dsouza123,

After some experimenting, I don't think the first 5 digits will help reduce the search.

Using the first 10 digits of RSA-640, we make a real number, like this: 310741.8240

then look for pairs of smaller reals, like this:

Code:

310.75 999.97 500.00 621.48 550.00 564.98
310.76 999.94 500.01 621.47 550.01 564.97
310.77 999.90 500.02 621.45 550.02 564.96
310.78 999.87 500.03 621.44 550.03 564.95
310.79 999.84 500.04 621.43 550.04 564.94

Because each of the smaller numbers has 90 digits trailing it, we can't treat them like 5 digit integers. So given 'a', c = a/b has a solution for any b.

We can only use the first 5 digits to finish constructing a factor pair after we find them using the last 5 digits.

But you have success, because we are thinking about it!