Why is 2/3 not seen as rational? [was "plotting even function"]

[John B. Thoo]

On March 19, 2005 4:20:04 PM PST I wrote:

> Hi.  I hope that I'm not embarrassing myself by asking the following.
>
> I believe that
>
>        3       2/3    1
>   y = --- (2 x)    + ---
>        4              2
>
> is an even function

And on Mar 19, 2005, at 12:36 PM, Thomas Shores replied:

> Here's the problem: how do you know that the exponent is a
> rational fraction, so that you can reinterpret it?  Octave
> rightfully doesn't.

I don't understand why  2/3  is not seen as rational.  At first I 
thought it may be because division by  3  results in a nonterminating 
decimal, but then I found that plotting  x^(2/5)  gives the same 
(formerly) unexpected result as plotting  x^(2/3)  (not symmetrical 
about the y-axix).  So, why is  m/n  not seen as rational?

TIA.

---John.
(a numerical computation and Octave novice)



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