Bounds

[Greenbank]

We need to keep:
1) The lowest factor found for a k n pair
2) the first factor found for a k n pair

And I would like to keep:
3) The factors with the lowest B1/B2 for a k n pair.

By looking at as many factors as possible I want to get an idea of the B1:B2 values required to find factors by p-1.

We've sieved everything up to 48 bits for some n ranges so we can use that info to work out what would be best for p-1 factoring.