Double factorial and primorial

[Joerg Arndt]

* Niels Möller <nisse at lysator.liu.se> [Dec 20. 2011 16:05]:
> Torbjorn Granlund <tg at gmplib.org> writes:
> 
> > then one could sort the indexes using a minheap.
> 
> Clever!
> 

A bit of a warning: I once used a heap to do essentially the same
thing, it turned that using a simple array was significantly faster
(factor 20 or so).  The array definitely needs to fit into level-1
cache.