911

911 is a prime number. It is a Sophie Germain prime because 2 x 911 + 1 = 1823 is a prime.

911 is both deficient and odious. The sum of all its positive divisors except itself is less than 911 (deficient), and its binary expansion has an odd number of 1s (odious).

Each term of a Fibonacci-like sequence is the sum of the previous two terms. If you start with 9 and 11, the 11th term in the sequence is 911.

911 is the sum of three consecutive primes: 911 = 293 + 307 + 311.

911 is a number that cannot be written as a sum of three squares.

911 is the emergency telephone number in the United States.

Source: Number Gossip