BARS::23 Miles From Jct. 79 in Borrego Springs, California

This road sign is on the Christmas Circle roundabout in Borrego Springs, California.

The road sign says...

   +---------------------+
   | CA Hwy-79        23 |
   | County Airport    3 |
   | Salton Sea       30 |
   +---------------------+

Finding road signs that contain at least three prime numbers each greater than 10 is non-trivial. These are called "triple prime road signs."

Off-by-one: If the 30 was 31, then this road sign would have four prime numbers with three of them greater than 10 (i.e. it would be a triple prime road sign).

   the numbers are:  79  23  3  30

   off-by-one math:  (sqrt(79 - 30) - 3)! = 23 + 1

   observe...
      79 - 30 = 49
      sqrt(49) = 7
      7 - 3 = 4
      4! = 4 * 3 * 2 * 1 = 24

   forced triple primes:  79, 23,  13 = spf(30) + 3

   observe...
      spf(30) = 2 + 3 + 5 = 10
      10 + 3 = 13 

Creator: Gerald Thurman [gthurman@gmail.com]
Created: 21 December 2007

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This work is licensed under a Creative Commons Attribution 3.0 United States License.