Number Gossip
(Enter a number and I'll tell you everything you wanted to know about it but were afraid to ask.)
Rare Properties of 6601
The composite integer n is a Carmichael number if bn-1 = 1 (mod n) for every integer b which is relatively prime with n.
Carmichael numbers behave like prime numbers with respect to the most useful primality test, that is they pretend to be prime.
Common Properties of 6601
To build the lucky number sequence, start with natural numbers. Delete every second number, leaving 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, ... . The second number remaining is 3, so delete every third number, leaving 1, 3, 7, 9, 13, 15, 19, 21, ... . The next number remaining is 7, so delete every 7th number, leaving 1, 3, 7, 9, 13, 15, 21, ... . The next number remaining is 9, so delete every ninth number, etc.
Those numbers were lucky they weren't crossed out.