There is a standard notation for number systems a little larger than 10 of using A, B, etc. for the additional digits. You see this most often in hexadecimal notation. But we wouldn't expect a crank to know that  that's one of the signs of a crank.
What the crank has found is a simple consequence of the fact that after the first few, all primes are 6k+1 or 6k+5. To see why this is so, calculate the sequence 2^n mod 144. It repeats at length 6 (64, 128, 112, 80, 16, 32, 64, ...). Only the cases corresponding to 128 and 32 can be prime (6k+1 and 6k+5), and these correspond to his two endings.
Last fiddled with by wblipp on 20070727 at 02:58
Reason: Fixed an error in my spreadsheet
