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It is a prime number, a Sophie Germain prime and the sum of three consecutive primes (293 + 307 + 311). It is an Eisenstein prime with no imaginary part and real part of the form 3n − 1. Since 913 is a semiprime, 911 is a Chen prime. It is also a centered decagonal number.
There are 911 inverse semigroups of order 7 (sequence A001428 in OEIS)
As "911" or "9/11", typically pronounced "nine-eleven", it is commonly used to refer the calendar dates November 9 or September 11, depending on which date notation is used. The latter usage most commonly refers to the September 11, 2001 attacks in the United States.
There were 911 days between the September 11, 2001 attacks and the March 11, 2004 Madrid train bombings, but counterterrorism experts disagree as to whether the date was chosen for that reason.