609

609 = 3 x 7 x 29.

100⁶⁰⁹ - 609 is a prime number.

609 is a strobogrammatic number. It appears the same whether viewed normally or upside down.

609 is a number n such that n² + 2 is a prime: 609² + 2 = 370883, a prime.

The sum of the digits of 609 is five times the number of digits: 6 + 0 + 9 = 15 = 5 x 3.

609 is a term that arises in the following sequence: Start with 1 and repeatedly reverse the digits and add 5 to get the next term.

609 is the telephone area code for Trenton, New Jersey.

Source: Prime Curios!