Try setting domain to complex:
(%i1) domain : complex$
(%i2) (x^2)^(1/3);
(%o2) (x^2)^(1/3)
(%i3) ((-1)^2)^(1/3);
(%o3) 1
(%i4) (-1)^(2/3);
(%o4) (-1)^(2/3)
(%i5) rectform(%);
(%o5) (sqrt(3)*%i)/2-1/2
Barton
-----maxima-bounces at math.utexas.edu wrote: -----
>To: maxima <maxima at math.utexas.edu>
>From: "Mike Hansen" <mhansen at gmail.com>
>Sent by: maxima-bounces at math.utexas.edu
>Date: 12/10/2007 05:49PM
>Subject: Maxima incorrectly simplifies powers
>
>Hello,
>
>The following issue was reported to Sage. Maxima currently simplifies
>(x^2)^(1/3) to x^(2/3) which is not correct for all values of x. For
>example, ((-1)^2)^(1/3) correctly gives 1, but (-1)^(2/3) also gives 1
>when it should give -1/2+sqrt(3)/2*I .
>
>--Mike
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