a question

[Torbjorn Granlund]

Your message is a bit vague, but I'll try to rely as well as I can.

So, you're using twos complement bignums.  Is the n you mention fixed?
If not, how come you're interested in just n x n -> 2n multiply, it
seems you would need n x m -> mn multiply.

If n is just a word, there are some tricks of converting between signed
n x n -> 2n and unsigned dito.  See GMP's longlong.h, somewhere not to
far from the end.

If n is arbitrary, I think it might be best to multiply the absolute
values.

-- 
Torbjörn