Carmichael number — the mathematical impostor

1105 is a famous Carmichael number — composite, yet passing Fermat's primality test. 1105 = 5×13×17. The mathematical 'impostor': looks prime, but isn't.

In 1105, the Crusader Kingdom of Jerusalem under Baldwin I was expanding its territories — the first European state in the Middle East experiencing a period of construction.

1+1+0+5=7. Seven-wisdom in a number that 'pretends' to be prime. 1105 knows more than it shows — like a sage concealing complexity behind a simple face.