ticbol
WangTaoSG beat me to the punch so I chose not show my answer anymore. But your latest/added question made me show my solution anyway. But it is still basically that of WangTaoSG.

"Now, what about irrational x and rational e^x ??"

Why, is logarithm of a rational number not irrational?
Is ln(3), say, not irrational enough?

e^x = (rational number)
Take the natural log of both sides,
x*ln(e) = ln(rational number)
x = ln(rational number) -----------***

Again, is ln(rational number) not irrational enough?
• Jul 29th 2005, 03:42 AM
Rebesques
not quite...
Ticbol, ln2 for example is irrational, and so are -I guess- most numbers we can just think of.
• Jul 29th 2005, 03:58 AM
ticbol
Yes, ln of rational numbers are irrational, so WangTaoSG already answered your added question. Or, the answer was already there.
• Jul 29th 2005, 05:55 AM
Rebesques
Quote:

x=ln(y), where y is rational, exp(x) is rational;

This does hold true, but it's only trickery :) (1)

I asked if we could determine forehand, which x provide rational/irrational e^x, and the irrationality has been quite dealt with.

(1) Addendum to this message: Say I 'd like to know if e^x is rational. With this reasoning, then, I would have to solve x=lny for y; Which means that I would have (again) to compute y=e^x, and see if it this is rational... This is why I called it a trick, 'cause it answers, but not unappealably :)
• Jul 30th 2005, 09:44 PM
ticbol
Yes, WangTaoSG just gave answers without explanations/solutions. He belongs to the group who thinks there is more prestige in giving only answers without enough explanations. I see that assumption a lot on Mathheads. Looks like, for them, it is demeaning if they show how they arrived at the answers. Maybe, for them, without explanations will maintain the "mystery" of their "mastery" of Math. Something for the students/lesser mortals to view these Mathheads in awe. ;-)

I don't belong to his group. In fact, even if I can get the answer my way but I cannot explain this way so that it will be understood by the seeker/poster of the question, then I will rather not give the answer.
It is more challenging, and rewarding, for me to show a good explanation/solution for the answer than in finding the answer. It is easier to just find the answer.

But then, hey, to each his own. Nobody is forced to copy anybody's style.
• Jul 30th 2005, 11:54 PM
Rebesques
Quote:

He belongs to the group who thinks there is more prestige in giving only answers without enough explanations. I see that assumption a lot on Mathheads. Looks like, for them, it is demeaning if they show how they arrived at the answers. Maybe, for them, without explanations will maintain the "mystery" of their "mastery" of Math. Something for the students/lesser mortals to view these Mathheads in awe. ;-)

You are kinda mean here :)

Those men, the mathheads, have given serious effort and precious time, to knowing math deeply. Their experience points out a way, when we just stand there gazing :( No need to be mad about it, maybe one day all math on this forum will seem elementary to us too.

...See ya! I 'm off for two weeks of sea and sun (...and bikinis hopefully :p )
• Jul 31st 2005, 12:38 AM
ticbol
Oh no, I am not mad at the Mathheads. Only at their attitudes towards us lesser mortals when it comes to Math. Like they saw/studied the Fountain of Knowledge on Math, and we did not, so we give them excuses when they don't explain Math at our level of understanding. Either we understand what they like to show us or they care not. If we don't get their lambdas and epsilons and phis, so much the better for them.

Most of them only, though. A few of them are still human enough.

Math, for me, will never be elementary. Math, in any of its branches, from lower than Arithmetic, (?), to whatever that I understand, will always be interesting and challenging. I get the same pleasure from any of the branches of Math that I understand. So I treat these equally.
If I don't practice, I will forget even pre-Algebra in no time.
That is why I join, and enjoy, Math forums like this one.