911 (nine hundred [and] eleven) is the integer following 910 and preceding 912. 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 the OEIS) 911 is obtained by concatenating its sum of digits and product of digits. 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 North American emergency telephone number, see 9-1-1, or the September 11, 2001 attacks in the United States. "911" is also the model number of the iconic Porsche 911 sports car which has been in production from September 1964 to the present.