99999989
This number is a prime.
 
According to Oliver Sacks, no practically realizable algorithm has yet been invented for rapidly identifying
primes in excess of 99999989 as apparently performed by the autistic twins that he observed. 
| The largest 8-digit prime is a member of a set of four primes, each of which has a maximum sum of digits. | 
89999999 99899999 99998999 99999989  | 
 
The largest prime serial number on a U.S. Bill. It is currently in the possession of Dave Undis of CoolSerialNumbers.com. 
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