I'll toss in my 2 cents and perhaps say something either stupid or obvious.  
That is that if you have a number of say 1024 bits then you can compute the 
cube root in 1024/3 operations where each operation in z^3.  I do not know why 
you need the number and I do not know if this is an acceptable cost.  




On Sat, Aug 06, 2005 at 09:26:09PM -0400, Victor Duchovni wrote:
> On Sat, Aug 06, 2005 at 05:36:52PM -0700, Anirban Banerjee wrote:
> 
> >  Can someone please let me have a pointer to how I may obtain a cube root 
> > of 
> > a BIGNUM
> 
> Wrong question. BIGNUMs are for high precision *integer* arithmetic,
> often in a finite ring (e.g. Z/pqZ). In this context cube roots either
> don't exist for most inputs (Z) or are computationally infeasible (Z/nZ).
> 
> For numbers that do have cube roots in the integer case, you can use a
> half-interval search. In the discrete (Z/nZ) case you are on your own,
> there are no known effective algorithms.
> 
> What problem are you trying to solve?
> 
> -- 
>       Viktor.
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