A019294(461)

[Don Reble]

Lambert Klasen wrote:
> any hint for stupid me, what kind of program you use,
> to compute this so fast ?

I find that there's usually little factoring to do. When you compute
Y=sigma(X) as a product of numbers, keep all of those numbers; and
at the next step, factor them instead of Y.
Also, Y's partial products are all (b^k-1)/(b-1), and usually their
factors can be found at http://www.cerias.purdue.edu/homes/ssw/cun
(I didn't use that, and so my program spent many minutes refactoring
2^263-1.)

-- 
Don Reble  djr at nk.ca