Kevin Wampler <wam### [at] uwashingtonedu> wrote:
> triple_r wrote:
> > http://projecteuler.net/index.php?section=problems&id=160
> >
> > I spent a lot of time on this, but was never able to get the right answer.  You
> > basically need a closed-form solution since you can't even iterate a simple
> > model that far in a reasonable amount of time.
>
> My guess would be that the solution isn't closed form but can be solved
> for in time which is logarithmic (or some other o(n) function) in n.
> Looks fun though (and tricky)!

Yes, I think you're right.  I should be more careful with what I mean by 'closed
form.'  I did feel better when I cheated and looked at the answer though. (#160)

http://www.haskell.org/haskellwiki/Euler_problems/151_to_160

I guess I just neglected to remember that the group of invertible elements of
the ring of integers modulo 10^d is isomorphic to the product of a cyclic group
of order 2 and another cyclic group.

It was so obvious!

 - Ricky

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