20543

This number is a prime.

The ZIP Code of the U.S. Supreme Court (1 First Street, N.E., Washington, D.C. 20543).

20543 is the smallest prime that generates three more primes by reiterative use of the following rule: next integer is equal to the sum of each digit powered to that digit, from the previous integer. The sequence goes this way: 20543, 3413, 311, 29. 0^0 = 1. No larger sequence of this type is known. [Rivera]