Dougy

I am really surprised that there are no "small" octos. The way I have defined them means that negative numbers, created through the 2^n-k calculation, rule that number out, so your interesting case has to remain as that.

But thank you for looking at the small case. I just find the result hard to believe, but the negative rule counts out a lot for small n, especially when k goes in multiples of 15 (almost 2^4), so maybe I should have realised.

Maybe you should post the full decimal value of this find to Chris Caldwell's Prime curios page:

http://primes.utm.edu/curios/
Regards

Robert Smith