I don't remember the name of the book unfortunately.
Great Simpsons episode where Homer is in an insane asylum:
Homer: What's his problem?
Michael Jackson:He's an idiot savant. he can multply any two numbes together.
Homer: (To savant) 9 times 5?
Savant: (Without skipping a beat) 45
Homer: Wow!
I've been doing a lot of Plains Indian beading lately, which involves a lot of counting out of teeny-tiny beads (Czech No. 11s). You use the same number of beads over and over again -- just in different color combinations. Mostly odd numbers. This particular pattern required 7s and 9s and 11s.
After I had beaded on this project for a couple of days, I started "seeing" 7s and 9s and 11s just by looking -- didn't have to count any more. I checked myself for awhile, but just quit counting because it was so much faster to "see" the numbers.
I guess these people are talented (or practiced) enough to "see" really big numbers, and primes, and stuff like that.