The Answer is an Emirp

November 13, 2019


Photographer: Greg Parker
Summary Authors: Jim Foster; Greg Parker

Which double-digit prime number will evenly divide into 111, 222, 333, 444, 555, 666, 777, 888, and 999? If you zoom in on the little open cluster above (click on the image to enlarge), in the center of this field you'll discover the answer. This is the 37 Cluster. This curious shaped, open cluster (NGC 2169) is found approximately 3,600 light-years away in the direction of Orion the Hunter. Not only is it a prime number but it's an emirp (prime spelled backward). With emirps, when the digits are reversed the resulting number is still a prime. By the way, if today’s date is written as 11/13/19, only 13 is an emirp; 11 is a palindromic prime and 19 an ordinary prime. Today's date, November 13, is also an emirp. Note that 2019 isn't a prime number since it can be divided evenly by 3 and also by 673. 

Image acquired in early January 2009 by Greg Parker at the New Forest Observatory (U.K) and processed by Noel Carboni in Florida (U.S.A.).

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The Answer is a Prime Number