611

611 = 13 x 47.

611 is a brilliant number. It is a semiprime (product of two primes) whose prime factors have the same number of decimal digits. These numbers are generally used for cryptographic purposes and for testing the performance of prime factoring programs.

611 is a number n such that n times n + 5 gives the concatenation of a number m and itself: 611 x 616 = 376376.

611 is a number with no zeroes that becomes a prime when any one digit is deleted: 61 and 11 are both primes.

For customers of some telephone companies in Canada and in the United States, 611 is the abbreviated dialing telephone number used to report a problem with telephone service, or with a payphone.

Source: The On-Line Encyclopedia of Integer Sequences