8k + 7


The following table demonstrates the form of the numbers required to be factors of a 2p-1 number. The table represents all 8k+a multiplication mods.

a01234567
000000000
101234567
202460246
303614725
404040404
505274163
606420642
707654321
We can determine from this table that the factors (not necessarily prime) for 2p-1 equal
(8k+1)(8k+7) or
(8k+3)(8k+5)

Well this is as far as I have gotten!